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HV exit exam questions and answers — Set 6 — High Voltage Safety Officer & Switchgear Operations

Set 6 of the High Voltage Safety Officer & Switchgear Operations (HV) exit exam bank — 30 of 428 questions, 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 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.

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