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BASIC NEC 2023

Electrical fuse Practice test

Free online electrical fuse practice test. 25 basic electrical questions on fuse operation, fusing current, HRC and rewireable fuses, time-delay fuses, and fuse selection. Good for electrician exam prep.

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Electrical Practice Questions

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A fuse is the simplest overcurrent protective device: a metal element with a low melting point that heats up and melts (blows) when current stays too high for too long, opening the circuit. Key ideas: the fusing current is always somewhat higher than the fuse's rated current, and a fuse should always be rated LOWER than the current-carrying capacity of the cable it protects. Fuses come in two broad families: rewireable (kit-kat) fuses, which are cheap but their fuse wire ages and weakens over time, and HRC (High Rupturing Capacity) cartridge fuses, which are sealed, consistent, and can safely break much larger fault currents.

25

Questions

38 min

Time

70%

Pass

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Electrical fuse Practice test — Question List

Question 1: An electrical fuse is a

Protective device
voltage limiting device
current limiting device
current controlling device

Question 2: In an electrical circuit the fuse is used to ...................

detect the fault only
protect the circuit from fault
minimize the fault only
detect and protect the circuit

Question 3: The breaking capacity of an electrical fuse compared to circuit breaker is ...............

less
more
same

Question 4: The operating time of an electrical fuse is about ...................

0.002 sec
1 sec
0.8 sec
1.5 sec

Question 5: Which of the following is the time current graph of an electrical fuse?

linear characteristic
inverse characteristic
straight line
parabola

Question 6: An electrical fuse should have high melting point.

True
False

Question 7: Which of the following fuse diameter will be high , 10 A fuse or 5 A fuse?

10 A
5 A
both are same

Question 8: Which following device only detects the fault in an electrical circuit?

fuse
circuit breaker
isolator
relay

Question 9: If the current rating of a fuse is 10 A the fusing current will be ..............

10 A
more than 10 A
less than 10 A
5 A

Question 10: If the maximum over current in a circuit should be 5 A. what should be the fuse rating?

5A
less than 5 A
more than 5A
2.5 A

Question 11: A fuse element (fuse wire) is normally made from a metal or alloy with:

A low melting point
The highest possible melting point
Zero electrical resistance
Magnetic properties like iron

Question 12: According to Preece's formula, the fusing current of a round fuse wire is roughly proportional to which power of its diameter?

1 (directly proportional to diameter)
1.5 (d to the power of 3/2)
2 (diameter squared)
3 (diameter cubed)

Question 13: What is the main advantage of an HRC (High Rupturing Capacity) cartridge fuse over a rewireable (kit-kat) fuse?

A consistent characteristic over time and a much higher breaking capacity
It never needs to be replaced after operating
It is always cheaper than a rewireable fuse
It has a visible fuse wire that can be reused

Question 14: The "ageing effect" sometimes seen in a rewireable fuse element is caused by:

Oxidation of the fuse wire, reducing its effective cross-sectional area over time
The fuse wire becoming thicker with age
The fuse base changing color
The supply frequency slowly changing

Question 15: A cartridge (HRC) fuse element is usually enclosed inside a sealed tube packed with:

Fine quartz sand (silica)
Plain water
Compressed air only
Loose cotton wool

Question 16: A "current-limiting" HRC fuse protects downstream equipment mainly by:

Clearing the fault before current reaches its full prospective peak
Allowing the full fault current to flow for several seconds
Increasing the fault current to trip other devices faster
Reducing the supply voltage automatically

Question 17: The total operating time of a fuse is made up of the pre-arcing time plus the:

Arcing time
Recovery time
Charging time
Rated voltage time

Question 18: The I²t rating of a fuse describes:

The total let-through energy the fuse allows to pass while clearing a fault
The insulation resistance of the fuse holder
The physical length of the fuse element
The color code printed on the fuse body

Question 19: When selecting a fuse to protect a cable, the fuse rated current should normally be:

At or above the load current, but not greater than the cable ampacity
Always exactly double the cable ampacity
Always less than the load current
Fuse rating has no relation to the cable size

Question 20: Why do power electronic devices such as thyristors and diodes often need special ultra-fast "semiconductor protection" fuses?

These devices can be destroyed by fault energy faster than a standard fuse can clear
Semiconductor devices never carry any current
Standard fuses are too fast for semiconductor devices
Semiconductor fuses are required only for lighting circuits

Question 21: "Discrimination" (selectivity) between two fuses in series means that, for a fault downstream, only the smaller (downstream) fuse should blow while the upstream fuse stays intact. This is normally achieved by:

Giving the fuses a sufficient rating ratio, commonly at least 2:1
Making both fuses exactly the same rating
Removing the downstream fuse completely
Using a much larger cable on the upstream side only

Question 22: A rewireable (kit-kat) fuse is called "rewireable" because:

A new fuse wire can be fitted in the same carrier after it blows
It automatically repairs itself after blowing
It never needs to be replaced
It changes its own current rating automatically

Question 23: Interrupting a DC circuit is harder for a fuse than interrupting an AC circuit mainly because:

DC current has no natural current zero to help the arc extinguish
DC fuses are never used in any application
DC voltage is always lower than AC voltage
DC circuits never carry fault current

Question 24: In many motor control circuits, a standard fuse is chosen mainly to protect against short circuits, while a separate device protects against motor overload. What is that separate device?

A thermal overload relay
A second identical fuse in parallel
A capacitor bank
A voltmeter

Question 25: A time-delay (dual-element) fuse is designed to:

Allow brief harmless overload current to pass, but open quickly on a true short circuit
Blow instantly at the slightest current increase, with no delay at all
Never blow under any fault condition
Only work correctly on DC circuits

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)