HV exit exam questions and answers — Set 10 — High Voltage Safety Officer & Switchgear Operations

Set 10 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.

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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.

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HV exit exam questions and answers — Set 10 — High Voltage Safety Officer & Switchgear Operations

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