HV exit exam questions and answers — High Voltage Safety Officer & Switchgear Operations

All 428 questions from the High Voltage Safety Officer & Switchgear Operations (HV) exit exam bank, with the correct option marked and a one-line explanation for every one. Read it end to end, jump to a single set, or search it — then sit the same bank as a timed paper. The real DG Shipping exit exam is 30 marks with 15 to pass.

These are the bank's original groupings. Practising deals the whole bank out again every time you reshuffle, so a practice “Set 3” is a different 30 questions each time — the set numbers below are the source file's, and are only used here so the key stays navigable. Options are listed in their original order with fixed letters; when you practise, those move too.

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Set 1 — 30 HV exit exam questions with answers

1. What voltage level is defined as High Voltage (HV) under marine SOLAS and STCW regulations?

A. AC voltage exceeding 1,000 Volts (1 kV) between conductors, or >600V to earth
B. > 24V
C. > 100,000V
D. > 110V

Answer: A — AC voltage exceeding 1,000 Volts (1 kV) between conductors, or >600V to earth

In marine electrical engineering, voltages exceeding 1,000V AC (e.g. 3.3 kV, 6.6 kV, 11 kV) are classified as High Voltage.

2. What is the primary hazard of working on High Voltage marine equipment compared to Low Voltage?

A. Magnetic tool attraction
B. Low sound
C. Severe electric arc flash explosion, high-energy blast, and arcing across air gaps
D. Slightly warm wires

Answer: C — Severe electric arc flash explosion, high-energy blast, and arcing across air gaps

High Voltage can ionize air and jump across gaps, creating explosive arc flashes with temperatures exceeding 10,000°C.

3. What safety document MUST be issued before performing maintenance on High Voltage switchgear?

A. Verbal approval
B. Deck log entry
C. High Voltage Permit to Work (PTW) and Sanction to Test
D. Hot Work Permit only

Answer: C — High Voltage Permit to Work (PTW) and Sanction to Test

HV PTWs enforce formal isolation, proving dead, earthing, key interlocking, and authorized signatures.

4. What mandatory step must be taken immediately after opening an HV circuit breaker and isolator?

A. Touch with bare hand
B. Spray water
C. Prove equipment dead using approved calibrated HV voltage detector, then apply safety earths
D. Turn on generator

Answer: C — Prove equipment dead using approved calibrated HV voltage detector, then apply safety earths

Testing with a certified HV voltage indicator ('prove dead') is mandatory before touching conductors or earthing.

5. What medium is commonly used inside modern High Voltage marine circuit breakers for arc quenching?

A. Water bath
B. Compressed oxygen
C. Vacuum (VCB) or Sulfur Hexafluoride gas (SF6)
D. Mineral oil

Answer: C — Vacuum (VCB) or Sulfur Hexafluoride gas (SF6)

Vacuum Circuit Breakers (VCB) and SF6 gas breakers rapidly extinguish high-energy electrical arcs.

6. What safety equipment must be worn when racking in or out an HV circuit breaker truck?

A. Leather apron
B. T-shirt and glasses
C. Cotton coveralls
D. Arc flash protection suit (rated cal/cm2), arc flash visor/hood, HV insulating gloves, and ear protection

Answer: D — Arc flash protection suit (rated cal/cm2), arc flash visor/hood, HV insulating gloves, and ear protection

Racking operations carry high risk of arc flash; full arc flash PPE suite is mandatory.

7. What is the purpose of earthing switches or portable earthing leads on isolated HV equipment?

A. Discharge residual capacitive charge and short-circuit conductors to ground for safety
B. Charge batteries
C. Increase voltage
D. Improve power factor

Answer: A — Discharge residual capacitive charge and short-circuit conductors to ground for safety

Earthing guarantees that conductors remain at ground potential even if accidentally re-energized.

8. What key interlock system is used to enforce strict mechanical sequence during HV switching operations?

A. Combination lock
B. Card swipe
C. Master padlock
D. Castell Key Interlock System

Answer: D — Castell Key Interlock System

Castell key interlocks physically prevent incorrect switching operations (e.g. earthing a live busbar).

9. What instrument is used to measure insulation resistance of High Voltage cables and machinery?

A. Hydrometer
B. High Voltage Megohmmeter (Megger - 2.5 kV or 5 kV DC test voltage)
C. Oscilloscope
D. Low voltage multimeter

Answer: B — High Voltage Megohmmeter (Megger - 2.5 kV or 5 kV DC test voltage)

HV insulation testing requires 2500V or 5000V DC test voltages to detect dielectric insulation weakness.

10. Who is authorized to issue a High Voltage Permit to Work on a ship?

A. Any deck rating
B. Certified High Voltage Authorised Person (HVAP) / Chief Engineer / Senior Electrical Officer
C. Chief Steward
D. Port Watchman

Answer: B — Certified High Voltage Authorised Person (HVAP) / Chief Engineer / Senior Electrical Officer

Only formally trained and certified Authorised Persons (HVAP) can issue HV permits and approve switching schedules.

11. What is creepage distance on High Voltage insulators?

A. Distance between bow and stern
B. Cable length
C. Busbar thickness
D. Shortest distance along outer surface of insulating material between two conductive parts

Answer: D — Shortest distance along outer surface of insulating material between two conductive parts

Insulators use corrugated fins to increase creepage surface distance, preventing surface flashover tracking.

12. What condition occurs when electrical stress breaks down insulating properties of air or solid insulation?

A. Magnetic hysteresis
B. Phase sync
C. Dielectric breakdown / Flashover
D. Resistance increase

Answer: C — Dielectric breakdown / Flashover

Dielectric breakdown creates conductive plasma paths, resulting in violent short-circuit arc flashes.

13. What is the function of a Potential Transformer (PT) / Voltage Transformer (VT) in HV switchboards?

A. Step up to 100 kV
B. Cool generators
C. Step down High Voltage (e.g. 6.6 kV) to safe Low Voltage (e.g. 110V) for metering and relays
D. Convert AC to DC

Answer: C — Step down High Voltage (e.g. 6.6 kV) to safe Low Voltage (e.g. 110V) for metering and relays

VTs provide accurate low-voltage signals for instruments and protective relays without exposing meters to HV.

14. What is the function of a Current Transformer (CT) in HV switchgear?

A. Store energy
B. Step up to 10,000A
C. Measure water temp
D. Step down high primary currents (e.g. 1000A) to standard low secondary currents (1A or 5A) for relays

Answer: D — Step down high primary currents (e.g. 1000A) to standard low secondary currents (1A or 5A) for relays

CTs isolate measurement circuits while converting heavy busbar currents into safe relay inputs.

15. What protective relay detects imbalances between incoming and outgoing currents in HV generator windings?

A. Differential Protection Relay (87G)
B. Under Voltage Relay (27)
C. Reverse Power Relay (32)
D. Over Frequency Relay (81)

Answer: A — Differential Protection Relay (87G)

Differential relays trip instantaneously if internal winding insulation fails, clearing internal generator faults.

16. What is the function of a Neutral Grounding Resistor (NGR) in marine HV electrical systems?

A. Ground antenna
B. Limit earth fault current to a safe value (e.g. 10A - 20A), preventing core damage during single phase-to-ground faults
C. Increase fault current
D. Heat engine room

Answer: B — Limit earth fault current to a safe value (e.g. 10A - 20A), preventing core damage during single phase-to-ground faults

NGRs restrict phase-to-ground fault currents to manageable levels while maintaining system protection tripping.

17. What hazard is associated with opening the secondary circuit of an energized Current Transformer (CT)?

A. Fuse blows
B. Extremely high induced open-circuit voltage causing lethal electric shock and explosive CT failure
C. CT cools down
D. Voltage drops to zero

Answer: B — Extremely high induced open-circuit voltage causing lethal electric shock and explosive CT failure

NEVER open-circuit a live CT secondary; high induced voltage spikes (> 1000V) cause fatal shocks.

18. What testing device is used to check insulation integrity of HV rubber gloves before use?

A. Manual air inflation pump / glove tester to check for pinhole leaks, plus visual inspection
B. Seawater dip
C. Pliers stretch
D. Flame check

Answer: A — Manual air inflation pump / glove tester to check for pinhole leaks, plus visual inspection

Air testing inflates gloves to reveal microscopic punctures that could pass high voltage currents.

19. What color is standard for High Voltage warning hazard signs on marine switchboards?

A. Green sign
B. Yellow background with black skull/lightning symbol and 'DANGER - HIGH VOLTAGE' text
C. Pink sign
D. White sign with blue text

Answer: B — Yellow background with black skull/lightning symbol and 'DANGER - HIGH VOLTAGE' text

High Voltage enclosures require high-visibility danger signs specifying operating voltage (e.g. DANGER 6.6 kV).

20. What protection is provided by a Reverse Power Relay (32) on a marine HV generator?

A. Protects high temp
B. Protects low oil
C. Trips generator breaker if electrical power flows BACK into generator (motoring the prime mover)
D. Protects low frequency

Answer: C — Trips generator breaker if electrical power flows BACK into generator (motoring the prime mover)

Reverse power relays prevent a failed engine from being driven as an electric motor by parallel generators.

21. What is the purpose of an Arc Flash Detection system inside HV switchboard cubicles?

A. Turn on cabin lights
B. Detect smoke
C. Take photos
D. Detect light flash via fiber-optic sensors and trip main breaker in milliseconds before pressure explosion develops

Answer: D — Detect light flash via fiber-optic sensors and trip main breaker in milliseconds before pressure explosion develops

Optical arc sensors combined with overcurrent relays trip switchgear in under 2–5 milliseconds to minimize arc flash damage.

22. What is partial discharge (PD) in High Voltage electrical apparatus?

A. Battery charge
B. Load shedding
C. Microscopic electrical sparks inside insulation voids that gradually erode dielectric strength
D. Complete breaker trip

Answer: C — Microscopic electrical sparks inside insulation voids that gradually erode dielectric strength

Partial discharge monitoring detects insulation degradation early, preventing catastrophic breakdown.

23. What insulation class is standard for marine HV electric motor windings (Class F or Class H)?

A. Wood insulation
B. Paper insulation
C. Cotton thread
D. Class F (155°C max temp) or Class H (180°C max temp) with vacuum pressure impregnation (VPI)

Answer: D — Class F (155°C max temp) or Class H (180°C max temp) with vacuum pressure impregnation (VPI)

Class F/H resin impregnation provides high dielectric strength and thermal resistance for marine motor windings.

24. What device is used to safely discharge stored electrical energy in large HV capacitor banks before maintenance?

A. Water spray
B. Screwdriver touch
C. Fan
D. Built-in discharge resistors and portable earthing/shorting probe

Answer: D — Built-in discharge resistors and portable earthing/shorting probe

Capacitors retain lethal charges long after power is disconnected; earthing probes ensure complete discharge.

25. What safety distance (air clearance) must be maintained from exposed un-insulated 6.6 kV conductors?

A. 1 mm
B. Minimum safe clearance distance (~150 mm to 250 mm depending on standard, plus safety margin)
C. Zero
D. 10 meters

Answer: B — Minimum safe clearance distance (~150 mm to 250 mm depending on standard, plus safety margin)

High Voltage requires minimum physical air clearance gaps to prevent flashover ionization.

26. What test is performed to measure moisture and gas breakdown in SF6 circuit breaker gas?

A. Titration
B. SF6 gas purity, dew point, and decomposition product analysis
C. Salt water test
D. Smoke test

Answer: B — SF6 gas purity, dew point, and decomposition product analysis

SF6 gas purity and moisture content must be verified to ensure high dielectric breakdown strength.

27. What is the function of a Synchroscope on an HV switchboard during generator paralleling?

A. Test insulation
B. Measure fuel
C. Indicate phase angle alignment and frequency difference between incoming generator and live busbar
D. Measure oil pressure

Answer: C — Indicate phase angle alignment and frequency difference between incoming generator and live busbar

Synchroscopes rotate to show when incoming generator frequency and phase match busbars for safe breaker closing.

28. What risk occurs if a generator breaker is closed out of phase (e.g. 180° out of phase) with live HV busbar?

A. Engine stops
B. Severe mechanical torque shock shearing generator shaft, massive fault current, and switchboard explosion
C. Slight light dim
D. Smooth speed up

Answer: B — Severe mechanical torque shock shearing generator shaft, massive fault current, and switchboard explosion

Out-of-phase paralleling generates massive mechanical and magnetic forces that can destroy engine mounts and switchgear.

29. What is an HV Competent Person (HVCP) under marine safety management systems?

A. Steward
B. Person trained and authorized to perform specific HV work under supervision of an HV Authorised Person
C. Any crew member
D. Deck apprentice

Answer: B — Person trained and authorized to perform specific HV work under supervision of an HV Authorised Person

Competent Persons have verified practical training to carry out specified HV switching or maintenance tasks safely.

30. What publication provides detailed rules for Electrical Installations on Ships including High Voltage (IEC 60092)?

A. IMDG Code
B. MARPOL Annex VI
C. IEC 60092 Standards / SOLAS Chapter II-1
D. STCW Table A-II/1

Answer: C — IEC 60092 Standards / SOLAS Chapter II-1

IEC 60092 series is the global engineering standard for marine electrical design, HV equipment, and safety.

Set 2 — 30 HV exit exam questions with answers

1. What safety enclosure rating (IP Rating) is required for High Voltage marine switchboard cubicles?

A. IP00
B. IP10
C. No rating
D. IP2X minimum (protection against touch) up to IP44/IP54 for splash protection

Answer: D — IP2X minimum (protection against touch) up to IP44/IP54 for splash protection

IP ratings prevent accidental tool or finger contact with live HV busbars.

2. What is the function of a High Voltage bus tie breaker?

A. Ties ropes
B. Connects or isolates two separate HV busbar sections for operational flexibility and redundancy
C. Connects battery charger
D. Regulates water

Answer: B — Connects or isolates two separate HV busbar sections for operational flexibility and redundancy

Bus tie breakers allow split-busbar operation so a fault on one side does not shut down the entire ship.

3. What device measures temperature continuously inside HV motor or generator stator windings?

A. Pressure gauge
B. Glass thermometer
C. Resistance Temperature Detectors (RTD - PT100) or Thermocouples
D. Hydrometer

Answer: C — Resistance Temperature Detectors (RTD - PT100) or Thermocouples

Embedded PT100 RTDs trigger alarm and trip signals if winding insulation temperature exceeds limits.

4. What procedure is followed during Sanction to Test on High Voltage equipment?

A. Temporarily remove safety earths under strict control to perform insulation or hi-pot testing, then re-earth
B. Remove warning signs
C. Turn on generator full load
D. Cancel permit

Answer: A — Temporarily remove safety earths under strict control to perform insulation or hi-pot testing, then re-earth

Sanction to Test allows controlled temporary removal of earths for insulation diagnostics.

5. What is High Potential (Hi-Pot) dielectric testing on HV cables?

A. Testing battery with bulb
B. Applying elevated DC/AC voltage (e.g. 2x rated voltage) to verify cable dielectric withstand strength
C. Measuring length
D. Checking cable color

Answer: B — Applying elevated DC/AC voltage (e.g. 2x rated voltage) to verify cable dielectric withstand strength

Hi-Pot testing proves that cable insulation can withstand over-voltage surges without breakdown.

6. What is the function of a surge arrestor connected to HV generator terminals?

A. Cool generator
B. Reduce noise
C. Divert high-voltage switching surges and lightning spikes safely to ground
D. Increase generator power

Answer: C — Divert high-voltage switching surges and lightning spikes safely to ground

Surge arrestors clamp fast transient voltage spikes, protecting expensive stator insulation.

7. What risk occurs if an HV circuit breaker is closed into a solid phase-to-phase short circuit fault?

A. Massive fault current flows; instantaneous overcurrent relays must trip breaker within cycles
B. Heats up slowly over hours
C. Voltage increases smoothly
D. Speed increases

Answer: A — Massive fault current flows; instantaneous overcurrent relays must trip breaker within cycles

Fault currents reach tens of thousands of Amperes; fast electromagnetic tripping clears the short circuit instantly.

8. What safety floor matting must be placed in front of marine High Voltage switchboards?

A. Aluminum plate
B. Steel mesh mat
C. Wet cloth rag
D. Tested dielectric insulating rubber matting (rated for operating voltage, e.g. 15 kV - 30 kV rating)

Answer: D — Tested dielectric insulating rubber matting (rated for operating voltage, e.g. 15 kV - 30 kV rating)

Certified insulating rubber mats provide secondary shock protection for operators standing at switchboards.

9. What type of fire extinguisher is mandatory near High Voltage switchboard rooms?

A. Foam pipe
B. Water jet
C. Carbon Dioxide (CO2) or Clean Agent non-conducting gas extinguishers
D. Saltwater hose

Answer: C — Carbon Dioxide (CO2) or Clean Agent non-conducting gas extinguishers

CO2 and clean gas agents extinguish electrical fires without leaving conductive or corrosive residue.

10. Who keeps primary key to safety padlocks securing an HV isolation point under a Permit to Work?

A. Messman
B. Anyone on deck
C. Pilot
D. The High Voltage Authorised Person (HVAP) / Person in Charge of work

Answer: D — The High Voltage Authorised Person (HVAP) / Person in Charge of work

Key control ensures no unauthorized person can remove padlocks or re-energize equipment during maintenance.

11. What is the function of a zero-sequence current transformer (ZSCT) in core-balance earth fault protection?

A. Control temp
B. Measure RPM
C. Detect vector sum of 3-phase currents; any non-zero value indicates earth leakage current
D. Measure battery

Answer: C — Detect vector sum of 3-phase currents; any non-zero value indicates earth leakage current

ZSCTs enclose all three phase cables; balanced current sums to zero, whereas earth leakage produces unbalance output.

12. What is the effect of harmonics generated by large High Voltage variable frequency drives (VFD)?

A. Cleans contacts
B. Reduces cable size
C. Increases efficiency 50%
D. Voltage waveform distortion, overheating of transformers/generators, and electronic interference

Answer: D — Voltage waveform distortion, overheating of transformers/generators, and electronic interference

VFD switching generates high-frequency harmonic currents requiring active/passive harmonic filters.

13. What maintenance step is required before cleaning HV switchboard busbar compartments?

A. Complete isolation, prove dead, apply portable earths, post warning signs, and issue HV PTW
B. Spray solvent live
C. Blow compressed air live
D. Wipe live with damp cloth

Answer: A — Complete isolation, prove dead, apply portable earths, post warning signs, and issue HV PTW

Busbar compartments MUST be totally de-energized, tested dead, earthed, and permitted before opening panels.

14. What is the function of an Automatic Voltage Regulator (AVR) on an HV generator?

A. Control generator field excitation to maintain steady terminal voltage under changing load
B. Measure oil pressure
C. Cool bearings
D. Control engine RPM

Answer: A — Control generator field excitation to maintain steady terminal voltage under changing load

AVRs adjust rotor field current to keep generated voltage stable despite load swings.

15. What is the purpose of an anti-condensation heater fitted inside HV electric motors and switchboards?

A. Heat engine room
B. Warm lube oil
C. Dry clothes
D. Maintain internal temperature slightly above ambient when stopped to prevent moisture condensation

Answer: D — Maintain internal temperature slightly above ambient when stopped to prevent moisture condensation

Anti-condensation space heaters energize automatically when equipment stops, keeping internal air dry.

16. What safety precaution must be observed when using a high voltage test probe to prove equipment dead?

A. Use low voltage light
B. Test probe on known live source FIRST, test target equipment, then re-test probe on known live source ('Live-Dead-Live' test)
C. Test on table
D. Assume indicator works

Answer: B — Test probe on known live source FIRST, test target equipment, then re-test probe on known live source ('Live-Dead-Live' test)

The 'Live-Dead-Live' protocol proves the voltage detector itself was working before and after the test.

17. What device trips an HV transformer if gas accumulation or oil surge occurs due to internal fault?

A. Speed governor
B. Thermocouple
C. Buchholz Relay
D. Reverse power relay

Answer: C — Buchholz Relay

Buchholz relays mounted on oil-immersed transformers detect internal arcing gas bubbles and oil surges.

18. What is the purpose of Lockout / Tagout (LOTO) padlocks on HV isolators?

A. Prevent theft
B. Hold door open
C. Decorate panel
D. Physically lock isolator handles in OPEN/OFF position with warning tags so no one can re-energize circuit

Answer: D — Physically lock isolator handles in OPEN/OFF position with warning tags so no one can re-energize circuit

LOTO padlocks prevent accidental or unauthorized re-closure of isolation switches.

19. What action must be taken immediately if a colleague contacts a High Voltage live conductor and collapses?

A. Throw water
B. Grab victim's arm bare hands
C. Run away
D. DO NOT touch victim directly; isolate HV power source immediately or use certified HV insulating rescue hook to pull victim away, then call emergency CPR

Answer: D — DO NOT touch victim directly; isolate HV power source immediately or use certified HV insulating rescue hook to pull victim away, then call emergency CPR

Touching an HV casualty before isolating power turns the rescuer into a second high-voltage casualty.

20. Who has overall responsibility for enforcing High Voltage safety management systems on board?

A. Ship cook
B. Port Agent
C. Shore electrician
D. The Chief Engineer and Ship's Master

Answer: D — The Chief Engineer and Ship's Master

The Chief Engineer leads technical HV management while the Master maintains overall safety responsibility.

21. Which document contains the shipyard's description of operations regarding the vessel's High Voltage (HV) equipment?

A. Preliminary operation manual
B. High voltage operation manual
C. Performance manual
D. Equipment manual

Answer: A — Preliminary operation manual

The Preliminary Operation Manual supplied by the shipyard provides the initial operational philosophy, system architecture, and operating parameters for the vessel's high-voltage equipment.

22. A low voltage protection circuit is used in electric motor starting equipment to:

A. Trip circuit contactor when the motor over-speeds due to low voltage
B. Trip circuit contactor when the motor develops short circuit due to low voltage
C. Allow the motor to restart automatically on restoration of voltage without manual resetting
D. Prevent motor from restarting automatically upon restoration of supply voltage

Answer: D — Prevent motor from restarting automatically upon restoration of supply voltage

Under-voltage (low voltage) protection prevents unexpected automatic restarting of motors upon voltage restoration, which protects personnel from mechanical injury and the electrical network from excessive starting surges.

23. In air blast circuit breakers, the compressed air pressure used for arc quenching is in the order of:

A. 100 mm Hg
B. 1 kg/cm²
C. 20 to 30 kg/cm²
D. 200 to 300 kg/cm²

Answer: C — 20 to 30 kg/cm²

Air blast circuit breakers (ABCB) utilize high-pressure dry air at approximately 20 to 30 kg/cm² (2 to 3 MPa / 20 to 30 bar) directed across the contacts to rapidly cool and deionize the arc.

24. What is the primary means by which an electrical maintenance worker is protected from electrical hazards while performing work on an electrical circuit?

A. Shutting down the necessary equipment only
B. Using personal protective equipment (PPE) alone
C. Posting safety warning signs
D. Performing a lockout / tagout (LOTO) and grounding procedure

Answer: D — Performing a lockout / tagout (LOTO) and grounding procedure

Lockout/Tagout (LOTO) coupled with proving dead and applying safety earths provides positive isolation, preventing accidental re-energization of circuits during maintenance.

25. What is the Polarisation Index (PI) in insulation resistance testing?

A. Ratio of 10-minute insulation resistance to 1-minute insulation resistance
B. Ratio of 1-minute insulation resistance to 10-minute insulation resistance
C. Ratio of 5-minute insulation resistance to 25-minute insulation resistance
D. Ratio of test voltage to leakage current after 1 minute

Answer: A — Ratio of 10-minute insulation resistance to 1-minute insulation resistance

Polarisation Index (PI) is defined as the ratio of insulation resistance measured at 10 minutes to the insulation resistance measured at 1 minute (R10 / R1). A PI value above 2.0 indicates clean, dry insulation.

26. The direction of mechanical force experienced by a current-carrying conductor placed in a magnetic field is determined by:

A. Fleming's left-hand rule
B. Fleming's right-hand rule
C. Helix rule
D. Corkscrew rule

Answer: A — Fleming's left-hand rule

Fleming's left-hand rule is used for electric motors and determining mechanical force on a conductor (Thumb = Force/Motion, Forefinger = Field, Middle finger = Current).

27. Which of the following insulating media can be used in sphere gaps and high-voltage test setups?

A. Liquid insulation (e.g., transformer oil)
B. Solid insulation (e.g., polyester, resin-bonded paper)
C. Gas insulation (e.g., SF6, N2, CO2, CCl2F2)
D. Any of the above

Answer: D — Any of the above

High voltage insulation testing and arc gaps can employ gaseous (air, SF6, N2), liquid (dielectric mineral oils), or solid insulating barriers depending on application requirements.

28. Cartridge and rewireable fuse protection in general distribution circuits is typically used for continuous current ratings up to:

A. 10 A
B. 20 A
C. 50 A
D. 100 A

Answer: D — 100 A

Standard general-purpose distribution rewireable/cartridge fuse holders are commonly rated up to 100 A, beyond which High Rupturing Capacity (HRC) bolted fuses or circuit breakers are preferred.

29. Which of the following circuit breakers provides high operating reliability and minimal maintenance requirements?

A. Air blast circuit breaker
B. SF6 gas circuit breaker
C. Vacuum circuit breaker
D. Bulk oil circuit breaker

Answer: B — SF6 gas circuit breaker

Sulphur Hexafluoride (SF6) circuit breakers operate in a sealed, non-corrosive environment with high dielectric regeneration, resulting in minimal contact erosion and extended maintenance intervals.

30. Which type of work procedure involves electrical installations that are completely de-energized, isolated, proven dead, and earthed?

A. Live working
B. Dead working
C. Working in the vicinity of live parts
D. Hot line working

Answer: B — Dead working

Dead working is defined as work carried out on electrical equipment or installations that are isolated from all sources of supply, proven dead, locked out, and earthed.

Set 3 — 30 HV exit exam questions with answers

1. Following the 2010 Manila Amendments, which regulatory body made High Voltage (HV) system training mandatory for senior engineering staff on vessels with installations above 1,000 V?

A. SOLAS
B. STCW
C. MARPOL
D. ISPS

Answer: B — STCW

The STCW (Standards of Training, Certification and Watchkeeping) 2010 Manila Amendments introduced mandatory HV training and certification for Electro-Technical Officers (ETO) and Management Level Marine Engineers (A-III/1, A-III/2).

2. Air blast circuit breakers are particularly preferred for applications requiring:

A. Short duty only
B. Intermittent duty only
C. Repeated frequent duty
D. DC traction sub-stations only

Answer: C — Repeated frequent duty

Because compressed air is continually replenished and leaves no chemical breakdown residues, air blast circuit breakers are well-suited for repeated duty and fast arc clearing.

3. High-voltage alternating current (AC) arc interruption is primarily achieved through:

A. High resistance interruption
B. Low resistance / zero-current interruption
C. Both A and B
D. Magnetic attraction only

Answer: C — Both A and B

Arc extinction can be achieved by either increasing arc resistance (lengthening, cooling, splitting) or exploiting natural AC zero-current crossing (low resistance method) to deionize the contact gap.

4. The desired protective tripping operation of a power circuit breaker under fault conditions is:

A. Manually initiated
B. Automatically initiated by protective relays
C. Warning buzzer alarm only
D. Delayed until watchkeeper reset

Answer: B — Automatically initiated by protective relays

Circuit breakers are designed to trip automatically and instantaneously upon receiving a trip command from associated protective relays (e.g., overcurrent, short circuit, differential).

5. Lower voltage systems supplied through step-down transformers from high-voltage systems must be protected against overvoltages. This is achieved by:

A. Direct earthing of the lower voltage system neutral
B. Appropriate neutral voltage limiters (surge arresters / spark gaps)
C. Both A and B
D. Increasing cable insulation thickness only

Answer: C — Both A and B

Protection against HV-to-LV breakdown in transformers is achieved by earthing the secondary neutral (solidly or via impedance) or installing neutral voltage limiters (surge arresters/spark gaps).

6. In the arc extinction process using the high resistance method:

A. Arc resistance is decreased with time
B. Arc resistance is increased with time
C. Arc resistance remains strictly constant
D. Arc resistance is reduced to zero

Answer: B — Arc resistance is increased with time

The high resistance method progressively increases the arc resistance by lengthening, cooling, and splitting the arc until the system voltage is insufficient to maintain the arc.

7. On high-voltage positive polarity conductors at elevated pressures, corona discharge visually appears as a:

A. Uniform bluish-white glowing sheath
B. Reddish glowing spot
C. Uniform greenish spot
D. Yellow pulsating flame

Answer: A — Uniform bluish-white glowing sheath

Positive corona produces a smooth, uniform bluish-violet/white glow around the conductor surface, whereas negative corona forms concentrated reddish or purplish glowing tufts/spots.

8. The heat produced at an electrical contact point owing to current flow is least affected by:

A. Temperature of the surrounding medium
B. Contact resistance
C. Magnitude of electric current flowing
D. Duration of current flow

Answer: A — Temperature of the surrounding medium

Heat generated at contact points is determined by Joule's Law (Q = I²·R·t). Surrounding ambient temperature has minimal direct influence on the heat generation rate compared to I, R, and t.

9. When procedures involve the temporary removal of circuit main earths under a Sanction-to-Test, the earths are secured with working locks. The keys to these locks are:

A. Retained by the Authorised Person (HV) who removes and replaces the earth as requested
B. Retained by the Chief Engineer
C. Retained by the Electro-Technical Officer only
D. Kept inside the engine control room safety locker

Answer: A — Retained by the Authorised Person (HV) who removes and replaces the earth as requested

Under marine HV safety rules, the Authorised Person (HV) who issued the Sanction-to-Test retains custody of safety lock keys and personally oversees the removal and re-application of earths.

10. What are the primary hazards associated with an Arc Flash event in High Voltage systems?

A. Sudden release of intense thermal energy, blinding light, and acoustic shockwaves
B. Severe burns and blast injuries that can be fatal even several meters away
C. Catastrophic structural equipment destruction and vaporized shrapnel
D. All of the above

Answer: D — All of the above

An arc flash generates plasma temperatures up to 20,000°C, explosive pressure waves (arc blast), intense light, molten metal shrapnel, and toxic gases, presenting catastrophic risk to personnel and equipment.

11. Electrical alternating current (AC) power is generated and transmitted at high voltage primarily to:

A. Safeguard against power theft
B. Minimise I²R transmission losses and reduce conductor size
C. Reduce the capital cost of generator construction
D. Eliminate the need for circuit breakers

Answer: B — Minimise I²R transmission losses and reduce conductor size

Transmitting at higher voltage reduces current for a given power level (P = √3·V·I·cosφ). Lower current drastically reduces transmission line I²R heat losses and copper conductor cross-section requirements.

12. When two diesel generator sets are running smoothly in parallel on the main switchboard during an engine room watch, what does this condition fundamentally imply?

A. Both generators have identical terminal voltage, system frequency, and phase sequence
B. Both diesels carry identical active kW, reactive kVAr, and line amperage at all times
C. Both diesels are operating at varying unsynchronized rotational speeds
D. Both generators are operating without voltage regulators

Answer: A — Both generators have identical terminal voltage, system frequency, and phase sequence

Parallel operation requires synchronization: identical terminal voltage, identical frequency, correct phase sequence, and identical phase angle at the moment of breaker closure.

13. In a 3-phase induction motor operating in normal motoring mode, the rotor rotates at a speed that is:

A. Equal to the synchronous speed of the stator rotating magnetic field
B. Slightly lower than the synchronous speed of the stator field
C. Higher than the synchronous speed of the stator field
D. Completely unrelated to the stator supply frequency

Answer: B — Slightly lower than the synchronous speed of the stator field

An induction motor requires relative motion (slip) between the rotating stator magnetic field and the rotor conductors to induce rotor current and develop torque. Hence, rotor speed is lower than synchronous speed.

14. For liquid-cooled static frequency converters on electric propulsion vessels, which additional monitoring arrangements must be provided?

A. Stand-by coolant pump automatic start / alarm
B. Coolant conductivity monitoring (de-ionized water)
C. Coolant temperature and flow rate alarms
D. All of the above

Answer: D — All of the above

Liquid-cooled marine power converters (drives) require monitoring of deionized water conductivity (preventing arcing), coolant flow, temperature levels, and automatic standby circulation pump cut-in.

15. Which of the following metals has the highest electrical conductivity at room temperature?

A. Copper
B. Aluminium
C. Silver
D. Iron

Answer: C — Silver

Silver possesses the highest electrical conductivity of all metals (~6.30 × 10⁷ S/m), followed closely by copper (~5.96 × 10⁷ S/m) and gold (~4.10 × 10⁷ S/m).

16. In a high-voltage circuit breaker, ionization of the contact gap medium is facilitated by:

A. Increase in electric field strength across the gap
B. Increase in mean free path of electrons
C. High temperature of the surrounding dielectric medium
D. All of the above

Answer: D — All of the above

Ionization between breaker contacts during contact separation is enhanced by high electric field gradients (voltage stress), thermal excitation at high temperatures, and longer electron mean free paths.

17. If two alternators are operating in parallel synchronism and the excitation voltage of one machine is suddenly increased:

A. Both machines will trip instantly
B. The over-excited machine will motorize
C. Circulating reactive current flows and synchronising torque maintains synchronism
D. The system frequency increases proportionally

Answer: C — Circulating reactive current flows and synchronising torque maintains synchronism

Increasing excitation on one alternator increases its generated EMF, driving a circulating lagging reactive current that produces demagnetizing armature reaction and synchronizing torque, preserving synchronism.

18. When a vessel suffers an unexpected total electrical blackout, what should be the watch engineer's initial priority action?

A. Notify the Chief Engineer and bridge watch officer immediately while emergency power initiates
B. Cancel audible alarms and wait for automation reset
C. Start stripping the oily water separator
D. Open all high-voltage generator isolators manually

Answer: A — Notify the Chief Engineer and bridge watch officer immediately while emergency power initiates

Upon blackout, immediate notification of the Chief Engineer and Bridge is critical while confirming emergency generator starting and busbar connection, followed by systematic power restoration.

19. What safety precautions must be strictly observed when working on marine High Voltage vessels?

A. Observing minimum approach distances corresponding to system voltage
B. Employing approved Permit to Work (PTW) and switching procedures
C. Using certified high-voltage PPE, face shields, and insulated tools
D. All of the above

Answer: D — All of the above

Marine HV safety compliance mandates strict adherence to approach boundaries, written PTW / Sanction-to-Test systems, calibrated HV voltage detectors, earthing sticks, and rated arc-flash PPE.

20. Power loss due to corona discharge on an overhead transmission line is directly proportional to:

A. Radius of conductor
B. Supply frequency (f + 25)
C. Spacing between conductors
D. Dielectric constant of air

Answer: B — Supply frequency (f + 25)

According to Peek's formula for corona loss, power loss P is proportional to (f + 25), where f is the system supply frequency.

21. Electrical injuries and fatalities on board vessels are commonly caused by:

A. Defective equipment or deteriorated insulation
B. Hazardous or damp working environments
C. Unsafe work practices and failure to isolate/ground
D. All of the above

Answer: D — All of the above

Marine electrical accidents stem from equipment degradation (salt, vibration, moisture), wet bilges/decks, and bypassing lock-out / tag-out / isolation safety protocols.

22. A synchronous alternator connected to an infinite bus is said to be overexcited when it operates at a:

A. Unity power factor
B. Lagging power factor (delivering reactive power to the grid)
C. Leading power factor (absorbing reactive power)
D. Zero power factor leading

Answer: B — Lagging power factor (delivering reactive power to the grid)

Overexcitation in a synchronous generator causes it to generate and export lagging reactive power (kVAr) to the electrical network, operating at a lagging power factor.

23. In a power transformer, the iron (core) loss remains practically constant from no-load to full-load because:

A. Transformation ratio is fixed
B. Permeability of the laminated core is infinite
C. Core magnetic flux remains practically constant across all load levels
D. Primary winding current is constant

Answer: C — Core magnetic flux remains practically constant across all load levels

Because primary supply voltage and frequency are constant, the main mutual core flux Φ remains virtually constant regardless of load current, keeping core hysteresis and eddy current losses constant.

24. Surge overvoltages in high-voltage power networks originate due to:

A. Atmospheric lightning discharges
B. System switching operations (e.g., tripping inductive/capacitive loads)
C. Sudden system earth faults and insulation breakdown
D. Any of the above

Answer: D — Any of the above

Transient surges are classified into external surges (direct/induced lightning) and internal switching surges (circuit breaker opening/closing on inductive or capacitive loads, fault transients).

25. Which of the following measuring instruments is used for measuring high-frequency AC surge and impulse peak voltages?

A. Peak voltmeter
B. Moving iron voltmeter
C. Dynamometer voltmeter
D. Thermocouple ammeter

Answer: A — Peak voltmeter

Peak voltmeters (using diode rectifiers and storage capacitors or digital peak detectors) accurately measure the maximum peak crest value of high-frequency and impulse waveforms.

26. Large-capacity marine high-voltage generators on diesel-electric propulsion vessels are typically designed to generate power at:

A. 220 V to 440 V
B. 6.6 kV to 11 kV
C. 132 kV to 220 kV
D. 400 kV

Answer: B — 6.6 kV to 11 kV

Marine high-voltage generation on cruise ships, LNG carriers, and drilling rigs is standardized at 3.3 kV, 6.6 kV, or 11 kV (class limits typically up to 15 kV).

27. Overvoltage protection devices (e.g., surge arresters and voltage surge limiters) are recommended for:

A. Hydro-electric generators
B. Steam turbine generators
C. Gas turbine and diesel generators
D. All of the above

Answer: D — All of the above

All major power generators regardless of prime mover type require overvoltage protection to protect stator insulation from lightning, switching surges, and load rejection overvoltages.

28. A Miniature Circuit Breaker (MCB) provides protection against:

A. Short circuits only
B. Overload conditions only
C. Both overload and short circuit conditions
D. Earth leakage current only

Answer: C — Both overload and short circuit conditions

MCBs integrate thermal protection (bimetallic strip for inverse-time overload protection) and magnetic protection (electromagnetic solenoid for instantaneous short-circuit protection).

29. The arcing contacts in high-voltage circuit breakers are predominantly manufactured from:

A. Copper-tungsten alloy
B. Porcelain ceramic
C. Pure aluminium
D. Phosphor bronze

Answer: A — Copper-tungsten alloy

Copper-tungsten (Cu-W) sintering combines the high electrical/thermal conductivity of copper with the extreme arc erosion resistance and high melting point of tungsten (3,422°C).

30. In AC circuit breakers, natural arc interruption occurs when the alternating current waveform reaches:

A. Peak positive voltage
B. Natural current zero (zero current)
C. Peak negative current
D. Maximum rate of voltage rise (dv/dt)

Answer: B — Natural current zero (zero current)

In AC systems, current naturally passes through zero twice per cycle. At natural current zero, energy input ceases, allowing the dielectric medium to rapidly deionize and prevent arc restrike.

Set 4 — 30 HV exit exam questions with answers

1. Under standard marine classification rules (e.g., IEC 60092-503), common marine high-voltage distribution networks are generally operated at voltages up to:

A. 3.3 kV
B. 6.6 kV
C. 11 kV to 15 kV
D. 33 kV

Answer: C — 11 kV to 15 kV

Marine HV systems normally operate at 3.3 kV, 6.6 kV, or 11 kV, with classification rules defining HV as nominal AC voltages exceeding 1,000 V up to 15 kV.

2. The electric field strength (voltage gradient) at which electrical breakdown occurs in an insulating medium is called its:

A. Breakdown voltage
B. Dielectric strength
C. Polarisation coefficient
D. Permittivity gradient

Answer: B — Dielectric strength

Dielectric strength is the maximum electric field an insulating material can withstand without electrical breakdown, typically expressed in kV/mm or MV/m.

3. What safety floor coverings are installed in front and rear of high-voltage switchboard panels?

A. Conductive grounding mesh
B. Certified electrical insulating rubber mats
C. Steel chequered plates
D. Ceramic unglazed tiles

Answer: B — Certified electrical insulating rubber mats

Tested and rated electrical insulation mats (IEC 61111 / Class 4) are mandatory along all operating pathways and rear maintenance passages of HV switchboards to isolate operators from ground.

4. In an AC synchronous alternator, when the load operates at unity power factor, the effect of armature reaction is:

A. Purely demagnetising
B. Purely cross-magnetising
C. Purely magnetising
D. Zero effect on main field flux

Answer: B — Purely cross-magnetising

At unity power factor, armature current is in phase with induced EMF, producing a cross-magnetising effect that distorts the main magnetic flux distribution across the pole shoes without net demagnetization.

5. Which of the following insulating materials is widely used for low and medium voltage marine electrical wiring and cables?

A. Polyethylene
B. Porcelain
C. Polyvinyl Chloride (PVC) / XLPE
D. Mica paper

Answer: C — Polyvinyl Chloride (PVC) / XLPE

Polyvinyl chloride (PVC) and cross-linked polyethylene (XLPE) with low-smoke zero-halogen (LSZH) sheathing are the most standard insulation materials for low/medium voltage marine cables.

6. For standard high-voltage laboratory calibration and voltage measurements, which spark gap configuration provides the highest measurement accuracy?

A. Sphere gaps
B. Rod gaps
C. Point-to-plane gaps
D. Needle gaps

Answer: A — Sphere gaps

Sphere gaps provide a nearly uniform electrostatic field (when gap spacing is less than sphere diameter), giving highly reproducible and standardized breakdown voltages per IEC 60052.

7. Which of the following is a recognized advantage or positive effect of corona discharge on power lines?

A. Increases power transmission efficiency
B. Eliminates radio interference completely
C. Attenuates steep-fronted voltage surges and reduces electrostatic stress concentration
D. Prevents conductor oxidation

Answer: C — Attenuates steep-fronted voltage surges and reduces electrostatic stress concentration

Corona increases the effective virtual diameter of conductors, which reduces surface electrostatic stress and dissipates high-frequency lightning/switching surge energy through localized ionization.

8. One of the primary functions of personal protective grounds (safety earthing leads) is to provide a _______ impedance path for short-circuit and induced currents.

A. High
B. Low
C. Inductive
D. Capacitive

Answer: B — Low

Personal protective grounds must have very low impedance to ensure that in the event of accidental energization, fault current is safely shunted to earth, instantly tripping upstream protection while limiting touch voltage.

9. A certified Safety Electrical One-Line Diagram must be utilized by authorized personnel to _______ all sources of electrical energy before issuing a Permit to Work.

A. Identify and verify
B. Estimate roughly
C. Bypass temporarily
D. Modify without approval

Answer: A — Identify and verify

A current single-line diagram is the primary engineering reference used to identify all primary, auxiliary, and back-feed power sources prior to planning switching, isolation, and grounding.

10. Work performed on or directly within reaching distance of uninsulated energized electrical components is classified as:

A. Energized (live) work
B. De-energized safe work
C. Sanction-to-test work
D. Dead working

Answer: A — Energized (live) work

Energized (live) electrical work involves working on or near exposed conductors that are not de-energized, locked out, tested dead, and grounded.

11. In electrical safety standards (such as NFPA 70E), crossing the Prohibited Approach Boundary is considered equivalent to making direct:

A. Insulation barrier
B. Visual observation
C. Contact with energized parts
D. Inductive grounding

Answer: C — Contact with energized parts

Crossing the Prohibited Approach Boundary is considered the same as making direct contact with energized electrical conductors, requiring the same PPE and qualifications as direct contact.

12. Under standard electrical installation codes, the minimum allowable clear workspace depth in front of electrical switchboards and control gear is typically:

A. 36 inches (approx. 0.9 to 1.0 m)
B. 48 inches
C. 24 inches
D. 12 inches

Answer: A — 36 inches (approx. 0.9 to 1.0 m)

Standard industrial codes (NEC / NFPA 70 / IEC) require a minimum clear workspace depth of 36 inches (0.9 m) in front of low/medium voltage panels to allow unimpeded escape during emergencies.

13. The secondary winding of an energized Current Transformer (CT) must never be _______ while the primary is carrying current.

A. Earthed
B. Open-circuited
C. Short-circuited
D. Connected to an ammeter

Answer: B — Open-circuited

Open-circuiting an energized CT secondary removes opposing secondary flux, causing the core to saturate and inducing dangerously high peak voltages (thousands of volts) across the open terminals.

14. What range of 50/60 Hz AC electrical current flowing through the human body is generally recognized as the 'can't let go' threshold (tetanic muscle contraction)?

A. 1 to 3 mA
B. 9 to 25 mA
C. 50 to 100 mA
D. 1 to 5 A

Answer: B — 9 to 25 mA

At 9 to 25 mA AC (typically ~10–16 mA for men, ~9–12 mA for women), involuntary muscle contraction prevents a person from releasing an energized conductor ('let-go threshold').

15. When performing high-voltage testing using portable instruments, proving the voltmeter (live-dead-live check) is mandatory on all voltage levels above:

A. 24 V
B. 50 V
C. 230 V
D. 600 V / 1000 V

Answer: D — 600 V / 1000 V

On power circuits above 600 V / 1,000 V, the Live-Dead-Live rule (test detector on known source, test circuit, re-test detector) is strictly mandatory to ensure detector functionality.

16. Which of the following is a generally recognized primary hazard of working on High Voltage electrical systems?

A. Arc Flash / Arc Blast
B. Electromagnetic induction
C. Static charge accumulation
D. Acoustic resonance

Answer: A — Arc Flash / Arc Blast

Arc Flash and electric shock represent the most severe life-threatening hazards during high-voltage switching and maintenance operations.

17. High-voltage insulating rubber gloves used for electrical protection must undergo routine electrical dielectric re-testing at least every:

A. 1 month
B. 6 months
C. 12 months
D. 24 months

Answer: B — 6 months

Under OSHA, NFPA 70E, and marine safety standards, electrical insulating gloves must be dielectrically tested at certified test laboratories every 6 months (or before first issue if older than 12 months).

18. Corona discharge on high-voltage conductors in darkness is visually identified by a:

A. Bright yellow spark
B. Faint violet / bluish glow
C. Continuous red flame
D. Green thermal plume

Answer: B — Faint violet / bluish glow

Corona discharge ionizes atmospheric nitrogen and oxygen molecules, emitting photons primarily in the ultraviolet and violet/bluish visible spectrum.

19. The phenomenon of corona discharge on power transmission conductors is typically accompanied by:

A. A loud explosive detonation
B. A continuous hissing or crackling noise and ozone smell
C. Mechanical vibration of the deck plates
D. Magnetic hum only

Answer: B — A continuous hissing or crackling noise and ozone smell

Corona produces a characteristic hissing/buzzing acoustic sound due to rapid localized ionization and produces ozone (O3) with its distinct pungent odor.

20. A Tesla coil is fundamentally classified as a:

A. Cascaded power transformer
B. Coreless audio transformer
C. High-frequency resonant air-core transformer
D. Low-impedance current transformer

Answer: C — High-frequency resonant air-core transformer

A Tesla coil is an electrical resonant transformer circuit used to produce high-voltage, low-current, high-frequency alternating-current electricity using air-core resonant tuning.

21. A switching surge in a high-voltage electrical network is defined as a:

A. Continuous steady-state high-voltage DC
B. Power frequency harmonic voltage
C. Short-duration transient overvoltage produced by switching operations
D. Permanent neutral shift

Answer: C — Short-duration transient overvoltage produced by switching operations

Switching surges are short-duration (microseconds to milliseconds) damped oscillatory overvoltages caused by breaker opening/closing, interrupting inductive currents, or fault clearing.

22. Insulators used in high-voltage installations are rigorously factory-tested for:

A. Power frequency withstand tests
B. Lightning and switching impulse tests
C. Both A and B
D. Radio wave absorption tests only

Answer: C — Both A and B

High voltage insulators must withstand normal operating power frequency voltages (dry and wet flashover tests) as well as steep lightning and switching impulse voltage surges (1.2/50 μs waveforms).

23. Impulse voltage testing of high-voltage power transformers is conducted primarily to determine the ability of:

A. Transformer tank to withstand structural vibration
B. Winding insulation and bushings to withstand transient overvoltage surges
C. Cooling radiators to dissipate oil heat
D. Core laminations to withstand magnetic saturation

Answer: B — Winding insulation and bushings to withstand transient overvoltage surges

Full-wave and chopped-wave impulse tests verify that the dielectric insulation system of windings, barriers, and bushings can withstand high-voltage atmospheric and switching transients.

24. High-voltage transformers can contribute to radio frequency interference (RFI) due to:

A. Corona discharges in air around external bushings
B. Internal partial discharges within solid/liquid insulation voids
C. Micro-sparking across loose hardware or connections
D. Any of the above

Answer: D — Any of the above

RFI from electrical equipment is generated by high-frequency electromagnetic radiation arising from external corona, micro-sparking, or internal partial discharges.

25. Compared to dry air at atmospheric pressure, the relative dielectric strength of Sulphur Hexafluoride (SF6) gas is approximately:

A. 1.0 times
B. 2.5 to 3.0 times
C. 10.0 times
D. 50.0 times

Answer: B — 2.5 to 3.0 times

SF6 gas has an electronegative molecular structure that captures free electrons, giving it a dielectric breakdown strength roughly 2.5 to 3 times that of air at identical pressure.

26. The electrical breakdown strength of dielectric insulating materials depends on:

A. Nature, magnitude, and rate of rise of applied voltage
B. Physical imperfections, voids, and impurities in the material
C. Ambient pressure, temperature, and relative humidity
D. All of the above

Answer: D — All of the above

Dielectric breakdown is a complex function of applied electrical field stress, test waveform duration, temperature, moisture content, pressure, and internal micro-voids.

27. Which of the following gases is most extensively used as the dielectric and arc-quenching medium in modern sealed High Voltage switchgear?

A. Nitrogen
B. Carbon dioxide
C. Sulphur Hexafluoride (SF6)
D. Argon

Answer: C — Sulphur Hexafluoride (SF6)

SF6 is widely used in Gas-Insulated Switchgear (GIS) and high-voltage circuit breakers due to its superior dielectric strength and rapid arc-quenching recombination properties.

28. High vacuum is utilized as an electrical insulating medium in all of the following applications EXCEPT:

A. Particle accelerators
B. Low-voltage domestic appliance cords
C. Field emission and electron tubes
D. X-ray tubes and vacuum interrupters

Answer: B — Low-voltage domestic appliance cords

Vacuum insulation requires sealed, rigid evacuated vessels (used in vacuum circuit breakers, X-ray tubes, particle accelerators), and is not applicable to domestic appliance flexible cords.

29. Dielectric insulating mineral oils in transformers and switchgear are generally operated up to continuous voltage stresses of approximately:

A. 500 V/cm
B. 5 kV/cm
C. 50 kV/cm
D. 500 kV/cm

Answer: C — 50 kV/cm

Standard transformer insulating oils have a breakdown strength of 30 to 60 kV across a standard 2.5 mm test gap (~120–240 kV/cm), operating safely under design working stresses up to ~50 kV/cm.

30. Electro-mechanical breakdown of solid insulating materials under intense electrostatic fields occurs primarily due to:

A. Magnetic hum
B. Chemical oxidation
C. Mechanical compressive stresses produced by strong electrostatic attraction
D. Ambient airflow turbulence

Answer: C — Mechanical compressive stresses produced by strong electrostatic attraction

Electro-mechanical breakdown occurs when the electrostatic compressive force (Maxwell stress) between opposite electrodes exceeds the mechanical compressive yield strength of the solid dielectric.

Set 5 — 30 HV exit exam questions with answers

1. All of the following are desirable properties of an insulating dielectric gas EXCEPT:

A. High dielectric breakdown strength
B. Chemical and physiological inertness
C. Extremely low molecular weight and low electron affinity
D. High thermal conductivity and non-flammability

Answer: C — Extremely low molecular weight and low electron affinity

Effective dielectric gases (like SF6) require high molecular weight and high electronegativity (high electron affinity) to capture free electrons and suppress electron avalanches.

2. Which of the following is an undesirable consequence of corona discharge on power transmission lines?

A. Generation of radio and television interference
B. Loss of electrical energy (power loss)
C. Production of ozone leading to conductor corrosion
D. All of the above

Answer: D — All of the above

Corona discharge wastes energy through I²R-equivalent power loss, generates high-frequency radio noise (RFI), and produces ozone/nitric acid that corrodes metallic fittings.

3. Which of the following instruments or techniques can be used for the precise measurement of high DC voltages?

A. Generating voltmeter (rotary electrostatic voltmeter)
B. Peak AC detector
C. Current transformer
D. Wheatstone bridge

Answer: A — Generating voltmeter (rotary electrostatic voltmeter)

A Generating Voltmeter (variable capacitance electrostatic field mill) measures high DC potentials without drawing continuous load current from the source.

4. All of the following methods are standard techniques for measuring high AC voltages EXCEPT:

A. Capacitive / Resistive potential dividers
B. Potential (voltage) transformers
C. Electrostatic voltmeters
D. Hall effect DC current clamps

Answer: D — Hall effect DC current clamps

Hall effect sensors measure magnetic fields to determine current, not direct high AC potential. High AC voltages are measured via PTs, voltage dividers, sphere gaps, or electrostatic meters.

5. Surge diverters (surge arresters) in power systems operate as:

A. Non-linear metal-oxide resistors that conduct rapidly to ground during overvoltage surges
B. Mechanical disconnecting switches
C. Inductive shunt reactors to correct low power factor
D. Harmonic filter capacitors

Answer: A — Non-linear metal-oxide resistors that conduct rapidly to ground during overvoltage surges

Modern surge arresters utilize non-linear Metal-Oxide Varistor (MOV, ZnO) elements that present near-infinite impedance at normal voltage, but switch to low resistance during surges to clamp overvoltages.

6. Standard standard impulse voltage waveforms (e.g., 1.2/50 μs lightning surge) are characterized by:

A. Polarity (positive or negative)
B. Peak crest voltage amplitude
C. Front time (T1) and time to half-value (T2)
D. All of the above

Answer: D — All of the above

Impulse voltages are defined by polarity, peak magnitude, front time (rise time to peak, e.g., 1.2 μs), and tail time (decay time to 50% peak, e.g., 50 μs) per IEC 60060-1.

7. The electrical breakdown voltage of gas gaps between electrodes is influenced by:

A. Gap distance between electrodes
B. Gas pressure and relative air density
C. Ambient temperature and humidity
D. All of the above

Answer: D — All of the above

Gas breakdown voltage depends on the product of gas pressure and gap distance (Paschen's Law), electrode geometry, air density factor, and ambient humidity.

8. 'At constant temperature, the breakdown voltage of a uniform gas gap is a function solely of the product of gas pressure and electrode gap distance (V_b = f(p·d)).' This is known as:

A. Townsend avalanche theorem
B. Thermal instability principle
C. Paschen's Law
D. Faraday's Law

Answer: C — Paschen's Law

Paschen's Law states that the breakdown voltage across a gas gap is an invariant function of the product of pressure (p) and gap distance (d).

9. Which of the following ground soil types typically exhibits the least (lowest) specific electrical resistivity?

A. Dry sand
B. Rocky soil
C. Gravel
D. Peat / organic marshy soil

Answer: D — Peat / organic marshy soil

Peat and marshy organic soils contain high moisture and dissolved ionic salts, resulting in the lowest electrical resistivity (typically 10 to 50 Ω·m), making them ideal for earthing.

10. Which of the following ground soil types typically exhibits the maximum (highest) specific electrical resistivity?

A. Clay
B. Dry sand and gravel
C. Peat
D. Loamy farm soil

Answer: B — Dry sand and gravel

Dry sand, gravel, and solid granite rock have very low moisture retention and lack dissolved ions, exhibiting exceptionally high resistivity (> 1,000 to 10,000 Ω·m).

11. In standard high-voltage measuring sphere gaps, the precision spheres are manufactured from:

A. Aluminium
B. Brass
C. Bronze or Copper
D. Any of the above (non-magnetic, corrosion-resistant conductive metals)

Answer: D — Any of the above (non-magnetic, corrosion-resistant conductive metals)

Calibrated sphere gap electrodes are precision-machined from brass, bronze, copper, or aluminium with polished, non-magnetic surfaces to ensure uniform electrostatic field distribution.

12. When a gas is heated or electrically energized into the 'plasma' state, it:

A. Loses electrical conductivity completely
B. Becomes highly electrically conductive due to free electrons and ions
C. Acts as a perfect dielectric insulator
D. Freezes into a crystalline structure

Answer: B — Becomes highly electrically conductive due to free electrons and ions

Plasma consists of a quasi-neutral gas of free ions and electrons, making it an excellent electrical conductor that sustains the electric arc in circuit breakers.

13. Which of the following statements concerning corona discharge on power transmission lines is INCORRECT?

A. Corona gives rise to radio and television interference
B. Corona results in active power loss on transmission lines
C. Corona discharge in air appears as a bright red luminescence
D. Corona is accompanied by a characteristic hissing and cracking noise

Answer: C — Corona discharge in air appears as a bright red luminescence

Statement C is incorrect: Corona in air visually appears as a pale bluish-violet glow, not bright red luminescence.

14. A high-voltage circuit breaker is fundamentally designed as a:

A. Power factor correction device
B. Transient voltage waveform generator
C. Continuous voltage booster
D. Mechanical switching device capable of making, carrying, and interrupting load and short-circuit currents

Answer: D — Mechanical switching device capable of making, carrying, and interrupting load and short-circuit currents

A circuit breaker is a mechanical switching device capable of making, carrying, and breaking currents under normal circuit conditions and specified abnormal/short-circuit conditions.

15. The primary function of a protective relay in a power circuit breaker protection scheme is to:

A. Dissipate arcing current thermally
B. Detect electrical faults and close trip contacts when measured parameters exceed predetermined thresholds
C. Regulate the generator field excitation
D. Extinguish the arc inside the interrupter chamber

Answer: B — Detect electrical faults and close trip contacts when measured parameters exceed predetermined thresholds

The protective relay constantly monitors system parameters (current, voltage, frequency, phase angle) and energizes the circuit breaker trip coil when an abnormal fault condition is detected.

16. Under standard electrical terminology and IEC standards, Low Voltage (LV) circuit breakers have a rated operating voltage of up to:

A. 220 V
B. 440 V
C. 1,000 V AC (or 1,500 V DC)
D. 10,000 V

Answer: C — 1,000 V AC (or 1,500 V DC)

IEC 60947 defines Low Voltage as nominal voltages up to and including 1,000 V AC or 1,500 V DC. Above 1,000 V AC is classified as High Voltage / Medium Voltage.

17. The total fault clearing time of a modern high-speed high-voltage circuit breaker (relay time plus breaker opening and arcing time) is typically:

A. Several minutes
B. 10 to 30 seconds
C. 5 to 10 seconds
D. A fraction of a second (typically 30 to 80 milliseconds / 2 to 4 cycles)

Answer: D — A fraction of a second (typically 30 to 80 milliseconds / 2 to 4 cycles)

Modern high-speed circuit breakers clear short-circuit faults within 2 to 5 cycles (approximately 30 to 100 milliseconds) from fault initiation.

18. The medium employed for arc extinction in an Air Circuit Breaker (ACB) is:

A. Sulphur Hexafluoride (SF6)
B. De-aerated mineral oil
C. Atmospheric air directed through arc splitter chutes
D. Deionized distilled water

Answer: C — Atmospheric air directed through arc splitter chutes

Air Circuit Breakers (ACBs) interrupt current in atmospheric air using magnetic blowout coils and arc chute splitters that lengthen, cool, and extinguish the arc.

19. Which of the following circuit breaker technologies is most widely preferred for Extra High Voltage (EHV/UHV) grid transmission substations?

A. Bulk oil circuit breaker
B. Air break circuit breaker
C. Minimum oil circuit breaker
D. SF6 gas circuit breaker

Answer: D — SF6 gas circuit breaker

SF6 puffer and self-blast circuit breakers dominate EHV applications (72.5 kV to 800 kV) due to their compact footprint, high interrupting capacity, and rapid dielectric recovery.

20. According to IEC standard specifications, the rated short-time withstand current (thermal short-circuit rating) for HV AC circuit breakers is rated for a duration of:

A. 0.01 seconds
B. 0.1 seconds
C. 1 or 3 seconds
D. 30 seconds

Answer: C — 1 or 3 seconds

Standard short-time withstand current ratings for switchgear and circuit breakers specify the thermal and electrodynamic endurance for a duration of 1 second or 3 seconds per IEC 62271-100.

21. Which of the following is NOT a standard mechanical operating mechanism type for circuit breakers?

A. Spring-charged mechanism
B. Pneumatic / Electro-pneumatic mechanism
C. Hydraulic mechanism
D. Vacuum operating actuator (vacuum is an interrupting medium, not an actuator mechanism)

Answer: D — Vacuum operating actuator (vacuum is an interrupting medium, not an actuator mechanism)

Vacuum refers to the arc quenching/interrupting medium inside interrupter bottles. Operating mechanisms that supply mechanical force are spring, pneumatic, hydraulic, or motor actuators.

22. In electrical contactors and switching devices, the interrupting medium may be:

A. Atmospheric air
B. Vacuum
C. SF6 gas or insulating oil
D. Any of the above

Answer: D — Any of the above

Contactors are manufactured in various technologies including air-break, vacuum contactors (standard in marine HV motor starters), and oil/gas immersed units.

23. The chemical compound abbreviation 'SF6' stands for:

A. Sulphur fluoride
B. Sulphur difluoride
C. Sulphur fluorite
D. Sulphur Hexafluoride

Answer: D — Sulphur Hexafluoride

SF6 is the molecular formula for Sulphur Hexafluoride, an inorganic, colourless, odourless, non-flammable, and highly stable greenhouse gas.

24. Which of the following is a physical and chemical characteristic of pure SF6 gas?

A. It is yellow in color
B. It possesses a pungent chlorine-like odor
C. It is highly combustible
D. It is colourless, odourless, non-toxic, and non-flammable

Answer: D — It is colourless, odourless, non-toxic, and non-flammable

Pure SF6 is non-toxic, non-flammable, colorless, and odorless. However, toxic decomposition byproducts (e.g., SF4, SOF2) can form during intense electrical arcing.

25. What is the density of Sulphur Hexafluoride (SF6) gas compared to ambient air?

A. It is lighter than hydrogen
B. It is lighter than atmospheric air
C. It has identical density to air
D. It is approximately 5 times denser than air

Answer: D — It is approximately 5 times denser than air

SF6 has a molecular weight of 146.06 g/mol, making it approximately 5 times heavier (denser) than air (~29 g/mol). It tends to accumulate in low-lying spaces and bilges.

26. In SF6 gas circuit breakers, the nominal gas operating pressure within the interrupter chamber is typically in the order of:

A. 100 mm Hg
B. 1 kg/cm² (atmospheric)
C. 3 to 6 kg/cm² (approx. 0.3 to 0.6 MPa / 3 to 6 bar)
D. 50 to 100 kg/cm²

Answer: C — 3 to 6 kg/cm² (approx. 0.3 to 0.6 MPa / 3 to 6 bar)

Modern single-pressure SF6 puffer circuit breakers operate at gas pressures between 3.5 to 6 bar (0.35 to 0.6 MPa) at 20°C.

27. When selecting an insulating gas for high-voltage circuit breakers, which essential properties must be considered?

A. High dielectric breakdown strength and rapid arc-recombination rate
B. High thermal conductivity and chemical stability
C. Non-flammability and non-toxicity in pure form
D. All of the above

Answer: D — All of the above

An ideal arc-quenching gas must possess high dielectric strength, high thermal transfer capability, chemical inertness, non-flammability, and rapid deionization upon current zero.

28. Among the following circuit breaker types, which one is typically limited to the lowest operational voltage ranges (up to 1,000 V)?

A. Air-break circuit breaker (plain break / miniature breaker)
B. Tank-type bulk oil circuit breaker
C. Air-blast circuit breaker
D. SF6 gas circuit breaker

Answer: A — Air-break circuit breaker (plain break / miniature breaker)

Plain air-break circuit breakers (without high-pressure blast or vacuum bottles) are generally restricted to low-voltage applications (up to 690 V / 1,000 V).

29. In modern high-speed EHV circuit breakers, the total operating opening time from receipt of trip command to contact separation is in the order of:

A. 0.001 sec (1 ms)
B. 0.015 to 0.030 sec (15 to 30 ms)
C. 0.50 sec (500 ms)
D. 2.0 sec

Answer: B — 0.015 to 0.030 sec (15 to 30 ms)

High-speed circuit breaker opening mechanisms accelerate the moving contacts to achieve separation within 15 to 30 milliseconds (1 to 1.5 cycles) after trip coil energization.

30. In a High Rupturing Capacity (HRC) fuse, the time elapsed between initial fuse element melting (cut-off) and final arc extinction at current zero is known as the:

A. Pre-arcing time
B. Arcing time
C. Total clearing time
D. Thermal relaxation time

Answer: B — Arcing time

Total operating time of a fuse equals Pre-arcing time (time for element to melt) plus Arcing time (time between element vaporization and final current zero extinction).

Set 6 — 30 HV exit exam questions with answers

1. The fusing factor of a protective fuse is defined as the ratio of:

A. Minimum fusing current to rated current
B. Rated current to minimum fusing current
C. Peak prospective fault current to rated current
D. Breaking capacity to making capacity

Answer: A — Minimum fusing current to rated current

Fusing factor is the ratio of Minimum Fusing Current (current that will melt the fuse) to Rated Current. For HRC fuses, this factor is typically 1.1 to 1.25.

2. Which of the following circuit breaker types does NOT utilize a compressed pneumatic operating mechanism?

A. Air-blast circuit breaker
B. SF6 blast circuit breaker
C. Gas-insulated switchgear
D. Bulk oil circuit breaker (commonly spring-driven or solenoid-operated)

Answer: D — Bulk oil circuit breaker (commonly spring-driven or solenoid-operated)

Bulk oil circuit breakers typically employ charged mechanical spring or electrical solenoid closing mechanisms rather than high-pressure pneumatic compressors.

3. The DC contact resistance across closed main contacts of a healthy high-voltage circuit breaker is typically in the order of:

A. 20 to 50 micro-ohms (μΩ)
B. 20 to 50 milli-ohms (mΩ)
C. 20 ohms (Ω)
D. 200 ohms (Ω)

Answer: A — 20 to 50 micro-ohms (μΩ)

Main contact resistance measured with a micro-ohmmeter (ductor test) must be very low (typically 20 to 50 micro-ohms) to prevent severe I²R overheating under full load currents.

4. Which of the following liquids presents definite technical objections to being used as an arc-extinguishing medium in high-voltage circuit breakers?

A. Mineral dielectric oil
B. Synthetic ester oil
C. Silicone oil
D. Water (due to high electrical conductivity and intense hydrogen dissociation)

Answer: D — Water (due to high electrical conductivity and intense hydrogen dissociation)

Water has high natural conductivity, causes rapid corrosion, and dissociates into explosive hydrogen and oxygen gases when exposed to high-voltage electric arcs.

5. If the insulation resistance between a circuit breaker phase terminal and earthed frame is found below acceptable limits, the probable root cause could be:

A. Moisture condensation on insulator surfaces
B. Dirt, salt, and conductive dust deposits on bushings
C. Carbon or copper particles deposited inside the interrupter chamber from arcing
D. Any of the above

Answer: D — Any of the above

Insulation resistance deterioration can be caused by surface contamination, humidity/moisture tracking, cracked porcelain, or metallic/carbon arc erosion residues.

6. If an electrically operated circuit breaker fails to open upon receiving an electrical trip command, possible causes include:

A. Defective or discharged trip spring mechanism
B. Open circuit in the 110V/24V DC trip coil circuit
C. Mechanical binding or jammed trip latch
D. Any of the above

Answer: D — Any of the above

Failure to trip can result from electrical faults (blown DC control fuse, open trip coil, faulty auxiliary switch) or mechanical faults (stuck latch, broken trip spring).

7. The power-frequency RMS voltage that appears across the poles of a circuit breaker after final arc extinction is defined as the:

A. Recovery voltage
B. Restriking voltage
C. Active surge voltage
D. Peak prospective voltage

Answer: A — Recovery voltage

Recovery voltage is the normal power-frequency (50/60 Hz) RMS voltage that appears across circuit breaker contacts after transient oscillations have damped out and the arc is extinguished.

8. The high-frequency transient voltage that appears across circuit breaker contacts at the precise instant of arc extinction at current zero is called the:

A. Recovery voltage
B. Restriking voltage (Transient Recovery Voltage - TRV)
C. Nominal system voltage
D. Demagnetising EMF

Answer: B — Restriking voltage (Transient Recovery Voltage - TRV)

Restriking voltage (TRV) is the transient voltage with high rate of rise (RRTRV) that appears across contacts immediately following arc extinction at current zero.

9. In a circuit breaker, the magnitude of the active recovery voltage depends upon:

A. System power factor (cosφ)
B. Armature reaction of the supplying generators
C. Circuit configuration and earthing conditions
D. All of the above

Answer: D — All of the above

Active recovery voltage is governed by the system power factor (higher at zero power factor lagging), generator armature reaction, and whether the system neutral is solidly grounded or isolated.

10. High Rupturing Capacity (HRC) fuses provide the most effective and rapid protection under conditions of:

A. Low-level sustained overloads
B. High-magnitude short-circuit faults
C. Slight voltage fluctuations
D. Phase imbalance only

Answer: B — High-magnitude short-circuit faults

HRC fuses excel at short-circuit protection because their silver elements vaporize in milliseconds during high prospective fault currents, providing rapid current-limiting cut-off.

11. For high-speed motor circuit breakers, the total closing time from command signal to contact touch is typically in the order of:

A. 3 to 10 milliseconds
B. 50 to 100 milliseconds
C. 1 to 2 seconds
D. 5 seconds

Answer: B — 50 to 100 milliseconds

High-speed vacuum and air motor circuit breakers typically have closing times between 40 to 100 milliseconds (0.04 to 0.10 s) to ensure rapid, bounce-free engagement.

12. Which type of protective relay is commonly used to protect electric motors against continuous moderate overload?

A. Impedance / Distance relay
B. Instantaneous electromagnetic relay
C. Bimetallic thermal overload relay
D. Buchholz gas relay

Answer: C — Bimetallic thermal overload relay

Thermal overload relays utilize bimetallic strips heated by motor load current to replicate the thermal heating curve of the motor windings and provide inverse-time tripping.

13. According to Preece's Law, the fusing current (I) of a fuse wire is related to its wire diameter (D) by the relationship:

A. I ∝ 1 / D
B. I ∝ D
C. I ∝ D^(3/2) (or D^1.5)
D. I ∝ D²

Answer: C — I ∝ D^(3/2) (or D^1.5)

Preece's formula (I = k·d^(1.5)) establishes that the fusing current for a round conductor is proportional to the diameter raised to the power of 3/2.

14. In a circuit breaker contact gap, ionization of the medium is NOT facilitated by:

A. High temperature of the surrounding dielectric medium
B. Chemical composition of the solid metallic contacts
C. Increase in electric field gradient across the gap
D. Increase in electron mean free path

Answer: B — Chemical composition of the solid metallic contacts

Ionization of the gas gap is driven by electric field strength, thermal kinetic energy, and mean free path. The bulk contact material itself does not increase gas ionization.

15. An ideal fuse wire material should possess which combination of physical properties?

A. High specific resistance and high melting point
B. High specific resistance and low melting point
C. Low specific resistance and high melting point
D. Low specific resistance and low melting point

Answer: D — Low specific resistance and low melting point

A good fuse material requires low specific resistance (to minimize normal I²R losses and heat) and a low melting point (to blow rapidly without reaching destructive arc temperatures).

16. For extra-high-voltage (EHV) transmission networks above 220 kV, which circuit breaker type is predominantly selected?

A. Bulk oil circuit breaker
B. Air-break circuit breaker
C. SF6 gas circuit breaker
D. Minimum oil circuit breaker

Answer: C — SF6 gas circuit breaker

SF6 circuit breakers are standard for EHV systems due to their superior dielectric strength, fast arc quenching, and compact gas-insulated switchgear (GIS) design.

17. The number of power-frequency AC cycles in which a modern high-speed circuit breaker completes its tripping and arc clearing operation is:

A. 2 to 5 cycles
B. 10 to 18 cycles
C. 20 to 30 cycles
D. 40 to 50 cycles

Answer: A — 2 to 5 cycles

Modern high-speed circuit breakers clear short-circuit faults within 2 to 5 cycles (30 to 100 ms on a 50/60 Hz system).

18. Which of the following metals is best suited for manufacturing precision HRC fuse elements due to its resistance to oxidation and low specific resistance?

A. Aluminium
B. Silver
C. Lead
D. Iron

Answer: B — Silver

Silver is the material of choice for HRC fuse elements because it does not oxidize in air under continuous full load, has high conductivity, and has consistent melting characteristics.

19. In a circuit breaker, the current that is flowing through the contacts at the precise instant of contact separation is known as the:

A. Restriking current
B. Surge current
C. Breaking current
D. Making current

Answer: C — Breaking current

Breaking current is the RMS value of the current flowing through a pole of the circuit breaker at the instant of contact separation when opening under fault conditions.

20. Merz-Price circulating current differential protection is primarily used for the protection of:

A. Overhead distribution feeders
B. Alternators and Power Transformers
C. Low-voltage motors
D. Direct current batteries

Answer: B — Alternators and Power Transformers

Merz-Price differential protection compares current entering and leaving alternator/transformer windings, providing fast, unit protection against internal stator phase-to-phase and phase-to-earth faults.

21. A 'Kick fuse' (time-lag or slow-blow fuse) is characterized by having:

A. Square law instantaneous response
B. Linear fast response
C. Inverse time-current characteristics
D. Constant delay regardless of current

Answer: C — Inverse time-current characteristics

Time-delay (slow-blow) fuses exhibit an inverse time-current characteristic: high overload currents trip rapidly while harmless motor starting current surges pass without blowing.

22. Air blast circuit breakers designed for 400 kV extra-high-voltage systems are engineered to extinguish the arc within:

A. 50 micro-seconds
B. 40 to 60 milli-seconds (approx. 2 to 3 cycles)
C. 500 milli-seconds
D. 5 seconds

Answer: B — 40 to 60 milli-seconds (approx. 2 to 3 cycles)

High-voltage air blast circuit breakers clear faults within 40 to 60 milliseconds (approx. 2 to 3 cycles on 50 Hz power networks).

23. The symmetrical short-circuit breaking capacity of a high-voltage power circuit breaker is standardly expressed in units of:

A. Amperes / kA or MVA (√3·V·I)
B. Volts / kV
C. Megawatts (MW)
D. Horsepower (HP)

Answer: A — Amperes / kA or MVA (√3·V·I)

Breaking capacity is defined by the rated symmetrical breaking current (in kA RMS) or apparent breaking power (in MVA, calculated as √3 × V_rated × I_breaking).

24. Sulphur Hexafluoride (SF6) gas in its un-ionized state is fundamentally an exceptional:

A. Conductor of electricity
B. Semiconductor
C. Chemically corrosive reagent
D. Dielectric electrical insulator

Answer: D — Dielectric electrical insulator

SF6 is an outstanding dielectric gas due to its high electronegativity, capturing free electrons to form heavy, immobile negative ions and preventing electrical breakdown.

25. The electrical contact resistance between closed circuit breaker contacts is least affected by which of the following parameters?

A. Mechanical contact pressure applied by springs
B. Surface profile and contour of the contact faces
C. Surface oxidation and contamination films
D. Surrounding ambient air temperature

Answer: D — Surrounding ambient air temperature

Contact resistance R_c is primarily governed by mechanical contact force, surface roughness/contact area, and oxide films. Moderate changes in ambient temperature have negligible direct effect on R_c.

26. The arc voltage developed across the separating contacts of an AC circuit breaker during arc burning is:

A. Leading the arc current by 90°
B. Lagging the arc current by 90°
C. Substantially in phase with the arc current
D. In phase opposition (180°) to the arc current

Answer: C — Substantially in phase with the arc current

Because an electric arc behaves predominantly as a non-linear resistive plasma column, the voltage across the arc is in phase with the arc current.

27. As the mechanical contact clamping force between two closed electrical contacts is increased, the contact resistance will:

A. Increase linearly
B. Increase exponentially
C. Remain completely unaltered
D. Decrease due to increased microscopic contact area

Answer: D — Decrease due to increased microscopic contact area

Increasing mechanical contact pressure crushes microscopic surface asperities and breaks surface oxide films, increasing the true metallic contact area and reducing contact resistance.

28. For electrical circuit breaker and switch contacts, which of the following parameters should ideally have the lowest possible value?

A. Electrical contact resistance
B. Thermal heat capacity
C. Thermal conductivity
D. Arc erosion resistance

Answer: A — Electrical contact resistance

Low contact resistance is essential to prevent excessive I²R heating and thermal runaway, while high thermal conductivity, high heat capacity, and high erosion resistance are desired.

29. Which of the following contact metals exhibits the lowest minimum arcing voltage threshold?

A. Carbon
B. Graphite
C. Tungsten
D. Silver (~12 V)

Answer: D — Silver (~12 V)

Silver has a low ionization potential and low work function, giving it a lower minimum arcing voltage (~12 V) compared to refractory materials like tungsten, carbon, or graphite.

30. If the minimum arcing voltage for platinum is 16 V, it can be concluded that at system voltages below 16 V:

A. It will be impossible to interrupt the circuit
B. It will be impossible to conduct current
C. Circuit current can be interrupted without sustaining an electric arc
D. Contacts will weld together upon opening

Answer: C — Circuit current can be interrupted without sustaining an electric arc

The minimum arcing voltage is the threshold below which an electric arc cannot initiate or be sustained in the contact gap, permitting arcless interruption.

Set 7 — 30 HV exit exam questions with answers

1. Submerging circuit breaker contacts in dielectric transformer oil is an effective method of:

A. Arc cooling, gas generation (hydrogen blast), and rapid deionization
B. Complete arc prevention
C. Electromagnetic induction shielding only
D. Radio frequency amplification

Answer: A — Arc cooling, gas generation (hydrogen blast), and rapid deionization

In oil circuit breakers, the intense arc decomposes oil into hydrogen gas (70%) and hydrocarbon vapors, creating a high-pressure bubble that violently cools and deionizes the arc path at current zero.

2. Which of the following is NOT an established method of arc extinction in electrical circuit breakers?

A. Oil immersion and gas blasting of contacts
B. Magnetic blowout coils and arc splitters
C. Insertion of silicon diode rectifiers into the arc column
D. De-ionization and lengthening of the arc path

Answer: C — Insertion of silicon diode rectifiers into the arc column

Arc extinction methods include lengthening the arc, magnetic blowout, blast cooling (air/SF6/oil), arc chute splitting, and vacuum deionization. Diode rectifiers cannot be inserted into an open plasma arc.

3. Which of the following is NOT a method of arc dispersion?

A. Oil immersion of contacts
B. Magnetic blow-out of arc
C. Use of rectifiers
D. De-ionisation of arc path

Answer: C — Use of rectifiers

Arcs at breaking contacts are extinguished by cooling and lengthening them, for example by oil immersion, magnetic blow-out into arc chutes, and de-ionising the arc path. Rectifiers convert AC to DC and play no part in arc dispersion.

4. The minimum arcing voltage for platinum is 16 V. It can therefore be concluded that when the voltage is below 16 V:

A. it will not be possible to interrupt the circuit
B. it will not be possible to pass the current
C. it will be possible to interrupt any value of current without arcing
D. it will be possible to interrupt any current without separating the contacts

Answer: C — it will be possible to interrupt any value of current without arcing

An arc cannot be sustained between contacts of a given material if the circuit voltage is below its minimum arcing voltage. Below 16 V, platinum contacts can therefore break the circuit without a sustained arc, whatever the current.

5. The minimum arcing voltage will be least in the case of:

A. Carbon
B. Graphite
C. Tungsten
D. Silver

Answer: D — Silver

Silver has a low minimum arcing voltage of about 12 V, lower than tungsten, platinum or carbon and graphite, which need around 15 to 20 V. Silver is therefore more prone to arcing but is valued for its high conductivity.

6. For contacts and their material, which of the following should have a low value?

A. Contact resistance
B. Thermal capacity
C. Thermal conductivity
D. All of the above

Answer: A — Contact resistance

Good contact material must have low contact resistance to minimise heating and voltage drop. High thermal capacity and high thermal conductivity are desirable so that heat generated at the contact surface is absorbed and conducted away.

7. As the force on the contacts is increased, the contact resistance will

A. Increase linearly
B. Increase exponentially
C. Remain unaltered
D. Decrease

Answer: D — Decrease

Greater contact pressure deforms the surface asperities so more microscopic points touch, enlarging the true contact area. The constriction resistance therefore falls as contact force rises, which is why breaker contact springs must keep correct pressure.

8. The arc voltage produced in an AC circuit breaker is

A. Leading the arc current by 90°
B. Lagging the arc current by 90°
C. In phase with the arc current
D. In phase opposition to the arc current

Answer: C — In phase with the arc current

The arc behaves essentially as a resistive path of ionised gas, so the voltage across it rises and falls with the current through it. The arc voltage is therefore in phase with the arc current.

9. Contact resistance is least affected by

A. The mechanical force applied
B. The shape of the contact faces
C. The amount of surface contamination
D. The ambient temperature

Answer: D — The ambient temperature

Contact resistance depends mainly on contact pressure, contact geometry and surface films such as oxides or dirt. Normal variations in ambient temperature have only a minor effect compared with these factors.

10. Sulphur hexafluoride (SF6) is a

A. Conductor of electricity
B. Semi-conductor
C. Highly flammable gas
D. Dielectric

Answer: D — Dielectric

SF6 is an electronegative, non-flammable gas with high dielectric strength and excellent arc-quenching ability. It is used as the insulating and interrupting medium in SF6 circuit breakers and gas-insulated switchgear.

11. The breaking capacity of a circuit breaker is usually expressed in

A. Amperes
B. Volts
C. MW
D. MVA

Answer: D — MVA

Symmetrical breaking capacity is commonly stated as the product of rated voltage and rated breaking current multiplied by root 3 for three-phase systems, giving a value in MVA that reflects the fault level the breaker can interrupt.

12. Air blast circuit breakers for 400 kV power systems are designed to operate in about

A. 50 microseconds
B. 50 milliseconds
C. 500 milliseconds
D. 50 seconds

Answer: B — 50 milliseconds

EHV air blast breakers are fast-acting, clearing faults within a few cycles. At 50 Hz, about 2.5 cycles corresponds to roughly 50 milliseconds, which is the typical design operating time for 400 kV systems.

13. In a circuit breaker, the current which exists at the instant of contact separation is known as the

A. Restriking current
B. Surge current
C. Breaking current
D. Recovery current

Answer: C — Breaking current

The breaking current is the value of current flowing at the instant the breaker contacts separate. Its symmetrical RMS value, together with system voltage, defines the breaking capacity of the breaker.

14. A material best suited for the manufacture of fuse wire is

A. Aluminium
B. Silver
C. Lead
D. Copper

Answer: B — Silver

Silver is preferred for fuse elements, especially in HRC fuses, because it does not oxidise readily, has low resistivity and gives consistent, predictable fusing characteristics with little deterioration over time.

15. The number of cycles in which a high speed circuit breaker can complete its operation is

A. 3 to 8
B. 10 to 18
C. 20 to 30
D. 40 to 50

Answer: A — 3 to 8

High speed circuit breakers are designed to clear faults within about 3 to 8 cycles of the supply frequency, limiting damage to equipment and preserving system stability.

16. For extra high voltage lines, which circuit breaker is preferred?

A. Bulk oil circuit breaker
B. Vacuum circuit breaker
C. SF6 gas circuit breaker
D. Minimum oil circuit breaker

Answer: C — SF6 gas circuit breaker

SF6 breakers combine high dielectric strength, rapid recovery and excellent arc quenching, making them the preferred choice for EHV duty. Vacuum breakers are generally limited to medium voltages up to about 36 kV.

17. A fuse wire should have

A. Low specific resistance and high melting point
B. Low specific resistance and low melting point
C. High specific resistance and high melting point
D. High specific resistance and low melting point

Answer: D — High specific resistance and low melting point

A fuse element must heat up and melt quickly when overcurrent flows. High specific resistance produces more heat for a given current and a low melting point ensures the element melts rapidly to open the circuit.

18. Ionisation in a circuit breaker is NOT facilitated by

A. High temperature of the surrounding medium
B. The material of the contacts
C. Increase of field strength
D. Increase of mean free path

Answer: B — The material of the contacts

Ionisation of the arc medium increases with high temperature, higher electric field strength and longer mean free path, which lets electrons gain more energy between collisions. The contact material itself does not promote ionisation of the medium.

19. The arcing contacts of circuit breakers are made of

A. Stainless steel
B. Hard pressed carbon
C. Porcelain
D. Copper-tungsten alloy

Answer: D — Copper-tungsten alloy

Copper-tungsten alloy combines the good conductivity of copper with the high melting point and arc-erosion resistance of tungsten, so arcing contacts resist burning and welding during repeated interruptions.

20. The fusing current I of a fuse wire is related to the wire diameter D as

A. I ∝ 1/D
B. I ∝ D
C. I ∝ D^(3/2)
D. I ∝ D^2

Answer: C — I ∝ D^(3/2)

According to Preece's law, the fusing current of a wire is proportional to its diameter raised to the power 3/2, because heat generated depends on cross-section while heat dissipated depends on surface area.

21. A relay used for protection of motors against overload is the

A. Impedance relay
B. Electromagnetic attraction relay
C. Thermal relay
D. Buchholz relay

Answer: C — Thermal relay

Thermal overload relays use bimetal elements heated by motor current, giving an inverse time characteristic that mimics motor heating. They allow short starting currents but trip on sustained overload.

22. HRC fuses provide the best protection in the case of

A. Open circuits
B. Short circuits
C. Overloads
D. None of the above

Answer: B — Short circuits

HRC fuses have very high breaking capacity and a current-limiting action, cutting off a fault current before it reaches its prospective peak. This makes them most effective against short circuits rather than small overloads.

23. The transient voltage that appears across the breaker contacts at the instant of arc extinction is called the

A. Recovery voltage
B. Restriking voltage
C. Supply voltage
D. Peak voltage

Answer: B — Restriking voltage

Immediately after current zero, a high-frequency transient voltage appears across the contacts and can re-ignite the arc. This is the restriking voltage; it later settles to the power-frequency recovery voltage.

24. The normal frequency RMS voltage that appears across the breaker poles after final arc extinction is the

A. Recovery voltage
B. Restriking voltage
C. Supply voltage
D. Peak voltage

Answer: A — Recovery voltage

Once the high-frequency restriking transient dies out, the voltage across the open contacts is the power-frequency recovery voltage, usually expressed as its RMS value.

25. If a circuit breaker does not operate on electrical command, the probable reason could be a

A. Defective spring
B. Open trip circuit
C. Defective trip latch
D. Any of the above

Answer: D — Any of the above

A breaker that fails to respond to an electrical trip or close command may have a broken or weak operating spring, an open or faulty trip coil circuit, or a sticking or defective latch. Each must be checked during fault finding.

26. In a circuit breaker, if the insulation resistance between a phase terminal and the earthed frame is less than the specified limit, the probable cause could be

A. Moisture
B. Dirty insulation surfaces
C. Carbon or copper particles sticking to internal surfaces
D. Any of the above

Answer: D — Any of the above

Moisture ingress, dirt on insulating surfaces and conductive carbon or copper deposits from arcing all create leakage paths to earth and lower insulation resistance. Cleaning and drying are required before the breaker is returned to service.

27. There is a definite objection to the use of which of the following media for extinguishing the arc in a circuit breaker?

A. Air
B. SF6 gas
C. Vacuum
D. Water

Answer: D — Water

Water is conductive when impure and would flash to steam in the arc, making it unsuitable as an arc-quenching medium. Air, SF6, oil and vacuum are the accepted interrupting media.

28. The contact resistance of a circuit breaker is of the order of

A. 20 micro-ohms ± 10
B. 20 milli-ohms ± 10
C. 20 ohms ± 10
D. 200 ohms ± 10

Answer: A — 20 micro-ohms ± 10

Healthy main contacts of a circuit breaker have very low resistance, typically tens of micro-ohms, measured with a micro-ohmmeter by injecting a high DC current. Higher values indicate worn, burnt or poorly pressured contacts.

29. Which of the following circuit breakers does NOT use a pneumatic operating mechanism?

A. Air blast circuit breaker
B. SF6 blast circuit breaker
C. Air break circuit breaker
D. Bulk oil circuit breaker

Answer: D — Bulk oil circuit breaker

Bulk oil circuit breakers are normally operated by spring or solenoid mechanisms, with the arc quenched by gas generated from the oil. Air blast and puffer-type gas breakers rely on compressed gas and pneumatic operation.

30. The fusing factor of an HRC fuse is

A. Minimum fusing current / current rating
B. Minimum fusing current / minimum rupturing time
C. Maximum fusing current / minimum fusing current
D. Minimum fusing current / prospective current of the circuit

Answer: A — Minimum fusing current / current rating

Fusing factor is the ratio of the minimum current that will melt the fuse element to its rated current. It is always greater than one, and for HRC fuses it is typically about 1.1 to 1.5.

Set 8 — 30 HV exit exam questions with answers

1. In an HRC fuse, the time between cut-off and final current zero is known as the

A. Total operating time
B. Arcing time
C. Pre-arcing time
D. Any of the above

Answer: B — Arcing time

Pre-arcing time runs from fault inception until the element melts (cut-off). The arcing time follows, from cut-off until the arc is finally extinguished at current zero; together they form the total operating time.

2. Out of the following circuit breakers, which one has the lowest voltage range?

A. Air break circuit breaker
B. Tank type oil circuit breaker
C. Air blast circuit breaker
D. SF6 circuit breaker

Answer: A — Air break circuit breaker

Air break circuit breakers stretch and cool the arc in atmospheric air using arc chutes, which limits them to low and medium voltages. Oil, air blast and SF6 breakers are used for progressively higher voltages.

3. While selecting a gas for a circuit breaker, the properties that should be considered are

A. High dielectric strength
B. Non-inflammability
C. Non-toxicity
D. All of the above

Answer: D — All of the above

An interrupting gas must insulate well, must not burn or support combustion in the arc, and must be safe for personnel. SF6 is chosen because it meets all these requirements along with good thermal stability.

4. The pressure of SF6 gas in circuit breakers is of the order of

A. 100 mm Hg
B. 1 kg/cm2
C. 3 to 5 kg/cm2
D. 30 to 50 kg/cm2

Answer: C — 3 to 5 kg/cm2

SF6 breakers normally operate with gas at a few bar, around 3 to 5 kg/cm2, which gives high dielectric strength without liquefaction at normal temperatures. Low gas pressure alarms and lockouts protect against leakage.

5. SF6 gas is

A. Sulphur fluoride
B. Sulphur difluoride
C. Sulphur hexafluorine
D. Sulphur hexafluoride

Answer: D — Sulphur hexafluoride

SF6 is sulphur hexafluoride: one sulphur atom bonded to six fluorine atoms. It is a heavy, inert, electronegative gas widely used in HV switchgear for insulation and arc quenching.

6. The interrupting medium in a contactor may be

A. Air
B. Oil
C. SF6 gas
D. Any of the above

Answer: D — Any of the above

Contactors are built with various interrupting media depending on rating and voltage, including air-break, oil-immersed, SF6 and vacuum types. Any of these media can be used to extinguish the arc.

7. For high voltage AC circuit breakers, the rated short-circuit current is passed for

A. 0.01 sec
B. 0.1 sec
C. 3 seconds
D. 30 seconds

Answer: C — 3 seconds

The rated short-time withstand current test passes the rated short-circuit current through the closed breaker for a standard duration, traditionally 3 seconds (1 second also used), to prove thermal and mechanical withstand.

8. The medium employed for extinction of the arc in an air circuit breaker is

A. SF6
B. Oil
C. Air
D. Water

Answer: C — Air

Air circuit breakers interrupt the arc in atmospheric air, lengthening and cooling it in arc chutes until it cannot be maintained. They are common on ships' main switchboards for low voltage systems.

9. The fault clearing time of a circuit breaker is usually

A. A few minutes
B. A few seconds
C. One second
D. A fraction of a second (typically milliseconds)

Answer: D — A fraction of a second (typically milliseconds)

Protective relays and circuit breakers together clear faults within tens to a few hundred milliseconds, limiting thermal and mechanical damage and maintaining stability of the power system.

10. Low voltage circuit breakers have a rated voltage of less than

A. 220 V
B. 400 V
C. 1000 V
D. 10,000 V

Answer: C — 1000 V

Low voltage is defined as up to 1000 V AC. Above 1000 V an installation is classed as high voltage, which is also the threshold for STCW high voltage training requirements on board ships.

11. The function of a protective relay in a circuit breaker scheme is

A. To earth any stray voltages
B. To close its contacts in the trip circuit when the actuating quantity reaches a certain predetermined value
C. To limit arcing current during the operation of the circuit breaker
D. To provide additional safety in the operation of the circuit breaker

Answer: B — To close its contacts in the trip circuit when the actuating quantity reaches a certain predetermined value

A protective relay senses an abnormal quantity such as current, voltage or impedance. When the quantity reaches the set value, its contacts close to energise the breaker trip coil, which opens the breaker and isolates the fault.

12. A circuit breaker is

A. A power factor correcting device
B. A device to neutralise the effect of transients
C. A waveform correcting device
D. A current interrupting device

Answer: D — A current interrupting device

A circuit breaker is a mechanical switching device that can make, carry and break normal currents and also interrupt abnormal currents such as short circuits, automatically under relay control.

13. Which of the following statements about corona is INCORRECT?

A. Corona gives rise to radio interference
B. Corona results in loss of power in transmission
C. Corona discharge can be observed as red luminescence
D. Corona is accompanied by a hissing noise

Answer: C — Corona discharge can be observed as red luminescence

Corona appears as a faint violet or bluish glow around conductors, not a red luminescence. It is accompanied by a hissing sound and ozone, causes power loss and produces radio interference.

14. In the plasma state, a gas

A. Loses electrical conductivity
B. Conducts electricity
C. Becomes a perfect insulator
D. Attracts moisture

Answer: B — Conducts electricity

Plasma is a highly ionised gas containing free electrons and ions, so it conducts electricity. The arc in a circuit breaker is a plasma column that must be deionised for interruption to succeed.

15. In sphere gaps, the spheres are made of

A. Aluminium
B. Brass
C. Bronze
D. Any of the above

Answer: D — Any of the above

Measuring spheres must have smooth, accurate surfaces and good conductivity. They are commonly made of brass, bronze or aluminium, with brass and bronze being most usual.

16. Which soil has the maximum specific resistance?

A. Black cotton soil
B. Sand
C. Peat
D. Loamy soil

Answer: B — Sand

Dry sand holds little moisture and few dissolved salts, so its resistivity is very high compared with loam, peat or clay-based black cotton soil. This matters when designing earthing systems.

17. At constant temperature, the breakdown voltage in a uniform field is a function of the product of gas pressure and the distance between the electrodes. This statement is known as

A. Electron avalanche
B. Thermal stability principle
C. Paschen's law
D. Breakdown voltage law

Answer: C — Paschen's law

Paschen's law states that the sparking voltage of a uniform-field gap is a function of the product of gas pressure and gap length. It explains why compressed gas raises breakdown strength.

18. The breakdown voltage of gases depends on

A. Distance between the electrodes
B. Relative air density
C. Humidity
D. All of the above

Answer: D — All of the above

Gas breakdown voltage depends on gap length and gas density, per Paschen's law, and is also affected by humidity. Sphere gap readings are therefore corrected for air density and humidity.

19. Impulse voltages are characterised by

A. Polarity
B. Peak value
C. Time to half of the peak value
D. All of the above

Answer: D — All of the above

An impulse wave is specified by its polarity, peak value, front time and time to half-value on the tail, for example the standard 1.2/50 microsecond lightning impulse.

20. Surge diverters are

A. Non-linear resistors in series with spark gaps which act as fast switches
B. Arc quenching devices
C. Shunt reactors to limit the voltage rise due to Ferranti effect
D. Over-voltages of power frequency harmonics

Answer: A — Non-linear resistors in series with spark gaps which act as fast switches

A gapped surge diverter uses non-linear resistor blocks in series with spark gaps. The gaps flash over on a surge and the resistors pass the surge to earth while limiting the follow current at normal voltage.

21. All of the following can be used for the measurement of high AC voltages EXCEPT

A. Potential dividers
B. Potential transformers
C. Electrostatic voltmeters
D. Hall effect generators

Answer: D — Hall effect generators

High AC voltages are measured with capacitive or resistive potential dividers, potential transformers, electrostatic voltmeters and sphere gaps. Hall effect generators measure magnetic field and current, not high voltage.

22. Corona results in

A. Improvement in power factor
B. Increased capacitive reactance of transmission lines
C. Radio interference
D. Better regulation

Answer: C — Radio interference

Corona discharges produce high-frequency current pulses that radiate electromagnetic noise, causing radio and communication interference, in addition to power loss and ozone formation.

23. All of the following are preferred properties of a dielectric gas EXCEPT

A. High dielectric strength
B. Physiological inertness
C. Low atomic number
D. Good heat transfer

Answer: C — Low atomic number

A good insulating gas should have high dielectric strength, thermal and chemical stability, non-toxicity and good heat transfer. Low atomic number is not a required property; SF6 is in fact a heavy molecule.

24. Electro-mechanical breakdown of solid insulating materials occurs due to

A. Magnetic hum
B. Vibrations
C. Mechanical stresses produced by the electric field
D. Electrical stresses produced by voltage fluctuations

Answer: C — Mechanical stresses produced by the electric field

In electro-mechanical breakdown, electrostatic attraction between electrodes compresses the insulation. When the compressive stress exceeds the mechanical strength of the material, it collapses and breakdown occurs.

25. Compared with air, the relative dielectric strength of sulphur hexafluoride is nearly

A. 1.5 times
B. 2.5 times
C. 4.0 times
D. 5.0 times

Answer: B — 2.5 times

At the same pressure, SF6 has a dielectric strength about 2.5 times that of air, owing to its electronegative nature, which captures free electrons and suppresses avalanche formation.

26. Transformers contribute to radio interference due to

A. Corona discharges in air
B. Internal or partial discharges in insulation
C. Sparking
D. Any of the above

Answer: D — Any of the above

Any discharge activity produces high-frequency pulses. Transformers can generate radio interference from external corona, partial discharges within the insulation and sparking at loose connections.

27. Impulse testing of transformers is done to determine the ability of

A. Bushings to withstand vibrations
B. Insulation to withstand transient voltages
C. Windings to withstand voltage fluctuations
D. All of the above

Answer: B — Insulation to withstand transient voltages

Impulse tests apply standard lightning or switching impulse waves to verify that the transformer insulation can withstand transient overvoltages without failure.

28. Insulators for high voltage applications are tested by

A. Power frequency tests
B. Impulse tests
C. Both power frequency and impulse tests
D. None of the above

Answer: C — Both power frequency and impulse tests

HV insulators must withstand both normal power frequency overvoltages, including wet and dry flashover tests, and lightning or switching impulses, so both power frequency and impulse tests are carried out.

29. A switching surge is a

A. High voltage DC
B. High voltage AC
C. Short duration transient voltage
D. Hyperbolically dying voltage

Answer: C — Short duration transient voltage

Switching operations such as breaker opening or line energisation create short duration transient overvoltages called switching surges, which insulation coordination must take into account.

30. The phenomenon of corona is generally accompanied by

A. A bang
B. A hissing sound
C. Magnetic hum
D. All of the above

Answer: B — A hissing sound

Corona produces a characteristic hissing or crackling sound, a faint violet glow and the smell of ozone, due to ionisation of air around highly stressed conductors.

Set 9 — 30 HV exit exam questions with answers

1. Corona effect can be identified by

A. Bushy sparks
B. A faint violet glow
C. Red light
D. Arcing between conductors and earth

Answer: B — A faint violet glow

When the air around a conductor is ionised by a high electric field, it emits a faint violet glow, most visible at night. This, along with hissing and ozone, identifies corona.

2. Rubber insulating gloves used for electrical protection must be electrically retested every

A. 3 months
B. 6 months
C. 12 months
D. Never

Answer: B — 6 months

Rubber insulating gloves must be electrically tested before first issue and at intervals not exceeding 6 months (OSHA 1910.137 / IEC 60903 practice), and air-tested and inspected before each use.

3. One of the generally recognised hazards of electrical work is ____.

A. Arc flash
B. Cuts
C. Falls
D. Concussion

Answer: A — Arc flash

Electrical work hazards are electric shock, arc flash and arc blast. An arc flash releases intense heat and light that can cause severe burns, which is why arc-rated PPE and approach boundaries are used.

4. Which is the 'can't let go' range of current flow through the body?

A. 3-9 mA
B. 9-25 mA
C. 25-60 mA
D. 1-3 mA

Answer: B — 9-25 mA

Around 9 to 25 mA, muscles contract involuntarily and the person may be unable to release the conductor. Lower currents are perceptible but releasable; higher currents risk respiratory paralysis and ventricular fibrillation.

5. The secondary of a current transformer must never be ____ while energised.

A. Earthed
B. Opened
C. Examined
D. Shorted

Answer: B — Opened

With an open secondary, there is no opposing secondary ampere-turns, so the primary current drives the core into saturation, producing dangerously high secondary voltage and overheating. CT secondaries are shorted before removing instruments.

6. The minimum allowable working space in front of electrical equipment is ____ inches deep.

A. 36
B. 48
C. 24
D. 30

Answer: A — 36

The US National Electrical Code (NEC 110.26) requires a minimum working space depth of 36 inches (about 0.9 m) in front of equipment likely to be examined or serviced while energised.

7. Crossing the Prohibited Approach Boundary is considered the same as making ____.

A. Insulation
B. Confluence
C. Contact
D. Isolation

Answer: C — Contact

Under NFPA 70E, crossing the prohibited approach boundary is treated as making contact with exposed energised parts, requiring the same protection and justification as direct contact.

8. Work performed on an electrical system within reaching distance of energised components is ____ work.

A. Energised
B. Essential
C. Safe
D. Unavoidable

Answer: A — Energised

Any work within reach of exposed energised parts is treated as energised work, requiring justification, an energised work permit, appropriate PPE and observance of approach boundaries.

9. A safety electrical one-line diagram should be used to ____ all sources of electrical energy.

A. Identify
B. Castigate
C. Evaluate
D. Modify

Answer: A — Identify

An up-to-date single line diagram shows every source of supply, including generators, shore connection and back-feeds, so that all sources can be identified and isolated before work begins.

10. One of the main functions of personal protective grounds is to provide a ____ impedance path for a short circuit.

A. High
B. Low
C. Smooth
D. Adequate

Answer: B — Low

Temporary protective earths bond and short the isolated conductors so that any accidental re-energisation produces a low impedance fault path, causing protective devices to operate quickly and limiting voltage at the work site.

11. For voltage measurement, the gap which gives the highest accuracy is the

A. Sphere gap
B. Field gap
C. Rod gap
D. None of these

Answer: A — Sphere gap

Sphere gaps give a nearly uniform field and consistent breakdown values, with standard calibration tables, so they are the accepted reference for peak voltage measurement with about ±3% accuracy.

12. The most commonly used insulating material for low and medium voltage cables is

A. Polyethylene
B. Porcelain
C. PVC
D. Vulcanised rubber

Answer: C — PVC

PVC is cheap, flexible, flame-retardant and durable, making it the most widely used insulation and sheathing for low and medium voltage cables. Porcelain is used for rigid insulators, not cables.

13. In an alternator, when the load power factor is unity, the armature reaction flux is

A. Demagnetising
B. Cross-magnetising
C. Reduced to zero
D. Of square wave form

Answer: B — Cross-magnetising

At unity power factor the armature current is in phase with the EMF, so the armature flux acts at right angles to the main field. This cross-magnetising effect distorts the field rather than weakening or strengthening it.

14. ____ are used for resilient flooring and walkways in front of high voltage panels.

A. Insulation gloves
B. Insulation mats
C. Insulation tapes
D. Insulation helmets

Answer: B — Insulation mats

Rubber insulating mats are laid in front of switchboards and HV panels to provide an insulated, resilient standing surface, reducing shock risk to personnel operating or working on the equipment.

15. The electric field strength at which breakdown occurs in a material is called its

A. Breakdown voltage
B. Dielectric strength
C. Tri-electric strength
D. Permittivity

Answer: B — Dielectric strength

Dielectric strength is the maximum electric field, usually expressed in kV/mm, that an insulating material can withstand before breakdown. Breakdown voltage depends on thickness, whereas dielectric strength is a material property.

16. Arc interruption in an AC circuit breaker takes place at

A. Zero voltage
B. Zero current
C. High current
D. High voltage

Answer: B — Zero current

In AC breakers the arc is extinguished at a natural current zero, when energy input to the arc is minimal. The breaker must then build dielectric strength faster than the restriking voltage rises.

17. An MCB protects a circuit from

A. Short circuit
B. Overload only
C. Both short circuit and overload
D. None of the above

Answer: C — Both short circuit and overload

A miniature circuit breaker has a thermal bimetal element for overload protection and a magnetic trip coil for instantaneous short-circuit protection.

18. Surge voltages originate in power systems due to

A. Lightning
B. Switching operations
C. Faults
D. Any of the above

Answer: D — Any of the above

Transient overvoltages arise from external causes such as lightning and internal causes such as switching operations, arcing faults and resonance. All must be considered in insulation coordination.

19. In an actual transformer, the iron loss remains practically constant from no load to full load because

A. The transformation ratio remains constant
B. The permeability of the core remains constant
C. The core flux remains practically constant
D. The primary voltage remains constant

Answer: C — The core flux remains practically constant

Iron losses (hysteresis and eddy current) depend on core flux density and frequency. Since supply voltage and frequency are constant, the mutual flux stays practically constant at all loads, so iron loss is constant.

20. An alternator is said to be over-excited when it is operating at

A. Unity power factor
B. Leading power factor
C. Lagging power factor
D. Zero power factor only

Answer: C — Lagging power factor

An over-excited alternator generates EMF greater than terminal voltage and supplies lagging reactive power to the system, i.e. it operates at a lagging power factor. An under-excited alternator works at a leading power factor.

21. Electrical injuries are commonly caused by

A. Unsafe equipment or installations
B. An unsafe environment
C. Unsafe work practices
D. All of the above

Answer: D — All of the above

Electrical accidents typically result from defective equipment or installations, unsafe environments such as wet or confined spaces, and unsafe work practices like working live without isolation. Controls must address all three.

22. What safety measures are to be followed while working on high voltage systems on ships?

A. Observing clearance distances for the voltage at which the qualified person will work
B. Proper precautionary work techniques
C. Proper use of PPE
D. All of the above

Answer: D — All of the above

Safe HV work on board requires observing minimum approach distances, following a permit-to-work with isolation, proving dead and earthing, and wearing appropriate PPE, as required by company SMS and STCW HV training.

23. In a circuit breaker, ionisation is facilitated by

A. Increase in field strength
B. Increase in mean free path
C. High temperature of the surrounding medium
D. All of the above

Answer: D — All of the above

Higher field strength and longer mean free path let electrons gain enough energy to ionise gas molecules on collision, while high temperature causes thermal ionisation. All three promote ionisation and sustain the arc.

24. Which of the following is the best conductor of electricity?

A. Copper
B. Aluminium
C. Silver
D. Iron

Answer: C — Silver

Silver has the lowest resistivity of all metals, slightly lower than copper. Copper is used more widely only because silver is far more expensive.

25. For liquid-cooled static converters, which monitoring arrangements shall be provided?

A. Stand-by start alarm of coolant pumps
B. Coolant conductivity
C. Coolant temperature
D. All of the above

Answer: D — All of the above

Classification rules require liquid-cooled converters to have monitoring of coolant flow and pump changeover, coolant temperature and coolant conductivity, since high conductivity could cause leakage currents and flashover.

26. In an induction motor, the rotor runs at a speed

A. Equal to the speed of the stator field
B. Lower than the speed of the stator field
C. Higher than the speed of the stator field
D. Having no relation with the speed of the stator field

Answer: B — Lower than the speed of the stator field

Torque in an induction motor depends on relative motion between the rotating field and rotor conductors to induce rotor current. The rotor therefore always runs slightly below synchronous speed; the difference is called slip.

27. When two diesel generator sets are running in parallel during your engine room watch, what does it simply mean?

A. That both generators have the same voltage and the same frequency
B. That both generators have the same kW load, the same kVAR load and the same current
C. That both diesels are running at the same speed
D. That both generators share exactly 50% of the active load in kW

Answer: A — That both generators have the same voltage and the same frequency

Generators connected in parallel to the same busbar must have the same terminal voltage and frequency. Load sharing in kW and kVAR depends on governor and AVR settings and need not be equal, and engines may differ in speed if pole numbers differ.

28. Alternating current power is transmitted at high voltage

A. To safeguard against pilferage
B. To minimise transmission losses
C. To reduce cost of generation
D. To make the system more reliable

Answer: B — To minimise transmission losses

For a given power, raising the voltage reduces the current, and since copper loss is proportional to current squared, losses and conductor size are greatly reduced. This is also why large ships adopt HV distribution.

29. What are the hazards of arc flash?

A. Sudden release of large amounts of heat and light energy at the point of the arc
B. Serious, even fatal, injuries to workers ten feet or more from the arc
C. Permanent damage to equipment
D. All of the above

Answer: D — All of the above

An arc flash releases intense heat, light, pressure and molten metal. It can cause fatal burns at considerable distance, blast injuries and permanent equipment damage, which is why arc flash boundaries and arc-rated PPE are specified.

30. Where procedures involve removal of circuit main earths for testing under a sanction-for-test, the earths are secured with working locks. The keys to these locks will be

A. Retained by the duty authorised person (HV), who will remove and replace the earths as required
B. Retained by the chief engineer
C. Retained by the electro-technical officer
D. Kept in the safety locker

Answer: A — Retained by the duty authorised person (HV), who will remove and replace the earths as required

Under HV safety rules, earthing devices locked under a permit or sanction-for-test remain under the control of the authorised person, who personally removes and replaces earths as testing requires, ensuring no one else can alter the safety arrangements.

Set 10 — 30 HV exit exam questions with answers

1. For high voltage conductors at high pressure, if the voltage is positive, the corona appears as a

A. Uniform bluish-white sheath
B. Reddish glowing spots
C. Uniform greenish spots
D. None of these

Answer: A — Uniform bluish-white sheath

Positive corona forms a uniform bluish-white glow along the conductor surface, while negative corona appears as reddish glowing spots distributed along the conductor.

2. What happens in arc extinction by the high resistance method?

A. Arc resistance is decreased with time
B. Arc resistance is increased with time
C. Arc resistance remains the same
D. Arc resistance is kept at zero

Answer: B — Arc resistance is increased with time

In the high resistance method, the arc is lengthened, cooled, constricted and split so that its resistance rises with time until the current falls to a value that cannot sustain the arc. It is used in DC and low-power AC breakers.

3. Lower voltage systems supplied through transformers from high voltage systems are to be protected against overvoltages. This may be achieved by

A. Direct earthing of the lower voltage system
B. Appropriate neutral voltage limiters
C. Both direct earthing and neutral voltage limiters
D. None of the above

Answer: C — Both direct earthing and neutral voltage limiters

Class and IEC 60092 rules require protection of LV systems against overvoltage from HV/LV insulation breakdown, achieved by direct earthing of the LV neutral, neutral voltage limiters or an earthed screen between windings.

4. The desired tripping of a circuit breaker under fault conditions is

A. Manual
B. Automatic
C. Only by giving a warning
D. None of these

Answer: B — Automatic

A circuit breaker must trip automatically, via protective relays or direct-acting releases, as soon as a fault is detected, because manual operation would be far too slow to prevent damage.

5. Air blast circuit breakers are preferred for

A. Short duty
B. Intermittent duty
C. Repeated duty
D. All of these

Answer: C — Repeated duty

Air blast breakers do not deteriorate an arc medium like oil, have low arc energy and fast operation, so they are well suited to frequent, repeated operation and rapid auto-reclosing duty.

6. After the 2010 Manila amendments, ____ made training in HV systems mandatory for senior engineering staff working on electrical power plants above 1000 volts.

A. SOLAS
B. STCW
C. MARPOL
D. IMO

Answer: B — STCW

The 2010 Manila amendments to the STCW Convention and Code introduced competence in operation and maintenance of high voltage systems above 1000 V for second engineers and chief engineers (Table A-III/2).

7. ____ type of work procedure involves electrical installations that are neither live nor charged.

A. Live working
B. Dead working
C. Working in the vicinity of live parts
D. None of the above

Answer: B — Dead working

Dead working is work on installations that have been isolated, proved dead, earthed and secured, so they are neither live nor charged. It is the preferred method under HV permit-to-work procedures.

8. Fuse protection is generally used for current ratings up to ____.

A. 10 A
B. 20 A
C. 50 A
D. 100 A

Answer: D — 100 A

Fuses are an economical form of protection for circuits with current ratings up to about 100 A. Above this rating, circuit breakers with adjustable protective relays are preferred because fuses become bulky and offer no adjustable or resettable protection.

9. Which insulating medium can be used to fill sphere gaps?

A. Liquid insulation (transformer oil)
B. Solid insulation (polyester, paper)
C. Gas insulation (SF6, N2, CO2, CCl2F2)
D. Any of the above

Answer: D — Any of the above

Sphere gaps used for high-voltage measurement and protection can operate in gaseous, liquid or solid dielectric media. The breakdown voltage depends on the medium and gap spacing, so any of these insulating media may be used.

10. What is the polarisation index?

A. Ratio of the 10-minute to the 1-minute insulation resistance
B. Ratio of the 1-minute to the 10-minute insulation resistance
C. Ratio of the 5-minute to the 25-minute insulation resistance
D. None of these

Answer: A — Ratio of the 10-minute to the 1-minute insulation resistance

The polarisation index is the insulation resistance measured after 10 minutes divided by the value after 1 minute at the same test voltage. A PI of about 2 or more indicates clean, dry insulation; a low PI suggests moisture or contamination in HV machine windings.

11. In air blast circuit breakers, the pressure of air is of the order of:

A. 100 mm Hg
B. 1 kg/cm2
C. 20 to 30 kg/cm2
D. 200 to 300 kg/cm2

Answer: C — 20 to 30 kg/cm2

Air blast circuit breakers use compressed air at roughly 20 to 30 kg/cm2, stored in a receiver, which is blasted across the contacts to cool and de-ionise the arc path and extinguish the arc rapidly.

12. An earth connection applied to apparatus after the circuit main earth has been applied, normally at the point of work, is known as a(n):

A. Additional earth
B. Circuit main earth
C. Neutral earth
D. Equipotential bond

Answer: A — Additional earth

Under HV safety rules, the circuit main earth is applied first at the isolation point under a permit to work. An additional earth is then applied, normally at the point of work, to give further protection against induced or accidental re-energisation.

13. In HV switchgear, low-voltage compartments for control and monitoring systems shall be:

A. Separated from the HV compartments so that contact with parts above 1000 V is impossible
B. Combined with the HV busbar compartment to save space
C. Accessible only when the HV busbars are energised
D. Fitted with doors interlocked to the shore connection

Answer: A — Separated from the HV compartments so that contact with parts above 1000 V is impossible

Class and IEC rules require low-voltage control and monitoring compartments to be segregated from high-voltage compartments so that persons working on control circuits cannot come into contact with parts at a rated voltage above 1000 V.

14. Under fault conditions, the higher voltages and fault-current levels of an HV system can develop:

A. Very large quantities of energy, causing arc flash and blast
B. Only small quantities of energy
C. No energy, as protection trips instantly
D. Energy only in the neutral conductor

Answer: A — Very large quantities of energy, causing arc flash and blast

Because fault energy is proportional to voltage and current, HV faults release very large amounts of energy as arc flash, heat and pressure blast. This is why HV switchgear must be arc-resistant and why strict safety procedures are required.

15. The supply voltage to portable tools and appliances on board should generally not exceed:

A. 250 V
B. 440 V
C. 690 V
D. 1000 V

Answer: A — 250 V

Class rules based on IEC 60092 limit the supply to portable appliances and tools to about 250 V, and lower safety voltages are preferred in damp or confined spaces. HV levels such as 1000 V are never used for portable equipment.

16. In a fully partitioned, air-insulated HV switchboard, all sections shall be partitioned from each other and their surroundings so that they are:

A. Arc-resistant
B. Open for natural ventilation
C. Electrically bonded to the busbars
D. Pressurised with nitrogen

Answer: A — Arc-resistant

Fully partitioned HV switchboards have each section segregated by metal partitions so that an internal arc fault is contained within its compartment. This arc-resistant construction protects personnel and adjacent sections from the effects of an internal arc.

17. In an HV shore connection system, the shore supply plug is interlocked with:

A. The ship-shore equipotential bonding (earthing) connection
B. The ship's steering gear
C. The emergency generator start circuit
D. The fire pump starter

Answer: A — The ship-shore equipotential bonding (earthing) connection

IEC/IEEE 80005-1 requires the HV shore connection to be interlocked so that power cannot be applied unless the equipotential bonding between ship and shore is established, and the plug cannot be withdrawn while energised, preventing dangerous touch voltages.

18. Which property of SF6 gas allows it to absorb the energy of an arc without much rise in temperature?

A. High thermal heat capacity
B. Low dielectric strength
C. High electrical conductivity
D. Low density

Answer: A — High thermal heat capacity

SF6 is used in HV breakers because it has high dielectric strength, is strongly electronegative and has a high thermal heat capacity. The last property lets it absorb arc energy with little temperature rise, aiding rapid arc quenching.

19. The polarisation index (PI) test is mainly carried out on:

A. HV generators and motors
B. LV lighting circuits
C. Lead-acid batteries
D. Cartridge fuses

Answer: A — HV generators and motors

The PI test, the ratio of 10-minute to 1-minute insulation resistance, is used to assess the insulation condition of HV rotating machines and transformers. It is not a routine test for low-voltage generators, which are checked by a simple insulation resistance test.

20. Before touching an exposed HV circuit, or before leaving it unattended, the circuit must be:

A. De-energised and earthed with an earthing (grounding) stick
B. Checked with a multimeter on the ohms range only
C. Isolated at the LV control panel only
D. Covered with a rubber mat while live

Answer: A — De-energised and earthed with an earthing (grounding) stick

Safe HV practice requires the circuit to be isolated, proved dead with an approved tester and earthed with an earthing stick before contact. Stored capacitive charge and induced voltages can remain after isolation, so earthing is essential.

21. The induced emf across a stationary conductor placed in a steady magnetic field is:

A. Zero
B. Maximum
C. Equal to the supply voltage
D. Half the peak value

Answer: A — Zero

By Faraday's law an emf is induced only when the flux linking a conductor changes, either by relative motion or a varying field. A stationary conductor in a steady field experiences no change of flux, so the induced emf is zero.

22. According to IEEE 43, which DC test voltage range is recommended for an insulation resistance test on the stator winding of a 6.6 kV HV motor?

A. 10,000 V to 15,000 V
B. 500 V to 1,000 V
C. 2,500 V to 5,000 V
D. 250 V to 500 V

Answer: C — 2,500 V to 5,000 V

IEEE 43 recommends 2,500-5,000 V DC for windings rated 5,001-12,000 V, so a 6.6 kV machine is normally tested at 5 kV. Lower voltages under-stress the insulation while much higher DC voltages risk damaging it.

23. For a 3.3 kV HV motor winding, the IEEE 43 recommended DC insulation resistance test voltage is:

A. 250 V
B. 5,000 V to 10,000 V
C. Equal to the peak of the rated AC voltage
D. 1,000 V to 2,500 V

Answer: D — 1,000 V to 2,500 V

IEEE 43 specifies 1,000-2,500 V DC for windings rated 2,501-5,000 V. A 3.3 kV winding therefore falls in this band; typically a 2.5 kV insulation tester is used on board.

24. For modern form-wound HV stator windings (Class B, F or H insulation), the minimum acceptable polarization index recommended by IEEE 43 is:

A. 1.0
B. 2.0
C. 5.0
D. 1.25

Answer: B — 2.0

IEEE 43 gives a minimum PI of 2.0 for Class B, F and H insulation (1.5 for Class A). A PI below this suggests moisture or contamination allowing leakage current to dominate the absorption current.

25. The Dielectric Absorption Ratio (DAR) of a winding is normally calculated as:

A. Insulation resistance at 1 minute divided by that at 15 seconds after discharge
B. Test voltage divided by leakage current
C. Insulation resistance at 60 seconds divided by that at 30 seconds
D. Insulation resistance at 10 minutes divided by that at 1 minute

Answer: C — Insulation resistance at 60 seconds divided by that at 30 seconds

The DAR is a short-duration version of the PI, taken as the 60-second reading divided by the 30-second reading. It is useful as a quick indicator where a full 10-minute PI test is not practicable.

26. A polarization index reading close to 1.0 on an HV motor winding most likely indicates:

A. An open-circuited stator winding
B. Moist or contaminated insulation where leakage current dominates
C. Excellent, perfectly dry insulation
D. The tester was set to too low a voltage

Answer: B — Moist or contaminated insulation where leakage current dominates

In good dry insulation the absorption current decays over 10 minutes so resistance keeps rising and PI exceeds 2. When leakage through moisture or dirt dominates, the resistance hardly changes with time, giving a PI near 1.

27. IEEE 43 states that the polarization index need not be used to judge a winding when the 1-minute insulation resistance is above approximately:

A. 100 kilohms
B. 10 megohms
C. 5,000 megohms
D. 1 megohm

Answer: C — 5,000 megohms

When the 1-minute IR exceeds about 5,000 MΩ the measured currents are so small that the PI becomes unreliable, and IEEE 43 says the PI may then be disregarded as an acceptance criterion.

28. Insulation resistance of a winding falls as temperature rises. As a rule of thumb used in IEEE 43, the IR approximately halves for every rise in temperature of:

A. 1°C
B. 100°C
C. 50°C
D. 10°C

Answer: D — 10°C

Insulation resistance has a strong negative temperature coefficient; IEEE 43 uses an approximate halving for every 10°C rise. Readings should therefore be corrected to a reference temperature of 40°C before trending or comparing results.

29. IEEE 43 recommends that insulation resistance readings be corrected to a common base winding temperature of:

A. 75°C
B. 40°C
C. 0°C
D. 20°C

Answer: B — 40°C

Readings taken at different winding temperatures cannot be compared directly, so IEEE 43 corrects them to a 40°C base. Trend records of HV machines should always state the temperature and corrected value.

30. After completing a DC insulation resistance test on an HV winding, the winding should be earthed for at least:

A. Ten seconds regardless of test duration
B. The same time as the test voltage was applied, divided by four
C. Four times the duration for which the test voltage was applied
D. No earthing is needed if the tester has auto-discharge

Answer: C — Four times the duration for which the test voltage was applied

HV windings and cables store charge, including slowly released absorption charge, which can re-appear as a dangerous voltage. IEEE 43 advises grounding for at least four times the voltage application time after the test.

Set 11 — 30 HV exit exam questions with answers

1. What is the purpose of the guard terminal on an HV insulation resistance tester?

A. To earth the instrument case
B. To protect the operator from arc flash
C. To increase the test voltage
D. To bypass surface leakage current so that only the current through the bulk insulation is measured

Answer: D — To bypass surface leakage current so that only the current through the bulk insulation is measured

Connecting the guard to a conductive band on the insulation surface diverts surface leakage away from the measuring circuit. This separates surface contamination effects from the true volume resistance of the cable or bushing insulation.

2. During a 10-minute insulation resistance test, the current that decays relatively slowly due to molecular polarization of the insulation is the:

A. Surface tracking current
B. Absorption current
C. Capacitive charging current
D. Conduction (leakage) current

Answer: B — Absorption current

The total test current comprises capacitive charging current (decays within seconds), absorption (polarization) current (decays over minutes) and steady conduction current. The PI essentially compares how much the absorption current has decayed between 1 and 10 minutes.

3. Before performing an insulation resistance test on an HV cable, which step is essential besides isolating and proving it dead?

A. Remove the cable screen earth connections permanently
B. Close the busbar earth switch at the far end and leave it applied during the test
C. Energise the cable at rated voltage for 1 minute
D. Disconnect or isolate the equipment at both ends, including VTs and surge arresters that could be damaged or affect the reading

Answer: D — Disconnect or isolate the equipment at both ends, including VTs and surge arresters that could be damaged or affect the reading

A cable has two ends and connected apparatus such as voltage transformers, surge arresters or electronics that can be damaged or give false low readings. Both ends must be isolated, and earths are removed only under a Sanction to Test.

4. Which safety document authorises the temporary removal of circuit main earths so that an insulation resistance test can be carried out on HV equipment?

A. Enclosed Space Entry Permit
B. Hot Work Permit
C. Sanction to Test
D. Limitation of Access

Answer: C — Sanction to Test

An IR test requires earths to be removed and voltage applied, which is not allowed under a Permit to Work. The Authorised Person issues a Sanction to Test so that testing proceeds under controlled conditions.

5. An IR trend for an HV generator shows readings steadily falling over several months after correction to 40°C. The most appropriate action is to:

A. Ignore it, as long as the reading is above 1 megohm
B. Raise the test voltage to obtain higher readings
C. Investigate and plan cleaning and drying of the winding before failure occurs
D. Increase the generator excitation

Answer: C — Investigate and plan cleaning and drying of the winding before failure occurs

The value of IR testing lies in trends of temperature-corrected readings. A steady decline indicates contamination, moisture or ageing, so cleaning, drying out and further tests such as PI should be planned before an earth fault or breakdown occurs.

6. Why is a polarization index test considered more reliable than a single spot insulation resistance reading for HV machines?

A. It is performed at a much higher voltage
B. It does not need the machine to be isolated
C. It is the only test that detects open circuits
D. It is a ratio, so it is relatively independent of winding temperature and machine size

Answer: D — It is a ratio, so it is relatively independent of winding temperature and machine size

Because PI is the ratio of two readings taken on the same winding minutes apart, temperature and geometry effects largely cancel. It therefore indicates the condition of insulation more consistently than a single absolute IR value.

7. In an IEEE 43 insulation test, the minimum recommended 1-minute insulation resistance (corrected to 40°C) for most form-wound AC windings built after 1970 is:

A. 100 megohms
B. 1 megohm
C. 5 megohms
D. 10,000 megohms

Answer: A — 100 megohms

IEEE 43 recommends a minimum IR1min of 100 MΩ for most DC armature and form-wound AC windings built after about 1970. Random-wound and form-wound windings rated below 1 kV have a lower 5 MΩ minimum.

8. When testing an HV cable insulation with a 5 kV insulation tester, the initial reading rises steadily for the first minutes mainly because:

A. The cable capacitance is charging and absorption current is decaying
B. Surface leakage is increasing with time
C. The cable conductor resistance is increasing
D. The tester battery is weakening

Answer: A — The cable capacitance is charging and absorption current is decaying

An HV cable has substantial capacitance. As the capacitive and absorption currents decay, the total current falls and the indicated resistance climbs; the reading should be allowed to stabilise or be recorded at fixed times.

9. Which HV equipment must normally be disconnected before an insulation test on a motor fed from a variable frequency drive?

A. Nothing; the drive can safely be included in the test
B. The drive output semiconductors, by disconnecting the motor cables at the drive
C. The motor anti-condensation heater thermostat only
D. The motor cooling fan guard

Answer: B — The drive output semiconductors, by disconnecting the motor cables at the drive

Power electronic devices in a VFD can be destroyed by kilovolt test voltages and would also distort the reading. The motor cables must be disconnected from the drive output terminals so only the cable and motor insulation are stressed.

10. In an insulated (unearthed) neutral HV system, a solid earth fault on one phase causes the voltage to earth of the two healthy phases to:

A. Rise to ten times the phase voltage
B. Rise to the line voltage, about 1.73 times the normal phase voltage
C. Remain at phase voltage
D. Fall to zero

Answer: B — Rise to the line voltage, about 1.73 times the normal phase voltage

With the neutral unearthed, the faulted phase is held at earth potential and the neutral shifts by a phase voltage. The healthy phases then sit at line voltage to earth, so cable and equipment insulation must be rated for this stress.

11. The main reason for using high-resistance earthing of the generator neutral on a ship's HV system rather than solid earthing is to:

A. Increase the voltage of the system
B. Increase earth fault current so fuses blow quickly
C. Eliminate the need for earth fault relays
D. Limit earth fault current to a low value, reducing damage while allowing selective detection of the faulty circuit

Answer: D — Limit earth fault current to a low value, reducing damage while allowing selective detection of the faulty circuit

Solid earthing would give very high, destructive earth fault currents at 6.6 kV or 11 kV. A neutral earthing resistor limits the current to a manageable value while still providing enough current for relays to identify and, if required, trip the faulted feeder.

12. A drawback of operating an HV system with a completely insulated neutral is the risk of:

A. Transient overvoltages from intermittent (arcing) earth faults
B. Loss of three-phase supply on the first earth fault
C. Very high earth fault currents tripping all breakers
D. Inability to run motors

Answer: A — Transient overvoltages from intermittent (arcing) earth faults

An intermittent arcing earth fault on an insulated system can repeatedly charge the system capacitance, producing transient overvoltages several times normal. These can break down insulation elsewhere, leading to a phase-to-phase fault, which is why resistance earthing is commonly used on HV.

13. When selecting the value of a neutral earthing resistor for an HV system, the resistive earth fault current is normally chosen to be:

A. Less than one tenth of the capacitive charging current
B. Not less than the system's capacitive charging current
C. Equal to the prospective three-phase fault current
D. Equal to the full-load current of the largest generator

Answer: B — Not less than the system's capacitive charging current

If the resistive fault current is at least equal to the capacitive charging current, the resistor damps the system and suppresses arcing-ground overvoltages. A much smaller current would leave the system behaving as unearthed.

14. In an insulated-neutral HV system, an earth fault can be detected by a neutral voltage displacement relay, which is typically connected to:

A. The DC control supply
B. The secondary of the generator CT on one phase only
C. The AVR output terminals
D. The open-delta (broken-delta) secondary winding of a voltage transformer

Answer: D — The open-delta (broken-delta) secondary winding of a voltage transformer

Under healthy conditions the residual voltage at an open-delta VT secondary is near zero. An earth fault displaces the neutral and produces a residual voltage there, operating the neutral voltage displacement (59N) relay.

15. A principal operational advantage of an insulated or high-resistance earthed HV system on board is that:

A. Earth faults are cleared instantaneously by fuses
B. Supply to essential services can continue after a single earth fault, giving time to locate it
C. Phase-to-earth voltages are halved
D. No insulation monitoring is required

Answer: B — Supply to essential services can continue after a single earth fault, giving time to locate it

With low earth fault current, a single earth fault need not trip the supply immediately, so critical consumers such as propulsion and steering keep running. The fault must still be located and repaired promptly before a second fault occurs.

16. What is a switching programme (switching plan) in HV safety documentation?

A. The PMS load-shedding table
B. A written, step-by-step sequence of switching, isolation and earthing operations prepared and checked before they are carried out
C. A list of generator running hours
D. A diagram of cable routes only

Answer: B — A written, step-by-step sequence of switching, isolation and earthing operations prepared and checked before they are carried out

Complex HV isolations are planned in advance using the single-line diagram, listing each operation in order with the device identity. This prevents errors such as operating the wrong breaker or earthing a live circuit, and each step is ticked off as completed.

17. A 'Limitation of Access' document is typically issued by the Authorised Person when:

A. A generator is to be synchronised
B. HV conductors are to be worked on directly
C. An insulation test requires removal of earths
D. Work is to be carried out in an HV area or near HV equipment where a Permit to Work is not required, defining the safe working limits

Answer: D — Work is to be carried out in an HV area or near HV equipment where a Permit to Work is not required, defining the safe working limits

A Limitation of Access defines the limits within which a person may work in an HV enclosure, for example cleaning or LV work in a switchroom. It warns of adjacent live equipment but does not authorise work on HV conductors.

18. A Permit to Work on HV equipment should only be issued after:

A. The equipment has been isolated, locked off, proved dead and earthed, with danger notices posted
B. The person in charge has started work
C. The circuit breaker has been opened at the bridge console
D. The Chief Engineer has verbally approved it

Answer: A — The equipment has been isolated, locked off, proved dead and earthed, with danger notices posted

A PTW certifies that the equipment is safe to work on. All isolation, locking, proving dead, application of circuit main earths and posting of notices must be completed before the Authorised Person issues it to the recipient.

19. While a Permit to Work is in force on an HV circuit, a Sanction to Test is required on the same circuit. The correct procedure is to:

A. Suspend or cancel the Permit to Work, with all personnel withdrawn, before issuing the Sanction to Test
B. Issue the Sanction to Test alongside the permit with work continuing
C. Let the workers remove earths themselves
D. Energise from the bus to test

Answer: A — Suspend or cancel the Permit to Work, with all personnel withdrawn, before issuing the Sanction to Test

A PTW and a Sanction to Test must not be in force at the same time on the same equipment, since testing removes earths and applies voltage. The permit is suspended or cancelled with all workers clear before testing begins.

20. On completion of work under an HV Permit to Work, the recipient signs the clearance section to confirm that:

A. The circuit main earths have been removed by the recipient
B. The equipment has been re-energised
C. The Authorised Person may leave the ship
D. All persons have been withdrawn and warned, and tools, materials and additional earths have been removed

Answer: D — All persons have been withdrawn and warned, and tools, materials and additional earths have been removed

Clearance confirms that the work party is clear, warned that the equipment is no longer safe to work on, and that additional earths and tools are removed. Only then does the Authorised Person cancel the permit and restore the system.

21. Who normally receives an HV Permit to Work and takes charge of the work party at the point of work?

A. The Master
B. The Competent Person in charge of the work
C. Any crew member on watch
D. The shore superintendent by email

Answer: B — The Competent Person in charge of the work

The Authorised Person issues the permit to a Competent Person, who reads, understands and signs for it and is responsible for the safety of the work party. The permit remains in the recipient's possession while work is in progress.

22. Which of the following must be clearly stated on an HV Permit to Work?

A. The ship's position
B. Only the name of the person issuing it
C. The fuel consumption of the generators
D. The exact equipment to be worked on, points of isolation, location of earths, and remaining hazards nearby

Answer: D — The exact equipment to be worked on, points of isolation, location of earths, and remaining hazards nearby

An effective PTW identifies precisely what is safe, where it was isolated and earthed, and which adjacent parts remain live. Ambiguity about equipment identity is a common cause of HV accidents.

23. Why should all HV switching operations and the issue and cancellation of safety documents be recorded in an HV log?

A. To provide a traceable record of the system state and of who authorised each operation
B. For calculating crew overtime only
C. Because it replaces the need for a Permit to Work
D. To satisfy MARPOL Annex VI

Answer: A — To provide a traceable record of the system state and of who authorised each operation

An HV log records each switching operation, safety document number, time and the persons involved. This allows the Authorised Person to know the current state of isolation and earthing and supports investigation and audit.

24. Before carrying out the switching programme for an HV isolation, the programme should be:

A. Approved only after the work is complete
B. Destroyed after the first step
C. Checked against the current single-line diagram and confirmed by a second competent person
D. Kept verbal to save time

Answer: C — Checked against the current single-line diagram and confirmed by a second competent person

Independent checking against an up-to-date single-line diagram catches errors in sequence or device identity. Switching from a checked written programme reduces the risk of mis-operation on live HV equipment.

25. Under typical HV safety rules, a Permit to Work that has been cancelled:

A. Can be re-used the next day for different equipment
B. Automatically authorises re-energising by any crew member
C. Cannot be re-used; a new permit must be issued if further work is required
D. Can be transferred verbally to another work party

Answer: C — Cannot be re-used; a new permit must be issued if further work is required

Each safety document applies to one defined job and is cancelled when finished. Any further work requires the safety precautions to be re-confirmed and a new document issued, preserving control over the isolation.

26. If the recipient of an HV Permit to Work has to hand over the job to another Competent Person during the work, the correct action is:

A. The first recipient simply passes the permit form to the second
B. Work continues without any document change
C. The permit is cleared and cancelled, and a new permit is issued to the new recipient by the Authorised Person
D. The Master signs the back of the permit

Answer: C — The permit is cleared and cancelled, and a new permit is issued to the new recipient by the Authorised Person

Responsibility under a permit is personal to the named recipient. A change of person in charge requires the Authorised Person to cancel the existing permit and issue a fresh one, so the new recipient is briefed on the precautions.

27. What is the main purpose of a multi-hasp locking device at an HV point of isolation?

A. To allow the breaker to be closed remotely
B. To replace the Permit to Work
C. To allow several persons or work groups each to fit a personal padlock, so the isolation cannot be removed until every lock is removed
D. To earth the busbars automatically

Answer: C — To allow several persons or work groups each to fit a personal padlock, so the isolation cannot be removed until every lock is removed

A multi-hasp has holes for several padlocks. Each person working applies their own lock, and the isolating device stays locked off until the last person removes their lock, protecting every member of the work party.

28. In a key safe system used for HV isolations, the keys of the safety locks applied at the points of isolation are:

A. Placed in a key safe, which is then locked so the isolation cannot be undone without the key safe being released
B. Handed to the bridge OOW
C. Left in the padlocks for convenience
D. Thrown overboard after use

Answer: A — Placed in a key safe, which is then locked so the isolation cannot be undone without the key safe being released

Placing the isolation lock keys in a key safe secures them in one controlled location. The key safe itself is locked under the control of the safety document holder, so isolations cannot be removed while work is in progress.

29. Locking off an HV circuit breaker in the 'isolated' or withdrawn position typically involves:

A. Opening the breaker using the push button
B. Only switching off the control supply MCB
C. Placing a 'Caution' tag on the bridge panel
D. Padlocking the busbar and cable spout shutters closed and locking the breaker truck from being racked in

Answer: D — Padlocking the busbar and cable spout shutters closed and locking the breaker truck from being racked in

When a withdrawable breaker is racked out, shutters cover the live spouts. Padlocking the shutters and preventing re-insertion of the truck ensures the isolation is physically secure, unlike reliance on control circuits.

30. Which sequence is correct for isolation before HV work?

A. Isolate, secure (lock off), post notices, prove dead, apply circuit main earths, issue permit
B. Apply earths, then isolate, then prove dead
C. Issue permit, then isolate, then earth
D. Prove dead, then isolate, then open the breaker

Answer: A — Isolate, secure (lock off), post notices, prove dead, apply circuit main earths, issue permit

Equipment is first disconnected from all sources and locked off with danger notices posted, then proved dead with an approved detector before earths are applied. Only then is the Permit to Work issued.

Set 12 — 30 HV exit exam questions with answers

1. When preparing an isolation of an HV transformer for work, the Authorised Person must ensure isolation from:

A. Both the HV and LV sides, to prevent back-feeding through the transformer
B. The LV side only
C. The cooling fan supply only
D. The HV side only, because the LV side is safe

Answer: A — Both the HV and LV sides, to prevent back-feeding through the transformer

A transformer can be energised from either winding. If the LV side is not isolated and earthed, back-feed from an LV source or generator could induce high voltage in the HV winding being worked on.

2. Before working on an HV motor, besides isolating the main HV supply, the Authorised Person must also isolate:

A. Separate LV supplies such as the anti-condensation heater and any auxiliary or control supplies
B. The ship's emergency generator
C. Only the main HV breaker; nothing else is needed
D. The main engine fuel pump

Answer: A — Separate LV supplies such as the anti-condensation heater and any auxiliary or control supplies

HV motors have heater, instrumentation and sometimes cooling fan circuits fed from separate LV boards. These can still be live after the HV breaker is isolated and must be identified on the isolation plan and locked off.

3. A risk assessment prepared for an HV isolation should specifically identify:

A. The vessel's bunker plan
B. The weather forecast
C. All sources of energy, including back-feeds, stored energy in capacitors and induced voltages
D. Only the name of the motor

Answer: C — All sources of energy, including back-feeds, stored energy in capacitors and induced voltages

HV isolation risk assessment considers every possible source: normal supply, alternative feeds, generators, transformers, capacitors and induced voltages from parallel cables. Each must be isolated, discharged or earthed in the isolation diagram.

4. An isolation and earthing diagram prepared for HV work should show:

A. The engine room ventilation layout
B. Generator running hours
C. Points of isolation, locks applied, circuit main earths and additional earths, and the limits of the safe zone
D. Only the location of fire extinguishers

Answer: C — Points of isolation, locks applied, circuit main earths and additional earths, and the limits of the safe zone

A marked-up diagram makes the isolation visible to everyone: where the circuit is isolated and locked, where earths are applied, and which adjacent equipment remains live. It is attached to or referenced in the Permit to Work.

5. Danger notices at an HV point of isolation are used to:

A. Replace the need for locks
B. Show the voltage rating only
C. Warn that the device must not be operated because people are working on the circuit
D. Indicate that the circuit is live and in normal service

Answer: C — Warn that the device must not be operated because people are working on the circuit

Danger or caution notices at isolation points warn everyone not to operate the device. They supplement, but never replace, physical locks and the control provided by safety documents.

6. Which is a principal advantage of diesel-electric propulsion over a conventional direct-drive diesel installation?

A. Generators can be run at optimum load by varying the number on line, improving fuel economy at part load
B. No switchboard is required
C. It allows lower voltage generators
D. Electrical losses are eliminated

Answer: A — Generators can be run at optimum load by varying the number on line, improving fuel economy at part load

In a diesel-electric plant the same generators supply propulsion and hotel load. Running only the number of sets needed keeps engines near optimum loading, improving fuel efficiency and redundancy, despite some conversion losses.

7. In a podded (azimuthing) propulsion unit, the propulsion motor is located:

A. On the bridge
B. In the engine room, driving through a long shaft line
C. In the steering gear room driving the rudder
D. Inside the pod outside the hull, driving the propeller directly

Answer: D — Inside the pod outside the hull, driving the propeller directly

A podded drive houses the electric motor in a streamlined pod beneath the hull, coupled directly to the propeller. The whole pod can rotate to direct thrust, which removes the need for a rudder and long shaft line.

8. How is electrical power transferred to the propulsion motor of a podded unit that can rotate continuously through 360 degrees?

A. Through a slip ring unit
B. Through flexible trailing cables only
C. Through the steering hydraulic system
D. By induction across the hull plating

Answer: A — Through a slip ring unit

A slip ring unit at the top of the pod transfers HV power, control signals and auxiliary supplies to the rotating pod. Slip rings are monitored, including arc monitoring, because faults in them can lead to serious damage.

9. A major manoeuvring advantage of podded propulsion is that:

A. It runs without electrical power
B. Thrust can be directed through 360 degrees, giving excellent manoeuvrability without a conventional rudder
C. It needs no steering control system
D. It doubles the ship's speed

Answer: B — Thrust can be directed through 360 degrees, giving excellent manoeuvrability without a conventional rudder

Because the pod azimuths, thrust can be vectored in any direction. This gives high manoeuvrability in port and for dynamic positioning, and the remote control system combines rpm and steering angle commands.

10. In a typical HV diesel-electric propulsion system, which component converts the fixed-frequency supply from the main switchboard into variable frequency for the propulsion motor?

A. The propulsion frequency converter (drive)
B. The AVR
C. The shaft earthing device
D. The emergency switchboard

Answer: A — The propulsion frequency converter (drive)

Propulsion motor speed is controlled by varying supply frequency. A frequency converter, such as a PWM VSI drive, LCI or cycloconverter, fed through propulsion transformers, supplies the motor with variable frequency and voltage.

11. Propulsion transformers supplying 12-pulse rectifiers usually have two secondary windings (star and delta) because the 30-degree phase shift:

A. Cancels the dominant 5th and 7th harmonic currents on the supply side
B. Allows DC to be fed directly to the motor
C. Doubles the output voltage
D. Eliminates the need for a circuit breaker

Answer: A — Cancels the dominant 5th and 7th harmonic currents on the supply side

A 6-pulse rectifier draws harmonics of order 5, 7, 11, 13 and so on. Two 6-pulse bridges fed from secondaries shifted by 30 degrees form a 12-pulse system in which the 5th and 7th harmonics cancel in the primary.

12. Why are harmonic filters often installed on ships with large electric propulsion drives?

A. To charge the emergency batteries
B. To provide earth fault protection
C. To increase propulsion motor speed
D. To reduce voltage distortion caused by the drives, which can overheat equipment and disturb other consumers

Answer: D — To reduce voltage distortion caused by the drives, which can overheat equipment and disturb other consumers

Thyristor and diode rectifiers draw non-sinusoidal current, distorting the busbar voltage. Filters or multi-pulse arrangements keep total harmonic distortion within class limits to protect generators, motors and sensitive electronic equipment.

13. Which type of electric motor is commonly used for very large, low-speed propulsion drives supplied by a synchroconverter (LCI) or cycloconverter?

A. Universal motor
B. Shaded-pole motor
C. Permanent-split capacitor motor
D. Synchronous motor

Answer: D — Synchronous motor

Large propulsion drives of many megawatts often use synchronous motors, which have high efficiency and can operate at controlled power factor. An LCI relies on the synchronous motor's back-EMF for natural commutation of its inverter thyristors.

14. Large HV propulsion motors are commonly cooled by:

A. Natural convection only
B. A closed air circuit with an air-to-water heat exchanger
C. Liquid nitrogen
D. Open sea water flowing through the windings

Answer: B — A closed air circuit with an air-to-water heat exchanger

Propulsion motors normally circulate internal air in a closed loop through an air-water cooler, keeping the windings clean and dry. Cooling water leakage and air temperatures are monitored with alarms.

15. In a podded propulsion remote control system, the operator's combined lever commands:

A. Only the ballast pumps
B. The shaft brake only
C. Only the generator voltage
D. Both propeller rpm and pod steering angle during manoeuvring

Answer: D — Both propeller rpm and pod steering angle during manoeuvring

Pod control stations provide commands for propeller speed and azimuth angle. During manoeuvring these are combined so that thrust magnitude and direction are controlled together, often integrated with joystick and DP systems.

16. In a diesel-electric propulsion system, the propulsion drive's power limitation function from the PMS is intended to:

A. Increase generator frequency above rated value
B. Disconnect the emergency generator
C. Prevent the bow thruster from starting at any time
D. Reduce propulsion power demand when available generator capacity is insufficient, preventing blackout

Answer: D — Reduce propulsion power demand when available generator capacity is insufficient, preventing blackout

Propulsion is often the largest load. If a generator trips or capacity is limited, the PMS signals the propulsion drive to reduce power rapidly so the remaining generators are not overloaded, avoiding a total blackout.

17. Which of the following is an advantage of electric propulsion for vessels such as cruise ships and research vessels?

A. Elimination of all harmonics
B. Generators can be operated without governors
C. No need for any protection relays
D. Lower noise and vibration and flexible location of generators

Answer: D — Lower noise and vibration and flexible location of generators

Because generators are not mechanically coupled to the propeller, they can be resiliently mounted and placed where convenient. This reduces noise and vibration and frees space, important for passenger comfort and acoustic research.

18. Which of the following is a disadvantage of diesel-electric propulsion compared to direct diesel drive?

A. Poor torque at low speed
B. Inability to reverse the propeller
C. Inability to use HV
D. Additional conversion losses in generators, transformers, converters and motors at full power

Answer: D — Additional conversion losses in generators, transformers, converters and motors at full power

Each conversion stage introduces losses, so at full design power a diesel-electric plant is typically somewhat less efficient than a direct-drive slow-speed diesel. Its benefits come from flexibility, redundancy and part-load economy.

19. An HV Permit to Work should be issued only by:

A. An Authorised Person (HV) appointed in writing for that system
B. The Master's representative from the deck department
C. The electrician who will do the work
D. Any engineer holding a CoC

Answer: A — An Authorised Person (HV) appointed in writing for that system

HV safety rules require that permits be issued by an Authorised Person formally appointed by the company or Chief Engineer for that particular HV system. This ensures the issuer is trained and familiar with the installation.

20. Before proving HV equipment dead, the voltage detector must be checked:

A. On a known live source or approved proving unit, both before and after the test
B. Only once a year by the manufacturer
C. By looking for visible sparks
D. Only after the test

Answer: A — On a known live source or approved proving unit, both before and after the test

An HV detector that has failed would wrongly show 'dead'. Checking it on a known live supply or proving unit immediately before and after the test confirms it was working when the equipment was tested.

21. Why must the identity of HV equipment be verified at the point of work before work begins under a permit?

A. Because the permit becomes invalid after one hour
B. To check the paint colour
C. Because many accidents occur when work is done on the wrong, still-live circuit or panel
D. To record the manufacturer's name for the PMS

Answer: C — Because many accidents occur when work is done on the wrong, still-live circuit or panel

Similar-looking cubicles or cables may belong to different circuits. Confirming equipment labels, cable identification and the presence of applied earths at the point of work ensures that the circuit worked on is the one isolated.

22. Additional earths applied at the point of work by the work party under an HV permit are intended to:

A. Protect against induced voltages or inadvertent energisation at the actual work location
B. Provide a supply for portable tools
C. Replace the circuit main earths
D. Allow testing without a Sanction to Test

Answer: A — Protect against induced voltages or inadvertent energisation at the actual work location

Circuit main earths are applied at the point of isolation. Additional earths at the work location give further protection from induced voltages or charge and must be removed and accounted for before the permit is cleared.

23. When an HV permit job is interrupted at the end of the day and is to continue next morning, the correct practice is that:

A. Earths are removed to allow the circuit to be used
B. Isolations and earths remain in place and the permit stays with the recipient or is suspended per the company procedure
C. The circuit is re-energised overnight and re-isolated in the morning without new documents
D. The permit is handed to the night watch for safe keeping and they may restore supply

Answer: B — Isolations and earths remain in place and the permit stays with the recipient or is suspended per the company procedure

Safety precautions must remain intact while a job is incomplete. Company HV rules define how a permit is held or suspended; supply may only be restored after formal clearance and cancellation by the Authorised Person.

24. Who is responsible for removing circuit main earths and restoring an HV circuit to service after a Permit to Work is cancelled?

A. The bridge watch officer
B. The PMS automatically
C. The Authorised Person, following the switching programme
D. Any member of the work party

Answer: C — The Authorised Person, following the switching programme

After the recipient clears and the AP cancels the permit, the Authorised Person removes the circuit main earths, locks and notices in the planned sequence and restores the circuit, recording each step in the HV log.

25. IEC 60092-503 applies to shipboard AC supply systems with voltages:

A. For DC systems only
B. Above 1 kV up to and including 15 kV
C. Up to 1,000 V only
D. Above 33 kV

Answer: B — Above 1 kV up to and including 15 kV

IEC 60092-503 covers the special features of shipboard AC systems above 1 kV up to and including 15 kV, which includes the common 3.3 kV, 6.6 kV and 11 kV systems. It is referenced by classification societies for HV installations.

26. Which part of SOLAS Chapter II-1 contains the main requirements for electrical installations on ships?

A. Part G
B. Part A-1
C. Part C
D. Part D

Answer: D — Part D

SOLAS Chapter II-1 Part D (Regulations 40 to 45) deals with electrical installations, including the main and emergency sources of power and precautions against shock, fire and other hazards of electrical origin.

27. STCW Table A-III/6 for Electro-Technical Officers includes which competence related to HV?

A. Plan and ensure safe loading of cargo
B. Operate and maintain power systems in excess of 1,000 V
C. Operate GMDSS equipment
D. Maintain a safe navigational watch

Answer: B — Operate and maintain power systems in excess of 1,000 V

Table A-III/6 includes the competence 'Operate and maintain power systems in excess of 1,000 volts', covering HV technology, safety precautions and procedures, electrical propulsion and safe operation and maintenance of HV systems.

28. Which international standard governs High Voltage Shore Connection (HVSC) systems used for cold ironing of ships?

A. ISO 9001
B. IEC/IEEE 80005-1
C. IMO Resolution A.741
D. IEC 60092-101 only

Answer: B — IEC/IEEE 80005-1

IEC/IEEE 80005-1 specifies requirements for HV shore connection systems, including safety interlocks, equipotential bonding, and shore and ship-side equipment. It enables ships to shut down generators at berth to reduce emissions.

29. STCW Section B-III/2 gives guidance on the training of engineers with management responsibility for HV plant. Which item is included?

A. Producing a switching strategy for isolating components of an HV system
B. Celestial navigation
C. Lifeboat launching
D. Cargo stowage calculations

Answer: A — Producing a switching strategy for isolating components of an HV system

Section B-III/2 lists the functional, operational and safety requirements of HV systems, assignment of qualified personnel, remedial action during faults, producing a switching strategy, selecting test apparatus, switching and isolation with safety documentation, and IR and PI testing.

30. A cycloconverter used in electric propulsion is best described as:

A. An inverter with a large DC link capacitor
B. A direct AC-to-AC frequency converter without a DC link, using thyristors
C. A rotating motor-generator set
D. A rectifier feeding a DC motor

Answer: B — A direct AC-to-AC frequency converter without a DC link, using thyristors

A cycloconverter builds a low-frequency output waveform directly from segments of the supply voltage using banks of thyristors, with no intermediate DC link. It suits large, low-speed, high-torque drives such as icebreaker and cruise ship propulsion.

Set 13 — 30 HV exit exam questions with answers

1. The maximum output frequency of a cycloconverter is practically limited to approximately:

A. Twice the supply frequency
B. One third to about 40% of the supply frequency
C. Ten times the supply frequency
D. Exactly the supply frequency

Answer: B — One third to about 40% of the supply frequency

Because the output is synthesised from portions of the input waveform, waveform quality degrades as output frequency rises. Cycloconverters are therefore limited to roughly one third to 40% of supply frequency, adequate for low-speed propulsion motors.

2. In a synchroconverter (load commutated inverter, LCI) drive, the inverter thyristors are commutated mainly by:

A. Forced commutation from IGBTs
B. The ship's emergency battery
C. The back-EMF of the synchronous motor
D. A large DC link capacitor

Answer: C — The back-EMF of the synchronous motor

An LCI uses a current-source DC link (inductor) and a thyristor inverter whose commutation relies on the leading-power-factor back-EMF of the over-excited synchronous motor. At very low speed this EMF is insufficient, so a pulsed start mode is used.

3. A pulse-width modulated (PWM) voltage source inverter drive is characterised by:

A. No rectifier stage
B. Direct AC-AC conversion
C. A DC link capacitor and fast switching devices such as IGBTs in the inverter
D. A DC link inductor and naturally commutated thyristors

Answer: C — A DC link capacitor and fast switching devices such as IGBTs in the inverter

A VSI holds the DC link voltage constant with a capacitor bank, and the IGBT inverter switches at high frequency to build a variable voltage and frequency output. It is widely used for induction motor propulsion and thruster drives.

4. A current source inverter (CSI) drive is identified by:

A. A large DC link capacitor
B. A large DC link inductor that keeps the DC link current nearly constant
C. Use of a cycloconverter
D. No DC link at all

Answer: B — A large DC link inductor that keeps the DC link current nearly constant

In a CSI, the DC link reactor smooths the current rather than voltage, and the inverter switches this current between motor phases. The LCI used for synchronous propulsion motors is a type of current source inverter.

5. Why is the voltage-to-frequency (V/f) ratio kept approximately constant when controlling the speed of an induction motor with a frequency converter?

A. To allow the motor to run without a supply
B. To increase slip to 100%
C. To keep the motor current at zero
D. To maintain constant magnetic flux in the motor so it can develop rated torque without saturating

Answer: D — To maintain constant magnetic flux in the motor so it can develop rated torque without saturating

Air-gap flux is proportional to V/f. Reducing frequency without reducing voltage would saturate the core and cause high magnetising current, while too low a voltage would weaken torque, so the drive varies both together.

6. Field-oriented (vector) control of an induction motor drive works by:

A. Switching the motor between star and delta
B. Varying only the voltage while frequency is fixed
C. Separately controlling the flux-producing and torque-producing components of stator current
D. Inserting resistance in the rotor circuit

Answer: C — Separately controlling the flux-producing and torque-producing components of stator current

Vector control resolves the stator current into components aligned with and perpendicular to the rotor flux. Controlling them independently gives dynamic performance similar to a separately excited DC motor.

7. Direct torque control (DTC) in a motor drive differs from conventional PWM vector control because it:

A. Controls motor torque and flux directly via hysteresis comparators and a switching table, without a separate modulator
B. Uses a mechanical gearbox
C. Requires a DC motor
D. Controls only the motor voltage magnitude

Answer: A — Controls motor torque and flux directly via hysteresis comparators and a switching table, without a separate modulator

DTC estimates stator flux and torque from measured voltages and currents and selects inverter switching states directly to keep them within hysteresis bands. This gives very fast torque response without a PWM modulator.

8. The synchronous speed of a 6-pole induction motor supplied at 60 Hz is:

A. 1,800 rpm
B. 900 rpm
C. 1,200 rpm
D. 3,600 rpm

Answer: C — 1,200 rpm

Synchronous speed Ns = 120f/P = 120 x 60 / 6 = 1,200 rpm. The induction motor rotor runs slightly below this speed by the slip needed to induce rotor current.

9. Large HV motors such as bow thrusters are often started by auto-transformer, soft starter or frequency converter rather than direct-on-line mainly to:

A. Reverse the motor direction
B. Increase the starting current
C. Reduce the motor's rated power
D. Limit the large starting current and the resulting voltage dip on the switchboard

Answer: D — Limit the large starting current and the resulting voltage dip on the switchboard

Direct-on-line starting draws several times full-load current, which on a ship's limited generating capacity can cause severe voltage dips and possible blackout. Reduced-voltage or converter starting limits this inrush.

10. Energising a large HV/LV transformer can cause a magnetising inrush current several times rated current. Transformer differential protection avoids tripping on this by:

A. Increasing the system frequency
B. Harmonic (second harmonic) restraint
C. Earthing the transformer neutral solidly
D. Removing the CTs during energisation

Answer: B — Harmonic (second harmonic) restraint

Magnetising inrush is rich in second harmonic current, while internal fault current is not. Differential relays measure this harmonic content and restrain operation during energisation, preventing nuisance tripping.

11. HV/LV distribution transformers on board ships are most commonly of which type?

A. Oil-filled transformers with conservator tanks
B. Dry-type (cast resin or air-insulated) transformers
C. Auto-transformers without isolation
D. Rotating transformers

Answer: B — Dry-type (cast resin or air-insulated) transformers

Dry-type transformers avoid the fire risk and leakage problems of oil in machinery spaces and require less maintenance. They are usually air-cooled, with winding temperature sensors for alarm and trip.

12. Before working inside a variable frequency drive cabinet after isolation, why must the specified waiting time be observed?

A. The DC link capacitors may retain a dangerous charge for several minutes
B. The fans must cool down
C. The PMS needs to reset
D. The cabinet door interlock opens only after an hour

Answer: A — The DC link capacitors may retain a dangerous charge for several minutes

DC link capacitors store considerable energy and discharge through bleed resistors over a period stated by the manufacturer. The DC link voltage must be confirmed dead with a suitable tester before touching any components.

13. Large HV synchronous motors and generators on board are usually excited by a brushless system in which:

A. Carbon brushes feed DC from batteries directly to the rotor
B. Permanent magnets alone provide excitation, which cannot be adjusted
C. The stator is fed with DC
D. An AC exciter and rotating diode rectifier on the shaft supply DC to the main field

Answer: D — An AC exciter and rotating diode rectifier on the shaft supply DC to the main field

In brushless excitation, an AC exciter's armature on the rotor feeds a rotating rectifier which supplies the main field winding. This eliminates brushes and slip rings and their maintenance.

14. Running an HV switchboard with the bus tie breaker open (split bus) during critical operations is done mainly to:

A. Reduce the number of generators needed
B. Prevent a single fault from blacking out the entire power plant
C. Increase fault level on each section
D. Allow closing generators without synchronising

Answer: B — Prevent a single fault from blacking out the entire power plant

Splitting the main switchboard into independent sections limits the effect of a short circuit or generator failure to one section. This redundancy is applied, for example, on DP vessels and during manoeuvring.

15. When a VSI drive decelerates a motor with high inertia, energy flows back into the DC link. If there is no regenerative front end, this energy is commonly dissipated by:

A. The AVR
B. The motor anti-condensation heater
C. A braking chopper and braking resistor
D. Opening the main breaker

Answer: C — A braking chopper and braking resistor

Regenerated energy raises the DC link voltage. A braking chopper switches a resistor across the DC link to dissipate the energy as heat; otherwise the drive trips on DC overvoltage. Active front ends can return the energy to the network.

16. HV motor stator coils are normally insulated using form-wound coils impregnated by:

A. Paraffin wax
B. Simple varnish dipping without vacuum
C. Vacuum pressure impregnation (VPI) with resin
D. Paper wrapped with no impregnation

Answer: C — Vacuum pressure impregnation (VPI) with resin

Form-wound HV coils are taped with mica-based insulation and vacuum pressure impregnated with epoxy or polyester resin. VPI removes voids that would otherwise cause partial discharge and gives good thermal and mechanical strength.

17. A synchronous motor that is over-excited operates at:

A. Zero speed
B. Leading power factor, supplying reactive power to the system
C. Lagging power factor, drawing reactive power
D. Speed above synchronous speed

Answer: B — Leading power factor, supplying reactive power to the system

Increasing a synchronous motor's field current beyond the normal value makes it supply reactive power, so it draws leading current. This property is used to support system voltage and for LCI commutation.

18. How is the vacuum integrity of a vacuum circuit breaker interrupter usually checked during maintenance?

A. By listening for gas leaks
B. By weighing the bottle
C. By applying a specified high test voltage across the open contacts and checking that no breakdown occurs
D. By measuring the contact resistance only

Answer: C — By applying a specified high test voltage across the open contacts and checking that no breakdown occurs

A healthy vacuum withstands a high voltage across the open contact gap. If the vacuum has deteriorated, the gap breaks down at the test voltage. This test is done with a vacuum tester or HV test set per the manufacturer's procedure.

19. Contact erosion in a vacuum circuit breaker cannot be seen directly. It is usually assessed by:

A. Measuring the insulation resistance of the control circuit
B. Opening the vacuum bottle
C. Counting the number of shutters
D. Checking the contact wear indicator or gap measurement on the moving contact mechanism against manufacturer's limits

Answer: D — Checking the contact wear indicator or gap measurement on the moving contact mechanism against manufacturer's limits

As the contacts erode, the moving contact travels further, reducing the gap at a wear mark or spring setting. Gap gauging against the manufacturer's limits shows when the interrupter must be replaced.

20. Main contact resistance of an HV circuit breaker is measured using:

A. A micro-ohmmeter injecting a high DC current through the closed contacts
B. A clamp meter
C. An analogue multimeter on the ohms range
D. An insulation resistance tester at 5 kV

Answer: A — A micro-ohmmeter injecting a high DC current through the closed contacts

Contact resistance is in the micro-ohm range. A micro-ohmmeter (ductor) passes a substantial DC current and measures the voltage drop, giving an accurate value; high readings indicate worn, dirty or poorly aligned contacts.

21. A withdrawable HV circuit breaker is in the 'test' position when:

A. It is fully connected to the busbar and closed
B. Its earthing switch is closed and the breaker is closed on load
C. Its main contacts are disconnected from the busbar and cable, but the auxiliary/control circuits are connected so its operation can be tested
D. It is removed from the switchboard

Answer: C — Its main contacts are disconnected from the busbar and cable, but the auxiliary/control circuits are connected so its operation can be tested

The test position separates the main circuits while keeping secondary plugs connected. The breaker can then be closed, opened and tripped to check mechanism, protection and interlocks without energising the HV circuit.

22. Racking an HV circuit breaker in or out is mechanically prevented unless:

A. The busbar earthing switch is closed
B. The cable is energised
C. The control supply is switched off
D. The breaker is in the open position

Answer: D — The breaker is in the open position

An interlock prevents a closed breaker from being moved between service and test or isolated positions, because separating the plug-in contacts under load would draw a dangerous arc. The breaker must be open first.

23. Thermographic (infrared) surveys of HV switchboards are carried out to detect:

A. Vacuum loss in VCBs
B. Overheating connections and components while the equipment is on load
C. Earth faults on de-energised cables
D. Insulation resistance values

Answer: B — Overheating connections and components while the equipment is on load

Loose or corroded busbar and cable connections produce local heating under load. Infrared surveys through inspection windows identify hot spots without opening live compartments, allowing repair before failure or fire.

24. An earth fault alarm appears on an HV system with high-resistance neutral earthing. The first step in remedial action is to:

A. Ignore the alarm until the next port
B. Remove the neutral earthing resistor
C. Identify the faulty feeder using earth fault relay indications and the switching plan, then isolate it in a controlled manner
D. Immediately open all generator breakers

Answer: C — Identify the faulty feeder using earth fault relay indications and the switching plan, then isolate it in a controlled manner

Earth fault relays on each feeder indicate where the fault is. The Authorised Person then plans the isolation of that circuit, transferring loads if necessary, so the faulty equipment can be safely tested and repaired without blacking out the ship.

25. When isolating an HV feeder using a withdrawable circuit breaker with an integral earthing switch, the correct sequence is:

A. Rack out the closed breaker, then open it
B. Open the breaker, rack it to isolated position and lock the shutters, prove dead, then close the earthing switch
C. Prove dead after closing the earthing switch only
D. Close the earthing switch, then open the breaker

Answer: B — Open the breaker, rack it to isolated position and lock the shutters, prove dead, then close the earthing switch

The breaker is opened, then withdrawn to the isolated position with shutters locked. The cable side is proved dead before the earthing switch is closed and locked, applying the circuit main earth.

26. Before closing an HV earthing switch on a circuit, it is essential to prove it dead because:

A. Closing an earth onto a live circuit causes a severe short circuit and arc flash
B. Earthing switches are only rated for live closing
C. The earthing switch will not close otherwise
D. The PMS requires it

Answer: A — Closing an earth onto a live circuit causes a severe short circuit and arc flash

Earthing a live conductor creates a bolted fault with very high fault current. Although earthing switches have a fault-making rating, proving dead and interlocks protect the operator from this danger.

27. An SF6 circuit breaker gives a 'low gas density' alarm followed by a lockout. The purpose of the lockout is to:

A. Increase the gas pressure automatically
B. Bypass the protection relays
C. Prevent the breaker from operating when it cannot interrupt current safely
D. Allow the breaker to operate faster

Answer: C — Prevent the breaker from operating when it cannot interrupt current safely

The dielectric strength and arc-quenching ability of SF6 depend on gas density. Below the lockout level the breaker may fail to interrupt a fault, so operation is blocked and the gas leak must be repaired and the gas refilled.

28. When opening an SF6 circuit breaker compartment that has interrupted faults, personnel should use PPE and ventilation because:

A. Arced SF6 decomposes into toxic and corrosive by-products and powders
B. Pure SF6 is explosive
C. SF6 is radioactive
D. SF6 is lighter than air and causes frostbite

Answer: A — Arced SF6 decomposes into toxic and corrosive by-products and powders

Pure SF6 is non-toxic, but arcing produces decomposition products such as sulphur fluorides and metal fluoride powders that are toxic and corrosive. Gas should be recovered and protective equipment worn while handling interrupter components.

29. During routine maintenance of HV switchgear, insulating surfaces such as bushings and spouts should be:

A. Greased heavily with ordinary grease
B. Cleaned with lint-free cloth and approved cleaner, and inspected for tracking marks
C. Sprayed with water
D. Painted with conductive paint

Answer: B — Cleaned with lint-free cloth and approved cleaner, and inspected for tracking marks

Dust, salt and moisture on insulation surfaces create leakage paths leading to tracking and flashover. Cleaning with approved materials and checking for carbon tracking are essential parts of HV switchgear maintenance.

30. On an HV circuit breaker with a spring-operated mechanism, the closing spring is normally:

A. Not used; the breaker closes by gravity
B. Charged only by the protection relay
C. Charged by the arc energy
D. Charged by a motor and released to close the breaker, which in turn charges the opening spring

Answer: D — Charged by a motor and released to close the breaker, which in turn charges the opening spring

A motor charges the closing spring, with manual charging available. Releasing it closes the contacts quickly and charges the opening spring, so the breaker is always ready to trip after closing.

Set 14 — 38 HV exit exam questions with answers

1. A protection relay has tripped an HV motor feeder on overcurrent. Before the breaker is reclosed, the engineer should:

A. Reclose immediately several times to see if it holds
B. Increase the relay setting
C. Bypass the relay
D. Investigate the cause of the trip and check the motor and cable, rather than repeatedly reclosing

Answer: D — Investigate the cause of the trip and check the motor and cable, rather than repeatedly reclosing

Reclosing onto a fault in HV equipment can cause severe damage and arc flash. The trip indication, motor, cable and driven load must be checked, and insulation tested if a fault is suspected, before restoring supply.

2. The 'switching strategy' for isolating a component of an HV system requires the engineer to consider:

A. The colour of the cubicle
B. Maintaining supply to essential services, possible back-feeds, generator capacity and sequence of operations
C. The time of the next meal
D. Only the shortest route to the switchboard

Answer: B — Maintaining supply to essential services, possible back-feeds, generator capacity and sequence of operations

A switching strategy plans how to isolate equipment while keeping essential services available, avoiding overloads and back-feeds, and ordering operations safely. STCW B-III/2 lists producing such a strategy as a management-level skill.

3. Which test equipment is suitable for proving an HV circuit dead before applying earths?

A. An insulation resistance tester
B. A neon screwdriver
C. A standard 600 V multimeter
D. An approved HV voltage detector rated for the system voltage

Answer: D — An approved HV voltage detector rated for the system voltage

Only a voltage detector designed and rated for the system voltage, such as a capacitive or contact-type HV indicator, may be used. LV instruments are unsafe and may give false readings or fail catastrophically.

4. An HV vacuum contactor feeding a motor is usually used in combination with HV HRC fuses because:

A. The contactor cannot carry normal load current
B. Fuses eliminate the need for overload protection
C. The fuses provide speed control
D. The fuses interrupt high short-circuit currents beyond the contactor's breaking capacity

Answer: D — The fuses interrupt high short-circuit currents beyond the contactor's breaking capacity

Vacuum contactors are designed for frequent switching of normal and overload currents but have limited breaking capacity. Current-limiting HRC fuses clear severe short circuits, while relays and the contactor handle overloads.

5. Partial discharge monitoring of HV machines and switchgear is valuable because:

A. It replaces the need for a Permit to Work
B. It measures earth resistance of the hull
C. It gives early warning of insulation deterioration while the equipment remains in service
D. It measures the speed of the motor

Answer: C — It gives early warning of insulation deterioration while the equipment remains in service

Partial discharges occur in voids and at surfaces of degrading insulation long before breakdown. Online PD monitoring trends the discharge activity, allowing maintenance to be planned before failure.

6. When removing portable earthing leads after HV work, the correct order is to:

A. Remove all ends simultaneously by hand
B. Disconnect the earth end first, then the conductors
C. Leave the earth end connected and energise
D. Disconnect the leads from the conductors first using an insulated stick, then disconnect the earth end last

Answer: D — Disconnect the leads from the conductors first using an insulated stick, then disconnect the earth end last

Earthing leads are connected earth end first and removed earth end last. This ensures that the lead is never connected to a conductor while its earth end is free, which could make the lead itself live.

7. In the event of an HV circuit breaker failing to trip on a fault, the system remains protected because:

A. The AVR will reduce generator voltage to zero immediately
B. The fault will clear itself
C. Earth fault relays always open all breakers instantly
D. Upstream breakers trip via breaker failure or backup protection after a time delay

Answer: D — Upstream breakers trip via breaker failure or backup protection after a time delay

Protection is coordinated so that if a breaker fails to clear a fault, breaker failure protection or time-graded backup relays trip the next upstream breaker, such as the generator or bus tie breaker, isolating the fault.

8. An HV cable is suspected of a fault after repeated tripping. Which test is most appropriate to confirm the insulation condition after isolation?

A. Measuring cable length with a tape
B. An insulation resistance test at the appropriate DC voltage under a Sanction to Test
C. Running the cable at overload
D. Visual check only

Answer: B — An insulation resistance test at the appropriate DC voltage under a Sanction to Test

After isolation and with the cable disconnected at both ends, an IR test at the correct DC voltage shows whether insulation has broken down. Where required, further tests such as fault location are carried out by specialists.

9. Why should the operation counter of an HV vacuum circuit breaker be recorded during maintenance?

A. To compare the number of operations with the manufacturer's mechanical and electrical endurance limits
B. To determine the generator's age
C. To calculate fuel consumption
D. To set the AVR

Answer: A — To compare the number of operations with the manufacturer's mechanical and electrical endurance limits

Breaker mechanisms and interrupters have rated numbers of operations. Recording the counter at each maintenance supports condition-based maintenance and ensures interrupters or mechanism parts are replaced before these endurance limits are exceeded.

10. For the same gap length in air, which electrode arrangement generally has the lowest breakdown voltage?

A. Point (needle) to plane
B. Uniform-field Rogowski electrodes
C. Parallel plates
D. Sphere to sphere with large spheres

Answer: A — Point (needle) to plane

Sharp electrodes concentrate the electric field at the tip, so ionisation and corona start at a lower voltage and breakdown follows sooner. Sphere and plane electrodes give a more uniform field and higher breakdown voltage for the same gap.

11. In HV insulation design, 'clearance' is defined as:

A. The thickness of cable insulation
B. The shortest distance through air between two conductive parts
C. The distance between the switchboard and the bulkhead
D. The distance along the surface of an insulator

Answer: B — The shortest distance through air between two conductive parts

Clearance is the shortest air path between conductors at different potentials and protects against air breakdown. Creepage is the path along the insulating surface, which is affected by pollution and moisture.

12. Why do HV insulators and bushings often have sheds or ribs on their surface?

A. To improve cooling of the conductor
B. To make them lighter
C. To increase the creepage distance and reduce surface leakage and flashover
D. To reduce clearance distance

Answer: C — To increase the creepage distance and reduce surface leakage and flashover

Sheds lengthen the surface path between conductor and earth without increasing the overall length. This increases creepage distance, so contamination and moisture are less likely to cause surface tracking and flashover.

13. Surface tracking on HV insulation is the formation of:

A. Rust on the switchboard frame
B. Carbonised conductive paths caused by leakage currents across a contaminated surface
C. Cracks due to mechanical shock only
D. Bubbles inside transformer oil

Answer: B — Carbonised conductive paths caused by leakage currents across a contaminated surface

Leakage current across a dirty or damp insulating surface causes small discharges that carbonise the material. These conductive tracks grow until they bridge the insulation and cause flashover.

14. A protection CT has a ratio of 1000/1 A. If the secondary current measured is 0.6 A, the primary current is:

A. 1,666 A
B. 600 A
C. 6,000 A
D. 60 A

Answer: B — 600 A

The primary current equals secondary current multiplied by the CT ratio: 0.6 A x 1000 = 600 A. Standard CT secondary ratings are 1 A or 5 A so meters and relays can be standardised.

15. A voltage transformer on a 6.6 kV switchboard with a secondary of 110 V has a ratio of:

A. 60:1
B. 110:1
C. 600:1
D. 6:1

Answer: A — 60:1

6,600 V divided by 110 V gives a ratio of 60:1. The VT steps down HV to a safe level for metering, synchronising and protection relays.

16. Unlike a current transformer, the secondary of an energised voltage transformer must never be:

A. Connected to a voltmeter
B. Open-circuited
C. Earthed at one point
D. Short-circuited

Answer: D — Short-circuited

A VT behaves like a voltage source; a short circuit on its secondary draws excessive current and can destroy it, so VT secondaries are fused. A CT secondary, by contrast, must never be opened while primary current flows.

17. A Circuit Main Earth (CME) is applied:

A. Only during insulation testing
B. At the point of isolation after proving dead, before a Permit to Work is issued
C. While the circuit is still live, to discharge it
D. At the point of work by the work party after the permit is issued

Answer: B — At the point of isolation after proving dead, before a Permit to Work is issued

Circuit main earths are applied by the Authorised Person at the point of isolation, using an earthing switch or approved earthing device, after proving dead. They are listed on the PTW, and any additional earths are applied later at the point of work.

18. Portable earthing equipment used for HV circuit main earths must be:

A. Made of the thinnest flexible wire available
B. Insulated only, with no conductor
C. Rated to carry the prospective fault current of the system for the specified time
D. Connected to the conductors before the earth point

Answer: C — Rated to carry the prospective fault current of the system for the specified time

If the circuit is accidentally energised, earthing leads must carry the full fault current until protection operates. Undersized leads could melt or explode, so equipment rated for the system's fault level is required.

19. During an HV laboratory demonstration, corona is observed at a sharp conductor edge before complete breakdown. This happens because:

A. The voltage is DC only
B. The local electric field at the edge exceeds the ionisation strength of air while the average gap field is lower
C. The conductor is cold
D. The air pressure is very high

Answer: B — The local electric field at the edge exceeds the ionisation strength of air while the average gap field is lower

At sharp points the field is strongly enhanced. Air ionises locally, producing corona glow and noise, while the rest of the gap still withstands the voltage. Increasing voltage further causes full breakdown.

20. In a ship's power management system, 'heavy consumer start request' logic is used to:

A. Prevent any motor from starting at sea
B. Trip the bow thruster when the main engine is running
C. Check that sufficient spare generator capacity is available, starting another generator if necessary, before a large motor is allowed to start
D. Start all generators whenever a fan starts

Answer: C — Check that sufficient spare generator capacity is available, starting another generator if necessary, before a large motor is allowed to start

Before starting a large load such as a bow thruster or cargo pump, the PMS verifies available power. If capacity is insufficient, it starts and connects another generator first, preventing overload and blackout.

21. Preferential tripping in a ship's electrical system is designed to:

A. Disconnect non-essential loads in stages when generators are overloaded
B. Increase generator speed
C. Trip essential services such as steering gear first
D. Synchronise incoming generators

Answer: A — Disconnect non-essential loads in stages when generators are overloaded

When a generator approaches overload, preferential trips shed non-essential consumers such as galley and air conditioning loads. This keeps essential services and generators online, avoiding blackout.

22. Following a blackout on a ship with PMS, the automatic sequence typically includes:

A. Automatic start of standby generator(s), closing on to the dead bus, and sequential restart of essential consumers
B. Opening of all generator breakers permanently
C. Starting the emergency generator only, with no further action
D. Manual start of all motors at once

Answer: A — Automatic start of standby generator(s), closing on to the dead bus, and sequential restart of essential consumers

The PMS starts standby generators and closes the first available one on to the dead busbar. Essential motors are then restarted in sequence to avoid excessive simultaneous starting current.

23. Which instrument on an HV switchboard continuously monitors insulation level on an insulated system and alarms on low insulation?

A. A kilowatt-hour meter
B. A synchroscope
C. A tachometer
D. An insulation monitoring device (earth fault monitor)

Answer: D — An insulation monitoring device (earth fault monitor)

On insulated or high-resistance earthed systems, an insulation monitoring or earth fault indication device continuously checks insulation to earth. It alarms when insulation falls, so faults can be located before a second fault occurs.

24. Capacitive voltage detecting systems (VDS) fitted on HV switchgear panels are used to:

A. Measure motor speed
B. Trip the breaker on overcurrent
C. Measure insulation resistance
D. Indicate the presence or absence of voltage on each phase without opening the compartment

Answer: D — Indicate the presence or absence of voltage on each phase without opening the compartment

Capacitive VDS indicators coupled to the busbar or cable bushings show voltage presence per phase. They allow a check before earthing, though the indicator itself must be proven with a suitable test device.

25. The load-dependent start/stop function of a PMS works by:

A. Running all generators at all times
B. Starting a further generator when load exceeds a set percentage and stopping one when load falls below a set level for a time
C. Stopping all generators at night
D. Varying the frequency to 40 Hz at low load

Answer: B — Starting a further generator when load exceeds a set percentage and stopping one when load falls below a set level for a time

Load-dependent start and stop optimises the number of generators on line. Time delays and hysteresis prevent hunting, keeping generators reasonably loaded and with spare capacity for sudden load increases.

26. In a metal-clad HV switchboard, each panel is divided into separate earthed metal compartments for:

A. Busbars, circuit breaker, cable termination and LV control equipment
B. Ventilation fans only
C. Only the control equipment
D. Spare parts storage

Answer: A — Busbars, circuit breaker, cable termination and LV control equipment

Metal partitions between compartments confine faults and arcs and allow safe access to one compartment while others remain live. The LV compartment is separated so control work can be done without exposure to HV parts.

27. HV switchgear is often tested for internal arc classification (IAC) according to IEC 62271-200 to demonstrate that:

A. The breaker can interrupt 100 kA
B. It is watertight
C. It can be operated without interlocks
D. Persons in front of or around the switchgear are protected from the effects of an internal arc

Answer: D — Persons in front of or around the switchgear are protected from the effects of an internal arc

IAC testing confirms that hot gases and debris from an internal arc are vented safely, for example through pressure relief flaps, and that covers and doors remain closed. It defines accessibility types for operator protection.

28. Access to HV switchboard rooms and HV equipment spaces on board should be:

A. Restricted only during port stay
B. Open to all crew for ventilation
C. Controlled by the galley staff
D. Restricted to authorised persons, with doors kept locked and warning signs posted

Answer: D — Restricted to authorised persons, with doors kept locked and warning signs posted

HV spaces are kept locked with keys under the control of the Chief Engineer or Authorised Person. Clear warning signs and access control prevent untrained persons coming into contact with HV hazards.

29. Why are HV switchboard operations increasingly performed remotely from a control room or with a remote racking device?

A. Because local controls are not permitted by SOLAS
B. To save electrical power
C. To keep the operator at a distance from the switchgear in case of an arc flash
D. To speed up synchronising

Answer: C — To keep the operator at a distance from the switchgear in case of an arc flash

Most arc flash injuries occur during switching or racking close to equipment. Remote operation keeps the operator outside the arc flash boundary, which is why the course stresses carrying out HV operations from a safe distance.

30. Pressure relief flaps or ducts on top of HV switchgear panels are provided to:

A. Drain condensation
B. Admit cooling air
C. Release the high-pressure hot gases of an internal arc in a controlled direction away from personnel
D. Allow inspection of busbars

Answer: C — Release the high-pressure hot gases of an internal arc in a controlled direction away from personnel

An internal arc causes a rapid pressure rise. Relief flaps open to direct the gases upward or into ducts, preventing the doors bursting open towards personnel standing in front of the panel.

31. A crew member has received an HV electric shock and is unconscious and not breathing normally after being freed from the supply. The immediate action is to:

A. Apply ice to the chest
B. Call for help, start CPR (30 compressions to 2 breaths) and use an AED as soon as available
C. Give water to drink
D. Wait for him to recover on his own

Answer: B — Call for help, start CPR (30 compressions to 2 breaths) and use an AED as soon as available

Electric shock commonly causes cardiac arrest from ventricular fibrillation. Current guidance is immediate CPR at 30:2 with compressions of 5-6 cm at 100-120 per minute and early defibrillation with an AED.

32. Electrical burns from an arc flash should be first treated by:

A. Cooling with cool running water for about 20 minutes and covering loosely with cling film or a sterile dressing
B. Applying butter or oil
C. Bursting any blisters
D. Rubbing with ice

Answer: A — Cooling with cool running water for about 20 minutes and covering loosely with cling film or a sterile dressing

Current first-aid guidance is to cool burns with cool running water for 20 minutes, remove constricting items, and cover loosely. Ice, creams or bursting blisters worsen injury. HV burns may hide deep tissue damage, so medical advice is essential.

33. In a case study, an HV breaker was closed while portable earths were still connected at the work site, causing a violent arc flash. The key lesson is that:

A. Breakers should be closed slowly
B. Only the Master may close breakers
C. Earths should never be used
D. All earths must be removed and accounted for under formal clearance before re-energisation

Answer: D — All earths must be removed and accounted for under formal clearance before re-energisation

Closing onto applied earths creates a bolted three-phase fault. Recording every earth on the permit, removing all additional earths at clearance and following the restoration programme prevents this type of accident.

34. Why must even a person who appears to have recovered from an HV electric shock be referred for medical assessment?

A. It is required to obtain a new CoC
B. Internal burns and cardiac rhythm disturbances may develop or remain hidden
C. The shock may have changed their blood group
D. They will need new PPE

Answer: B — Internal burns and cardiac rhythm disturbances may develop or remain hidden

HV current passing through the body can cause deep tissue damage along its path and delayed heart arrhythmias. Visible injuries may be small, so medical evaluation and observation are needed.

35. An arc flash incident occurs in the HV switchroom. Which action should be taken first by the responding engineer?

A. Rush in immediately without PPE
B. Use water to cool the switchboard
C. Isolate the supply from a safe location and raise the alarm before entering
D. Open the faulty panel door to inspect

Answer: C — Isolate the supply from a safe location and raise the alarm before entering

Rescuers must not become victims. The supply is isolated remotely or from upstream breakers, the alarm is raised and appropriate PPE and fire-fighting means are prepared before entering the space.

36. Under HV safety rules, 'live working' on HV conductors on board ships is:

A. Permitted with rubber gloves only
B. Permitted for voltages up to 11 kV
C. Not permitted; HV equipment must be made dead, isolated and earthed before work
D. Permitted if the Master agrees

Answer: C — Not permitted; HV equipment must be made dead, isolated and earthed before work

Shipboard HV safety codes require that no work be done on or near exposed HV conductors unless they are dead, isolated, proved dead and earthed under a Permit to Work. Live testing is carried out only under strict procedures.

37. When proving an HV circuit dead, the test should be carried out:

A. Only at the switchboard, not at the point of work
B. Only on the neutral
C. On one phase only
D. On all phases at the point of work, after checking the detector on a known live source

Answer: D — On all phases at the point of work, after checking the detector on a known live source

Each phase may be fed differently or have induced voltage, so all phases must be tested. Testing at the point of work confirms the specific equipment is dead, with the detector proved before and after.

38. A Competent Person (HV) in shipboard HV safety rules is a person who:

A. Is any crew member on board
B. Is authorised to issue all safety documents
C. Has sufficient technical knowledge and experience to avoid danger and may receive safety documents and carry out work under them
D. Is responsible only for fire-fighting

Answer: C — Has sufficient technical knowledge and experience to avoid danger and may receive safety documents and carry out work under them

The Competent Person is trained and experienced enough to work safely on the isolated equipment and to lead a work party. The Authorised Person, formally appointed, issues and cancels safety documents.

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HV exit exam questions and answers — High Voltage Safety Officer & Switchgear Operations

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