Deck 7: Circuit Breakers and Fuses

Question 1

Why do the SF6 gases have an excellent heat transfer property?

A)Low gaseous viscosity.
B)High dielectric strength.
C)Higher molecular weight.
D)Both (a) and (c)

Answer 1

Both (a) and (c)

Question 2

What is the breaking capacity of the air blast circuit breaker?

A)5000 MVA
B)6000 MVA
C)7000 MVA
D)10000 MVA

Answer 2

7000 MVA

Question 3

In axial blast type of CB, expansion of air takes place from ______________.

A)High pressure to low pressure.
B)Low pressure to high pressure.
C)Always in high pressure.
D)Always in low pressure.

Answer 3

High pressure to low pressure.

Question 4

The air blast circuit breakers for 400 kV systems are designed to operate in how much time?

A)0.1 s
B)0.5 s
C)50 ms
D)100µs

Question 5

What is the type of air blast in cross blast type CB?

A)The blast of air is along the ar(C)
B)The blast of air cuts across the ar(C)
C)Both (a) and (b)
D)None of the above.

Question 6

Why is the resistance switching used in a air blast circuit breaker?

A)Reduce the magnitude of fault current.
B)Control the CB operating time.
C)Damp out the fast transient.
D)Change the fault current power factor.

Question 7

When using air blast circuit breaker, current chopping is a phenomenon observed when

A)A long overhead line is switched off.
B)A bank of capacitors is switched off.
C)A transformer on no load is switched off.
D)A heavy load is switched off.

Question 8

Which type of air is used in air blast circuit breaker?

A)Ionised air
B)Air free from moisture
C)Air should have least CO₂
D)Air must have oil mist.

Question 9

What is the maximum current upto which fuses can be used?

A)25 A
B)50 A
C)75 A
D)100 A

Question 10

What is fusing factor?

A)The ratio of current rating of the fuse to the minimum fusing current
B)The ratio of minimum fusing current to the current rating of the fuse
C)The ratio of maximum fusing current to the current rating of the fuse
D)The ratio of minimum fusing current to the voltage rating of the fuse

Question 11

For a current upto 10A which material is used as the fusing element?

A)Copper
B)Silver
C)Alloy of lead and tin
D)Zinc

Question 12

Which among these is the least expensive protection for over current in low voltage system?

A)Rewirable fuse.
B)Isolator.
C)Circuit breaker.
D)Air breaker switch.

Question 13

Which among the following statement is true related to fuse?

A)Greater the current smaller is the time taken by the fuse to blow out.
B)Greater the current greater is the time taken by the fuse to blow out.
C)The current is directly proportion to the blow out time of fuse.
D)Is dependent on the temperature and atmospheric conditions.

Question 14

How is the restriking voltage measured?

A)RMS value
B)Peak value
C)Instantaneous value
D)Average value

Question 15

What is the interrupting medium in the contactor?

A)Air at atmospheric pressure.
B)SF6 gas
C)Oil
D)All of the above.

Question 16

Recovery voltage is the value of rms voltage that reappears across the poles of a circuit breaker before_______________

A)Restriking voltage
B)Final arc extinction
C)Rise of voltage
D)All of the above

Question 17

Why is it difficult to interrupt a capacitive circuit?

A)The current has a leading power factor.
B)The restriking voltage can be high.
C)Current magnitude is very small.
D)Stored energy in the capacitor is very high.

Question 18

Rate of rise of restriking voltage depend on_____________?

A)Type of circuit breaker.
B)Capacitance of the system.
C)Inductance of the system.
D)Both (b) and (c)

Question 19

On what factor does the rate of rise of restriking voltage (RRRV) depend?

A)System voltage.
B)Circuit power factor only
C)Switching conditions
D)Both (b) and (c).

Question 20

In a circuit breaker, the active recovery voltage depends upon which among these?

A)Circuit conditions.
B)Power factor.
C)Both (a) and (b)
D)None of the above

Question 21

What is restriking voltage?

A)The transient voltage that appears across the circuit breaker contacts at the instant of arc extinction.
B)The transient voltage that appears across the circuit breaker contacts at the end of arc extinction.
C)Both (a) and (b)
D)None of the above

Question 22

For a high speed circuit breaker what will the total clearing time?

A)Few minutes.
B)Few seconds.
C)1 to 2 cycles.
D)5 to 20 cycles.

Question 23

The interrupting time of a circuit breaker is the period between the instant of _________.

A)Initiation of short circuit and the arc extinction on an opening operation.
B)Energizing the trip circuit and the arc extension on an opening operation.
C)Initiation of short circuit and the parting of primary arc contacts.
D)Energizing of the trip circuit and the parting of primary arc contacts.

Question 24

In a circuit breaker, ionisation is facilitated by_________________.

A)Increase in field strength.
B)Increase in mean free length.
C)High temperature of the surrounding medium.
D)All of the above.

Question 25

Which of the following should have low value for the contacts and their material?

A)Thermal capacity.
B)Contact resistance.
C)Thermal conductivity.
D)None of above.