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BASIC NEC 2023

Circuit Breaker Practice Test 2

Free online circuit breaker practice test, part 2. 25 basic electrical questions on MCB, MCCB, ACB, trip mechanisms, protection relays, and switchgear practice. Good for electrician exam prep.

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This test looks at circuit breakers from a practical, day-to-day view: the different breaker families you will meet in the field, and how they detect and clear a fault. A circuit breaker does not usually sense a fault by itself — a protective relay (fed by current transformers) senses the fault and sends a trip signal to the breaker's trip coil. Common low-voltage families include the MCB (miniature circuit breaker, for final circuits), the MCCB (molded case circuit breaker, for larger feeders with adjustable settings), and the ACB (air circuit breaker, for main switchboard incomers at high current).

25

Questions

38 min

Time

70%

Pass

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Circuit Breaker Practice Test 2 — Question List

Question 1: When a fault occurs in a power system, the circuit breaker ................

opens automatically
opened manually
opened by remote control

Question 2: On its own, a circuit breaker usually cannot detect a fault. What normally senses the fault and tells the breaker to open?

A protective relay fed by current transformers
The breaker enclosure color
The supply company by phone call
Nothing — large breakers never need any sensing device

Question 3: The "rated normal current" of a circuit breaker is best described as:

The maximum current it can carry continuously without overheating
The maximum fault current it can interrupt once
The voltage rating of the breaker
The number of poles it has

Question 4: The "trip coil" (shunt trip) of a circuit breaker is:

An electrically operated coil that releases the tripping mechanism when energized
A mechanical spring that only closes the breaker
A cooling fan mounted on the breaker
A device used only to measure voltage

Question 5: A modern power circuit breaker typically clears a fault (from trip signal to full arc extinction) in about:

3 to 5 cycles
3 to 5 minutes
3 to 5 hours
It never fully clears within any set time

Question 6: A "TPN" (triple pole and neutral) circuit breaker is normally used to switch:

A three-phase circuit along with its neutral conductor
A single-phase circuit only
Only the ground (earth) conductor
A DC circuit only

Question 7: An MCB (Miniature Circuit Breaker) is mainly used to:

Protect final low-voltage circuits like lighting and socket outlets
Switch extra-high-voltage transmission lines
Replace transformers in a substation
Generate electrical power

Question 8: Compared to an MCB, an MCCB (Molded Case Circuit Breaker) is typically used for:

Larger feeders and industrial loads, with adjustable trip settings
Only very small lighting circuits under 5 A
Only DC battery circuits
Household smoke detectors

Question 9: A modern low-voltage Air Circuit Breaker (ACB) is typically installed as the:

Main incomer breaker on a low-voltage switchboard at high current
A tiny 6 A breaker for a single light fixture
A hand-held test instrument
A device that only measures air pressure

Question 10: A typical MCB or MCCB uses a "thermal-magnetic" trip mechanism. What does each part do?

Thermal part gives delayed overload trip; magnetic part gives instant short-circuit trip
Thermal part cools the breaker; magnetic part heats it
Both parts only work on DC circuits
Thermal part trips instantly; magnetic part never trips

Question 11: MCB trip curve "Type C" is designed to instantly (magnetically) trip at about 5 to 10 times rated current, higher than Type B (3 to 5 times). Type C is usually chosen for circuits with:

Motors and other inductive loads with higher inrush current
Only simple resistive heaters with no inrush current
Only DC circuits
Circuits that never carry any current

Question 12: On an MCCB nameplate, what is the difference between Icu (rated ultimate breaking capacity) and Ics (rated service breaking capacity)?

Icu is the maximum interrupt rating (may need service after); Ics is a lower value the breaker survives fully usable
They are always exactly equal in every breaker
Icu is the normal continuous current rating
Ics only applies to fuses, never to breakers

Question 13: Unlike a fuse, a circuit breaker can normally also be used for:

Routine, repeated manual switching of the circuit on and off
Generating electrical power
Storing electrical energy long term
Converting AC to DC

Question 14: A "trip-free" circuit breaker mechanism means:

The breaker will trip open on a genuine fault even if the handle is held ON
The breaker can never be closed once it has tripped
The breaker will never trip no matter how large the fault
The handle detaches completely when the breaker trips

Question 15: How does an RCD (Residual Current Device, also called a GFCI) differ in purpose from a standard overcurrent circuit breaker?

An RCD detects earth leakage current for shock protection; a breaker protects against overload/short circuit
They do exactly the same job with no difference
An RCD only works on DC circuits
A standard breaker also detects earth leakage current in milliamps

Question 16: An AFCI (Arc-Fault Circuit Interrupter) is designed to protect against:

Dangerous arcing faults that could start an electrical fire
Only lightning strikes on overhead lines
Only over-voltage from the utility supply
Only mechanical damage to the breaker enclosure

Question 17: A withdrawable (draw-out) type circuit breaker in switchgear allows the breaker to be:

Racked out for maintenance without shutting down the whole busbar
Physically destroyed after every single operation
Only used for residential lighting circuits
Permanently bolted in place with no movement possible

Question 18: Vacuum circuit breakers are mainly used up to around medium voltage (about 36 kV), while SF6 breakers are preferred for:

Higher voltage levels, including high and extra-high voltage systems
Only household 120 V circuits
Only battery-powered DC circuits
SF6 is never used for any voltage level

Question 19: In a substation with two busbar sections, the "bus coupler" circuit breaker is used to:

Connect or isolate the two bus sections for flexible operation
Permanently weld the two busbars together
Generate reactive power only
Measure the busbar temperature

Question 20: A circuit breaker's "mechanical endurance" rating (number of no-load open/close operations) compared to its "electrical endurance" rating (operations under rated current) is normally:

Much higher than the electrical endurance rating
Always exactly equal to the electrical endurance rating
Much lower than the electrical endurance rating
Not something manufacturers ever specify

Question 21: The "anti-pumping" feature in a circuit breaker control scheme prevents:

Repeated close-trip-close cycling while a fault condition is still present
The breaker from ever closing at all
Water from entering the breaker enclosure
The breaker from being opened manually

Question 22: An under-voltage release (UVR) fitted to a circuit breaker automatically trips the breaker when:

The supply voltage drops below a set threshold
The supply voltage rises above rated value
The ambient temperature rises above 40 °C
The breaker has been closed for more than 24 hours

Question 23: The "rated frequency" marked on a circuit breaker (such as 50 Hz or 60 Hz) matters because:

Arc interruption design depends on current zero crossings set by system frequency
Frequency has no effect on breaker operation at all
It only affects the color of the breaker label
It only matters for breakers used on DC systems

Question 24: "Time grading" between circuit breakers in series, using different relay time-delay settings, is done so that:

The breaker closest to the fault trips first, keeping the outage as small as possible
All breakers in the system trip at exactly the same instant
The breaker farthest from the fault always trips first
Time grading has no effect on which breaker trips

Question 25: Unlike a fuse, a circuit breaker can normally be reset and reused after it clears a fault (as long as it was not damaged by the fault).

True
False

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)