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HV exit exam questions and answers — Set 7

Set 7 of the High Voltage Safety Officer & Switchgear Operations (HV) exit exam bank — 13 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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13 of 13 shown · Set 7

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Set 7 — 13 HV exit exam questions with answers

1. 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.

2. 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.

3. 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).

4. 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).

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

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