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.
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
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.