In general α is selected as 2.5 for ordinary circuit breakers . But for points very close to generator and as GCB it will be 3.0 . It is probable this C.B. has had malfunction due to high peak of subtransient short circuit current .
On the other hand these types of CBs has the advantage of current limiting ,namely , in case of short circuit the down stream CB will be fronted with less short circuit current . Probably , the designer has sized the outgoing feeders according to this phenomenon . But , now the function and duty of CB has changed and it is not outgoing feeder any longer . It is incoming breaker .