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)

Accepted Answer

SF6 gas has excellent heat transfer properties due to its low gaseous viscosity and higher molecular weight. These characteristics facilitate efficient heat dissipation and make it a preferred choice in applications requiring high thermal stability.

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

Accepted Answer

The breaking capacity of an air blast circuit breaker can reach up to 7000 MVA, making it suitable for high-capacity applications.

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.

Accepted Answer

In an axial blast type of circuit breaker (CB), the expansion of air occurs from high pressure to low pressure. This is because the arc generated during the interruption process heats the air, causing it to expand rapidly from a region of high pressure near the arc to the surrounding lower pressure areas, aiding in extinguishing the arc.

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

Accepted Answer

Air blast circuit breakers for 400 kV systems are designed to operate very quickly to interrupt fault currents, and a typical operation time is around 50 milliseconds (ms). This rapid operation is crucial for protecting electrical equipment and maintaining system stability.

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.

Accepted Answer

The blast of air in a cross blast type circuit breaker (CB) cuts across the arc, which helps in extinguishing it more effectively by cooling and elongating the arc path.

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.

Accepted Answer

Resistance switching in an air blast circuit breaker is used to damp out the fast transients that occur during the interruption process. This helps in reducing the stress on the circuit breaker and the power system.

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.

Accepted Answer

Current chopping occurs when an air blast circuit breaker interrupts a small current, such as when a transformer on no load is switched off. This can lead to overvoltages due to the sudden interruption of current flow.

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.

Accepted Answer

Air free from moisture is used in air blast circuit breakers to ensure efficient operation and to prevent the formation of conductive ion paths that could lead to flashovers or faults.

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

Accepted Answer

Fuses can be used for currents up to 100 A, beyond which circuit breakers are typically used for higher current ratings.

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

Accepted Answer

The fusing factor is the ratio of the current rating of the fuse to the minimum fusing current.

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

Accepted Answer

An alloy of lead and tin is commonly used as a fusing element for currents up to 10A due to its low melting point, which allows it to melt quickly and interrupt the circuit when excessive current flows.

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.

Accepted Answer

Rewirable fuses are considered the least expensive option for overcurrent protection in low voltage systems due to their simplicity and low manufacturing costs. They consist of a replaceable wire or strip that melts when the current exceeds a certain level, breaking the circuit.

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.

Accepted Answer

A) Greater the current, smaller is the time taken by the fuse to blow out. This is because a fuse is designed to melt and break the circuit when the current exceeds a certain threshold, protecting the circuit from damage. Higher currents will heat the fuse wire more quickly, causing it to melt faster.

Question 14

How is the restriking voltage measured?
A)RMS value
B)Peak value
C)Instantaneous value
D)Average value

Accepted Answer

The restriking voltage is measured in terms of its peak value because it is important to understand the maximum voltage stress that the circuit breaker will experience during interruption.

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.

Accepted Answer

The interrupting medium in the contactor can be air at atmospheric pressure, SF6 gas, or oil.

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

Accepted Answer

Recovery voltage is the value of rms voltage that reappears across the poles of a circuit breaker before final arc extinction, indicating the voltage the circuit breaker needs to withstand after interrupting the current.

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.

Accepted Answer

The restriking voltage can be high in capacitive circuits, making it difficult to interrupt the circuit as it can cause arcing and damage to the circuit components.

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)

Accepted Answer

The rate of rise of restriking voltage depends on both the capacitance and inductance of the system. Capacitance affects the rate at which the voltage can change, while inductance affects the rate at which current changes, influencing the restriking voltage's behavior.

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).

Accepted Answer

The rate of rise of restriking voltage (RRRV) depends on both the circuit power factor and the switching conditions. The power factor influences the rate at which the current changes, affecting the RRRV. Switching conditions, including the type of switchgear and the manner in which the circuit is interrupted, also significantly impact the RRRV.

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

Accepted Answer

The active recovery voltage in a circuit breaker is influenced by both the circuit conditions and the power factor. Circuit conditions can affect the rate at which the voltage recovers after interruption, and the power factor influences the phase angle between the current and voltage, affecting the recovery voltage characteristics.

Quiz 7: Circuit Breakers and Fuses

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

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

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

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

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

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

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

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

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

What is fusing factor?

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

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

Which among the following statement is true related to fuse?

How is the restriking voltage measured?

What is the interrupting medium in the contactor?

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

Why is it difficult to interrupt a capacitive circuit?

Rate of rise of restriking voltage depend on_____________?

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

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