CHAPTER 5: FUSES QUESTIONS
Identification
1. Fuses are widely used electrical protection.
2. Rewireable type are the fuses that can be rewired or replaced once it fails.
3. Fusible wire used to be contained in an asbestos tube to prevent splashing of volatile metal.
4. Cartridge type. A silver and quartz combine to give a very good insulator and prevent arc from
re-striking.
5. Arc and fault energy contained within insulating tube-prevents damage.
6. Cartridge type can handle 100 000 A
7. Semi-open type can handle 4000 A.
8. Fuse capacity is measured in ‘𝐼^2t,’ where ‘I’ is current and ‘t’ is time.
9. Prospective current is the RMS current when a fuse is replaced with a solid link.
10. British Standard. A standard lays down definite limits of temperature rise, fusing factor =
minimum fusing current/current rating, breaking capacity.
11. The energy released at the fault is therefore colossal compared with that let through by a fuse.
Damage is therefore extensive.
12. For circuits with steady loads like lamps, you can use fuses for overload protection since the load
doesn’t change much during switching on or operation.
13. Extra capacity or fusing factor.
14. Select fuse rating of 25% or greater than the full-load rating of the circuit to allow for the extra
heating by capacitance effect.
15. Starting current surge normally lasts for 20 seconds.
16. 75% tap auto-transformer takes about 4 times full-load current.
17. Striker pin. This special type is most commonly used on medium- and low-voltage circuits.
18. A coil (instead of a fuse) is connected into the primary circuit and magnetism is used to lift a
plunger to trip a circuit breaker.
19. Dashpot. The one that gives a time/current characteristic like a fuse.
20. Amps-turns are a measurement of magnetism.
Multiple choice
1. The curves in operation characteristic of fuses help to choose the right fuse ratings for
.
A. Proper Grading
B. Proper Function
C. Proper Location
2. Fusing factor is equal to?
A. 1.4
B. 1.3
C. 1.2
3. British Standard .
A. 88: 1954
B. 88: 1952
C. 88: 1957
4. FUSE SELECTION DEPENDS ON A NUMBER OF FACTORS:
A. Maximum fault kVA of circuit to be protected.
B. Voltage of circuit.
C. Both A and B
5. Extra capacity is known as the fusing factor which can range from times the normal
rating.
A. 1.5 to 1.6
B. 1.25 to 1.6
C. 1.4 to 2.6
6. Direct-on-line takes about how many times to full-load current?
a. 7 times
b. 8 times
c. 9 times
7. Star/delta starting takes about how many times to full-load current?
A. 2.5 times
B. 5 times
C. 7.5 times
8. In overload protection, the recommend ratio to give satisfactory discrimination is?
A. 2:1
B. 3:2
C. 4:2
9. This is used to hit a tripping mechanism on a three-phase switch-fuse unit.
A. Striker pin
B. Striker needle
C. Striker thread
10. Used mainly on rural distribution systems. when fuse blows, isolating the circuit and
giving line patrolman easy indication of fault location.
A. Drops out
B. Drops in
C. Drop twice
11. The fuse acts as and .
A. Both intruder and fault starter
B. Both fault detector and interrupter
C. Both fault starter and detector
12. This coil must carry the full fault current and if this is high then the heating effect (𝐼^2 ) may be
so great as to burn out the insulation.
A. Current Rating
B. Thermal Rating
C. Fuse Rating
13. High fault currents induce tremendous magnetic forces inside the trip coil tending to force the
windings apart.
A. Magnetic Stresses
B. Dynamic Stresses
C. Static Stresses
14. The device consists of two main current conducting parts, namely the main conductor and the
fuse.
A. Is Limiter
B. Vs Limiter
C. Cs Limiter
15. Detects the short-circuit current.
A. Voltage Transformer
B. Resistance Transformer
C. Current Transformer
16. Measures the current and provides the triggering energy.
A. Measuring and locating device
B. Application device
C. Measuring and tripping device
17. Converts the tripping pulse to bus bar potential.
A. Pulse Transformer
B. Voltage Transformer
C. Step Transformer
18. Conducts the operating current and limits the short-circuit current.
A. Insert Holder
B. Exert Holder
C. ID Holder
19. Currents of values up to can be interrupted in 1 ms.
A. 210 kA (11 kV)
B. 214 kA (11 kV)
C. 201 kA (11 kV)
20. Edison’s “lead safety wire” in was finally recognized as the first fuse.
A. 1881
B. 1882
C. 1883
True. 1. Fuses operate very quickly and can cut-off fault current long before it reaches its first peak.
True. 2. Fuses are like motors.
True. 3. Using a higher-rated fuse might take longer to blow during short circuits, which can affect the
system based on the value and duration of the short circuit.
True. 4. When choosing fuses, it’s important to pick ones with a slightly higher rating than the maximum
normal current in the system.
True. 5. Fuses need to handle short bursts of high current, like when motor start without blowing or
getting damaged which is crucial for motor circuits.
True. 6. It’s important to determine if fuses need to coordinate with other protective devices to isolate
only faulty circuits.
True. 7. There is no general rule laid down for the application of fuses.
True. 8. Transient switching surges – take next highest rating above full-load current.
True. 9. Protective device have limitation, it can only detect faults that are associated with excess
current.
True. 10. Operating characteristic (i.e. current/time relationship) cannot be adjusted or set in protective
device.
True. 11. Because of limitations, fuses are normally used only on relatively unimportant, small power,
low- and/or medium-voltage circuits.
True. 12. A very ‘special’ type of fuse is the IS-limiter.
True. 13. IS-limiter is originally developed by the company ABB.
True. 14. The 𝐼s-limiter is intended to interrupt very high short-circuit currents very quickly, in order to
protect the system against these high currents.
True. 15. The overvoltage occurring due to the interruption of current is relatively low due to the fact
that the magnitude of current on the instant of interruption is still relatively low.
True. 16. As electrical systems grew, it was discovered that conductor faults were related to inconsistent
diameters.
True. 17. Today, fuses remain crucial, especially in medium and high-voltage distribution systems.
True. 18. Deterioration of element as it is open to the atmosphere is a disadvantage of rewireable type of
fuses.
True. 19. One of the advantage of cartridge type of fuse is it has correct rating and characteristic fuse
always fitted to a circuit-not open to abuse as rewireable type.
True. 20. Cartridge are normally sealed therefore not affected by atmosphere hence gives more stable
Characteristic-reliable grading.
