Example 5 A 20 MVA transformer which may be called upon to operate at 25 overload feeds 11kV busbars through a circuit breaker other circuit breakers supply outgoing feeders The transformer circuit breaker is equipped with 10005 A CTs and the feeder circuit breakers with 5005 A CTs and all sets of CTs feed inductiontype overcurrent relays The relays on the feeder circuit breakers have a 125 plug setting and a 04 time setting If a three phase fault current of 7500 A flows from the transformer to one of the feeders find the operating time of the feeder relay the minimum plug setting of the transformer relay and its time setting assuming a discriminative time margin of 05 second The timecurrent characteristic of the relays is same as shown in Fig 53