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HV exit exam questions and answers — Set 7 — High Voltage Safety Officer & Switchgear Operations

Set 7 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 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.

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