Why Making Current of Circuit Breaker is More than Breaking Current?

What is Making and Breaking Current of Circuit breaker?

The circuit breaker is used for switching on and off the circuit and also for the isolation of the faulty section in case of a fault. The Making and Breaking Current of circuit breaker both these terms are very important for selection of circuit breaker for any electrical installation. The making and breaking current capacity parameter of circuit breaker has prime importance when the circuit breaker opens at the time of fault and also when the circuit breaker closes at fault.

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The braking current capacity of the circuit breaker is the maximum fault current that can flow through the circuit breaker for a short period of time from the time occurring short circuit till the clearing of the fault without any permanent damage. The breaking current capacity or short circuit breaking current of the breaker is expressed in KA(RMS). The circuit breaker of 40 KA short circuit interrupting capacity can break the 40 KA current without getting damaged. 

The making current of the circuit breaker is the maximum peak value of the current that the breaker can interrupt without any damage if the breaker is closed at fault.

The making current of the circuit breaker is always more than the breaking current. To understand why making current capacity of the breaker should be more than its breaking current capacity, let’s discuss how fault current behaves, and what is the characteristics of the fault current.

When a fault occurs in the electrical network, the AC and DC current is present. The DC current magnitude is about 60 to 80 % of the AC current. The symmetrical AC components of the fault current are as given below.

Why Making Current of Circuit Breaker is More than Breaking Current?

There are three periods of time after the occurrence of the fault.

Sub-transient period:

The AC current is very large but decays very fast. The DC current is about 60 to 80 % and gets decayed within 1 to 2 cycles.

Transient period:

The current falls at a slower rate.

Steady-state period:

In a steady-state period, the current reaches a steady value.

The magnitude of the current during the sub-transient period:

The magnitude of the current during the sub-transient period is maximum and if the circuit breaker is made when the fault persists must be capable to interrupt the fault current without getting damaged.

Relationship between Making and Breaking current: 

Let the symmetrical breaking current be I Amperes( RMS)

The peak value of the symmetrical breaking current is      

= √2 x I
=1.414 x I

During the sub-transient period, the DC current is also present for one to two cycles. The magnitude of the DC current is approximately equal to the peak value of the symmetrical breaking current.

The peak value of the current during the sub-transient period is 

Making current = 1.414 x I x 1.8
= 2.55 x I
Making current = 2.55 x Symmetrical breaking current

If the symmetrical braking current capacity of the breaker is 40 KA(RMS), the making current of the breaker must be 2.55*40=102 KA(peak). 

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