Suggest me some theory about elcircut breakers

Your question is a bit unclear. Can you rephrase it? What do you want to know about a circuit breaker?

How to determine the interrupt rating of a breaker that is protecting a 50 kva transformer 3 phase

The interrupt rating for a circuit breaker protecting a 50 kVA transformer will depend on the impedance of the transformer. That is marked on the enclosure. Tell me that and the voltage and I can tell you exactly what the short circuit current at the secondary will be...

Also consider the SC level at the point of connection of transformer