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HV exit exam questions and answers — Set 6

Set 6 of the High Voltage Safety Officer & Switchgear Operations (HV) exit exam bank — 30 of 193 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. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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).

9. In a modern Vacuum Circuit Breaker (VCB), the vacuum pressure maintained inside the sealed interrupter bottle is in the order of:

  • A. 10 mm Hg
  • B. 10⁻² mm Hg
  • C. 10⁻⁶ mm Hg
  • D. 10⁻⁶ to 10⁻⁹ Torr (approx. 10⁻⁴ to 10⁻⁷ Pa)
Answer: D — 10⁻⁶ to 10⁻⁹ Torr (approx. 10⁻⁴ to 10⁻⁷ Pa)
Vacuum interrupters are sealed under ultra-high vacuum of 10⁻⁴ to 10⁻⁷ Pa (10⁻⁶ to 10⁻⁹ mmHg/Torr) where the mean free path of electrons exceeds the contact gap distance.

10. 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.

11. 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).

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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).

18. 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.

19. 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.

20. 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.

21. 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.

22. 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.

23. 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.

24. 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.

25. 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.

26. 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).

27. 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.

28. 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).

29. 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.

30. 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.

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