MCCB and its Ics and Icu

Is this another exam question?

no this is not.

It depends on what you want.

Icu is the "ultimate" breaking capacity of the breaker, but the breaker does not need to survive the attempt, meaning it can be sacrificed in the task, so long as the fault is interrupted.

Ics is the "service" interrupting capacity, meaning it will interrupt the fault, but will be able to be re-used (after inspection).

Here in North America, the concept of Icu is irrelevant (actually non-existent), our rules require that the breaker be able to re-close and trip again, at least once. So take that for what it's worth.

so many thanks for your complete answer.

then for your aria Ics>=Ik is ok. am i right?

Yes.

The definition I have (UK)for Icu is O -t- CO, that is open on short, wait 3 minutes and close onto the fault and open successfully. I think that is much what JRaef wrote. One should always look at the particular standard or ask the manufacturer.

Ics requires that the breaker be suitable for further service after the fault. Ics is basically "refer to manufacturer" but for domestic service type MCB the rated value must be Ics, since a domestic user will reclose several times before resorting to an electrician.

BS EN60947-2, which is commercial & industrial european standard, defines both Icu & Ics. Ics may be 25/50/75/100% of Icu according to that standard.

In domestic & similar service here, there is always an HBC, current limiting, fuse upstream of MCBs which chops off current before it actually reaches the peak value of Ik, that is break before first half-cycle peak. The distribution board & MCBs are actually designed around a stated maximum ""fuse size & type standard in the supply company's incoming cable head.