sir
we have in our Motor control centre motors of various capacities running.The starters are of DOL type and type 2 coordinated.we dont have ground fault protection for each motors.
in case of a insulation failure in any of the motors the incoming feeder to the mcc trips and the whole section gets affected.The fuses in the motor feeder also fails but the incomer breaker motor control centre mcc trips very fast. how to isolate the faulty motor alone
ram

You need to perform a system coordination study, a process by which an electrical engineer determines the proper fusing and breaker trip settings to ensure that a fault is interrupted at the lowest possible level.

thanks sir
since we have different ratings of motor is it possible to corelate all the fuse ratings with the breaker.Also i understand that whenever there is problem in the motor(ground fault) the overload relay should trip the contactor . on the otherhand if there is problem in the cable leading to motor the fuse should blow and the breaker should trip and the contactor is not supposed to open. your comments
ram

An overload relay is not designed to trip on a ground fault specifically. It might trip if the ground current exceeds the setting, but often times if the ground fault has high resistance or acring, the ground current will be too low to be picked up by the OL relay. The best method is to use a ground fault relay of some sort, or possibly use a solid state OL relay that includes residual ground fault protection. Some do, some do not. You need to check for sure.