HV - High Voltage Safety Officer & Switchgear Operations Exit Exam Questions - Set 7

Question 1

High Rupturing Capacity (HRC) fuses provide the most effective and rapid protection under conditions of:

Accepted Answer

B: High-magnitude short-circuit faults

Answer

A: Low-level sustained overloads

Answer

C: Slight voltage fluctuations

Answer

D: Phase imbalance only

Question 2

For high-speed motor circuit breakers, the total closing time from command signal to contact touch is typically in the order of:

Accepted Answer

B: 50 to 100 milliseconds

Answer

A: 3 to 10 milliseconds

Answer

C: 1 to 2 seconds

Answer

D: 5 seconds

Question 3

Which type of protective relay is commonly used to protect electric motors against continuous moderate overload?

Accepted Answer

C: Bimetallic thermal overload relay

Answer

A: Impedance / Distance relay

Answer

B: Instantaneous electromagnetic relay

Answer

D: Buchholz gas relay

Question 4

According to Preece's Law, the fusing current (I) of a fuse wire is related to its wire diameter (D) by the relationship:

Accepted Answer

C: I ∝ D^(3/2) (or D^1.5)

Answer

A: I ∝ 1 / D

Answer

B: I ∝ D

Answer

D: I ∝ D²

Question 5

In a circuit breaker contact gap, ionization of the medium is NOT facilitated by:

Accepted Answer

B: Chemical composition of the solid metallic contacts

Answer

A: High temperature of the surrounding dielectric medium

Answer

C: Increase in electric field gradient across the gap

Answer

D: Increase in electron mean free path

Question 6

An ideal fuse wire material should possess which combination of physical properties?

Accepted Answer

D: Low specific resistance and low melting point

Answer

A: High specific resistance and high melting point

Answer

B: High specific resistance and low melting point

Answer

C: Low specific resistance and high melting point

Question 7

For extra-high-voltage (EHV) transmission networks above 220 kV, which circuit breaker type is predominantly selected?

Accepted Answer

C: SF6 gas circuit breaker

Answer

A: Bulk oil circuit breaker

Answer

B: Air-break circuit breaker

Answer

D: Minimum oil circuit breaker

Question 8

The number of power-frequency AC cycles in which a modern high-speed circuit breaker completes its tripping and arc clearing operation is:

Accepted Answer

A: 2 to 5 cycles

Answer

B: 10 to 18 cycles

Answer

C: 20 to 30 cycles

Answer

D: 40 to 50 cycles

Question 9

Which of the following metals is best suited for manufacturing precision HRC fuse elements due to its resistance to oxidation and low specific resistance?

Accepted Answer

B: Silver

Answer

A: Aluminium

Answer

C: Lead

Answer

D: Iron

Question 10

In a circuit breaker, the current that is flowing through the contacts at the precise instant of contact separation is known as the:

Accepted Answer

C: Breaking current

Answer

A: Restriking current

Answer

B: Surge current

Answer

D: Making current

Question 11

Merz-Price circulating current differential protection is primarily used for the protection of:

Accepted Answer

B: Alternators and Power Transformers

Answer

A: Overhead distribution feeders

Answer

C: Low-voltage motors

Answer

D: Direct current batteries

Question 12

A 'Kick fuse' (time-lag or slow-blow fuse) is characterized by having:

Accepted Answer

C: Inverse time-current characteristics

Answer

A: Square law instantaneous response

Answer

B: Linear fast response

Answer

D: Constant delay regardless of current

Question 13

Air blast circuit breakers designed for 400 kV extra-high-voltage systems are engineered to extinguish the arc within:

Accepted Answer

B: 40 to 60 milli-seconds (approx. 2 to 3 cycles)

Answer

A: 50 micro-seconds

Answer

C: 500 milli-seconds

Answer

D: 5 seconds

Question 14

The symmetrical short-circuit breaking capacity of a high-voltage power circuit breaker is standardly expressed in units of:

Accepted Answer

A: Amperes / kA or MVA (√3·V·I)

Answer

B: Volts / kV

Answer

C: Megawatts (MW)

Answer

D: Horsepower (HP)

Question 15

Sulphur Hexafluoride (SF6) gas in its un-ionized state is fundamentally an exceptional:

Accepted Answer

D: Dielectric electrical insulator

Answer

A: Conductor of electricity

Answer

B: Semiconductor

Answer

C: Chemically corrosive reagent

Question 16

The electrical contact resistance between closed circuit breaker contacts is least affected by which of the following parameters?

Accepted Answer

D: Surrounding ambient air temperature

Answer

A: Mechanical contact pressure applied by springs

Answer

B: Surface profile and contour of the contact faces

Answer

C: Surface oxidation and contamination films

Question 17

The arc voltage developed across the separating contacts of an AC circuit breaker during arc burning is:

Accepted Answer

C: Substantially in phase with the arc current

Answer

A: Leading the arc current by 90°

Answer

B: Lagging the arc current by 90°

Answer

D: In phase opposition (180°) to the arc current

Question 18

As the mechanical contact clamping force between two closed electrical contacts is increased, the contact resistance will:

Accepted Answer

D: Decrease due to increased microscopic contact area

Answer

A: Increase linearly

Answer

B: Increase exponentially

Answer

C: Remain completely unaltered

Question 19

For electrical circuit breaker and switch contacts, which of the following parameters should ideally have the lowest possible value?

Accepted Answer

A: Electrical contact resistance

Answer

B: Thermal heat capacity

Answer

C: Thermal conductivity

Answer

D: Arc erosion resistance

Question 20

Which of the following contact metals exhibits the lowest minimum arcing voltage threshold?

Accepted Answer

D: Silver (~12 V)

Answer

A: Carbon

Answer

B: Graphite

Answer

C: Tungsten

Question 21

If the minimum arcing voltage for platinum is 16 V, it can be concluded that at system voltages below 16 V:

Accepted Answer

C: Circuit current can be interrupted without sustaining an electric arc

Answer

A: It will be impossible to interrupt the circuit

Answer

B: It will be impossible to conduct current

Answer

D: Contacts will weld together upon opening

Question 22

Submerging circuit breaker contacts in dielectric transformer oil is an effective method of:

Accepted Answer

A: Arc cooling, gas generation (hydrogen blast), and rapid deionization

Answer

B: Complete arc prevention

Answer

C: Electromagnetic induction shielding only

Answer

D: Radio frequency amplification

Question 23

Which of the following is NOT an established method of arc extinction in electrical circuit breakers?

Accepted Answer

C: Insertion of silicon diode rectifiers into the arc column

Answer

A: Oil immersion and gas blasting of contacts

Answer

B: Magnetic blowout coils and arc splitters

Answer

D: De-ionization and lengthening of the arc path

High Voltage Safety Officer & Switchgear Operations (HV) (Set 7)

High Rupturing Capacity (HRC) fuses provide the most effective and rapid protection under conditions of:

Questions in HV Set 7

Question Discussion & Notes

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