Short Circuit Withstand Current and Duration

Do you do any of your own homework?

i thought you have studied ECT167 as i mentioned...? There is a mention of time-based-discrimination there, where you can understand the reason for SC withstand time of CBs. Here is a nice paper on the same subject.... isolating_the_fault_with_time_based_discrimination

Do your short circuit calculations and find out the magnitude of the "wosrt-scenario" fault currents at each of the Switchgear/Busbar/Equipment location in the Substation.

Do your Protective Relay Co-ordination Studies and arrive at the co-ordinated trip times of various switchgear at various locations in the sub-station.

Each switchgear/Busbar/equipment shall be rated for the worst anticipated magnitude of short circuit current at its location and for the worst anticipated trip time of the uppermost stream breaker for that particular fault current.