# Circuit Breaker Interview Questions And Answers Part-1

Circuit breaker interview questions and answers are very important in view of the job interviews. We shall discuss in detail the questions in this post.

What is a Circuit Breaker?

The circuit breaker is a device that opens and closes the circuit under normal and fault conditions.

What is the making current of the circuit breaker?

Making current is the magnitude of peak current flows through the circuit breaker when we close the breaker under fault conditions.

Name the material used for the contact circuit breaker?

Copper-Bismuth, Copper-lead, Copper-tellurium, Silver-bismuth, Silver-lead, and Silver-tellurium are some of the alloys suitable as contact materials in the vacuum circuit breakers.

Which type of circuit breaker do we use in the HV application?

SF6 breaker is suitable for HV application.

Name the duties of the circuit breaker.

• Interruption of small inductive currents
• Switching of capacitor banks and reactors
• Interruption of terminal faults
• Interruption of short line faults
• Asynchronous switching

What is rupturing capacity of the circuit breaker?

The circuit breaker handles the maximum current at fault. The breaker interrupts the fault current to isolate the faulty circuit.

The maximum power interrupting capacity of the circuit breaker at fault is rupturing capacity. We express the rupturing capacity is MVA(Mega volt-ampere).

Rupturing capacity = Short circuit current of CB X Rated voltage of CB

What is the short circuit MVA of the circuit breaker?

Short circuit MVA of the circuit breaker is the maximum rupturing capacity under fault. We express the short circuit capacity in kA in rms.

Short circuit MVA =Symmetrical breaking current(kA) x Rated Voltage(kV)

What is breaking current?

The breaking current of the circuit breaker is the maximum current that the breaker can interrupt without being damaged.

What is the relationship between making current and breaking current of the circuit breaker?

Making current(peak) = 2.55 X symmetrical breaking capacity of the circuit breaker.

If the symmetrical breaking capacity of the circuit is 40 kA(RMS), then what is the making current capacity?

Making current capacity = 2.55 X 40 = 102 kA (peak)

Why is symmetrical fault current is more than the asymmetrical fault current?

In symmetrical fault, all three-phase get short circuits and the fault current is the same in each phase. In asymmetrical fault, the healthy phase keeps feeding the fault current for some time. As a result, the asymmetrical fault current is more than the symmetrical fault current.

What are the duties of Circuit Breakers?

• Interruption of small inductive currents
• Switching of capacitor banks and reactors
• Interruption of terminal faults
• Interruption of short line faults
• Asynchronous switching

What are the different types of circuit breakers?

1. Air Break Circuit Breaker
2. Oil Circuit Breaker
3. Minimum Oil Circuit Breaker
4. Air Blast Circuit Breaker
5. Vacuum Circuit Breaker
6. SF6 Circuit Breaker

What are the periods of the fault?

There are three periods occurs after fault.

1. Sub transient period
2. Transient period

We shall discuss more Circuit Breaker Interview Questions And Answers in the next parts.