Circuit Breakers and Breaking Currents

To answer this question, let's first describe couple of concepts:

Breaking Current – The r.m.s value of the current at the instant of the contact separation is called the breaking current of the circuit breaker and it is expressed in kA.

Making Current: The peak value of the current at the time of making the contact of circuit breaker is called making current and it is expressed in kA.

Nature of Short Circuit Current: Over the time, the short circuit current goes with different state – (a) first 2 or 3 cycles is called sub-transient state, (b) next few cycles is transient state, (b) finally the steady state.

Generally, the short circuit current in sub-transient state is 1.8 to 2 times more than that of the steady state.

Circuit Breaker Fault Clearing Time: The circuit breaker has its fault clearing time which is the time elapsed between the instant of instance of fault occurrence and the instant of contact separation with final current zero.

Because of this time, actual contact separation of the circuit breaker occurs at time when the fault current reaches at its steady state. So, the breaking current of a circuit breaker considers the r.m.s value of the steady state of fault current.

Breaking current = r.m.s value of the steady state fault current

But, if a fault pre-exists and the circuit breaker is closed at this condition, the circuit breaker makes the contact with the fault current at the sub-transient state (or at best transient state).

Making current = The peak value of the sub-transient state fault current

= √2 x r.m.s value of the sub-transient state fault current

= √2 x 1.8 x r.m.s value of the steady state fault current

= 2.5 x r.m.s value of the steady state fault current

= 2.5 x Breaking current

So, the making current is almost 2.5 time of the breaking current.

Hope it helps.

-MS