Quantcast

How many times can a circuit breaker be re-closed after a fault?

FA272237

01 March 2021

Following is some information relating to types of faults and the number of times a breaker can clear these faults.


Icu - Rated Ultimate Short Circuit Breaking Capacity

The test for Icu is described in clause 8.3.5 of AS3947.2-1997

Breaker must open on this fault, then after a time delay be reclosed onto the fault, which it again

must open and clear. After the second opening, the breaker will :

- be able to isolate

- the poles must be able to close (no open circuit)

- the trip unit must still work properly

But there is a risk that the circuit breaker will not be able self protect, there is a risk of thermal

overheating which could lead to the destruction of the breaker. Therefore the breaker should be

replaced before being reused in normal conditions.

Ics - Rated Service Short Circuit Breaking Capacity

The test for Ics is described in clause 8.3.4 of AS3947.2-1997

Similar to Icu except that after clearing the fault the breaker will be reclosed onto the fault two more

times. As well as this the breaker must be able to pass a temperature rise test.

After the third opening, the breaker will :

- be able to isolate

- no open circuit on the poles

- trip unit works properly

- breaker can be put back into service : it can assume its normal protective function.

Although the breaker may be put back into service, its replacement must be organised as soon as

possible, as it might not be able to clear a 4th short circuit.

Overloads

The breaker are designed to be able to withstand 12 overloads at 6xIn (those that are UL certified can

do 50 overloads at 6 times).

When evaluating the life span take the worse case :

- if the overload is under 6 times assume it is 6 times

- if the overload is over 6 times but less than Ics, take Ics

- etc..

Any breaker that clears a fault at Icu or Ics should be replaced as per the duty cycle 

Note: You should never leave a breaker that has cleared a fault at those levels in service because you don’t know how may times it has happened in the past. Clearing at Icu or Ics damages the breaker.