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BASIC NEC 2023

circuit breaker theory quiz question and answers

Free online circuit breaker theory practice test. 25 basic electrical questions on arc quenching, breaking and making capacity, SF6 and vacuum breakers, restriking voltage, and switchgear basics. Good for electrician exam prep.

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A circuit breaker opens a circuit automatically when a fault (like a short circuit) occurs, and can also be closed and opened manually for normal switching. The hardest part of its job is stopping the ARC that forms between the contacts as they separate — different breaker types quench this arc using oil, compressed air, SF6 gas, or a vacuum. Two ratings matter most: breaking capacity (the maximum current it can safely interrupt) and making capacity (the maximum current it can safely close onto, which is higher because of the current peak right after a fault starts).

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38 min

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circuit breaker theory quiz question and answers — Question List

Question 1: The circuit breaker is placed in a power system …………………………..

To interrupt the current flow system
To interrupt the voltage flow in system
To reduce the reactive power in system
To interrupt the active power flow in system

Question 2: If the arc in the circuit breaker is absent during the opening of circuit contacts ……………………….

A high switch over voltage will flow in the circuit
A high current will flow in the circuit
High temperature in the circuit breaker cabinet damages the breaker enclosure
High reactive power flows in the circuit

Question 3: Which of the SF6 property specifies that it has high arc quenching medium?

Electronegativity
Electropositivity
High thermal conductivity
High density

Question 4: Which of the following circuit breaker should not be used in high voltage power systems?

Minimum oil circuit breaker
air blast circuit breaker
SF6 circuit breaker
Oil circuit breaker

Question 5: The voltage that presents at breaking contact at the instant of arc extinction is called as ……………….

Restriking voltage
Recovery voltage
Arc voltage
Active recovery voltage

Question 6: The major function of the relay in circuit breaker is…………………………

To interrupt the current flow
To activate the trip signal to open the contacts
To protect the circuit break from high short circuit currents
To help in faster switching

Question 7: The air circuit breaker can be used up to the voltage level of ………………. KV

15
21
6.6
11

Question 8: Low resistance method of arc interruption is applicable for only …………………

AC circuits
DC circuits
AC circuits with low voltage
DC circuits with low voltage

Question 9: Which of the following circuit breakers are used for extra high voltage power systems?

Oil circuit breakers
Air circuit breakers
SF6 circuit breakers
Bulk oil circuit breakers

Question 10: What does GIS stands for?

Gas insulated switchgear
Gas insulated system
Gas interrupted switch gear
Gas interrupted system

Question 11: The "rated breaking capacity" of a circuit breaker is defined as:

The maximum RMS current it can safely interrupt at rated voltage
The maximum voltage it can be closed onto
The normal continuous current rating only
The weight of the breaker in kilograms

Question 12: Why is the rated "making capacity" of a circuit breaker higher than its rated breaking capacity?

It must withstand the high asymmetrical current peak when closing onto a fault
Making capacity is actually always lower, not higher
It only matters for DC breakers
It depends on the color of the breaker enclosure

Question 13: In a vacuum circuit breaker, the arc is quenched (extinguished) using:

A high vacuum inside a sealed interrupter chamber
Mineral oil surrounding the contacts
A blast of compressed air
Sulphur hexafluoride (SF6) gas

Question 14: An air blast circuit breaker extinguishes the arc by:

Blowing high-pressure compressed air across the contacts
Submerging the contacts in transformer oil
Using a magnetic field only, with no gas or air
Cooling the contacts with liquid nitrogen

Question 15: How does a MINIMUM oil circuit breaker differ from a BULK oil circuit breaker?

Minimum oil breakers use oil only for arc quenching; bulk oil breakers also use oil for insulation
Minimum oil breakers use no oil at all
Bulk oil breakers use less oil than minimum oil breakers
There is no real difference between the two types

Question 16: Interrupting a DC circuit is harder for a circuit breaker than interrupting an AC circuit mainly because:

DC current has no natural zero crossing to help extinguish the arc
DC circuits never carry any fault current
DC voltage is always lower than AC voltage
DC breakers are not allowed by any electrical code

Question 17: An "auto-reclosing" circuit breaker on an overhead distribution line is used to:

Automatically close again after a brief trip to restore supply after a temporary fault
Permanently lock out the line after any trip
Increase the system voltage after a fault
Only work on underground cable systems

Question 18: The arc chute (arc splitter) fitted in many low and medium voltage circuit breakers works by:

Splitting the arc into smaller arcs and cooling it so it extinguishes faster
Increasing the size of the arc to burn out the fault current faster
Storing electrical energy for later reuse
Preventing the breaker from ever opening

Question 19: The "rated short-time withstand current" of a circuit breaker describes its ability to:

Carry a stated fault current for a short time without damage, without opening
Interrupt current forever without ever needing maintenance
Operate only in a cold climate
Store spare parts inside the breaker enclosure

Question 20: Which of the following are common operating mechanisms used to open and close circuit breaker contacts?

Spring-operated, pneumatic, and hydraulic mechanisms
Only hand-crank mechanisms are used today
Only chemical reaction mechanisms
Breakers have no operating mechanism at all

Question 21: What is the key difference between an isolator (disconnector) and a circuit breaker?

An isolator cannot interrupt load current; a circuit breaker can safely interrupt fault or load current
They are exactly the same device with different names
An isolator is always rated for a higher fault current than a breaker
A circuit breaker cannot be opened manually

Question 22: One major advantage of an SF6 circuit breaker over an oil circuit breaker is:

Much longer maintenance intervals and no oil fire risk
It is always cheaper to buy than an oil breaker
It cannot be used above 1000 volts
It requires topping up with water every month

Question 23: Modern power systems are steadily replacing oil circuit breakers with SF6 and vacuum circuit breakers.

True
False

Question 24: A high "rate of rise of restriking voltage" (RRRV) across a circuit breaker's contacts after arc extinction makes the breaker's job:

Harder, since the arc can re-strike before the gap regains dielectric strength
Easier, since high RRRV always helps quench the arc faster
It has no effect on breaker performance
It only matters for isolators, not breakers

Question 25: The rated voltage of a circuit breaker selected for a system should be:

Equal to or greater than the system nominal voltage
Always exactly half the system voltage
Lower than the system voltage to save cost
Voltage rating does not matter for breaker selection

EXAM CARD BASIC

Questions25

Time limit38 min

Pass score70%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

Timer starts immediately. Progress is auto-saved.

Source: NEC 2026 (NFPA 70)