Motor Protection

With all due respect Fredski, he does have the fuses ahead of the contactor, he is just asking if he should set the instantaneous trip of the very high end motor protection relay to a value that will open the contactor on a short circuit ahead of the fuse clearing level in a attempt to keep from blowing the very expensive MV fuses if possible.

Minu,

The answer lies in further investigation. Typically, a MV motor controller is designed such that the let-through current of the current limiting fuses is below the withstand rating of the vacuum contactor, but the thermal trip curve of the relay is meant to provide the running overload protection to prevent the fuses clearing on long time over current. The setting of the instantaneous trip of the protective relay can be set below the fuse short circuit curve to help pervent clearing of the fuses if possible, but must still be below the withstand rating of the vacuum contactor as well.

Relay has to be set for over current protection, starting protection, unbalanced and earth fault protection and stall protection only when the motor is controlled thru contactor

For High short circuit currents it will be better to allow the fuse to blow out instead of opening the contactor, as breaking the contactor for short circuit current will damage the contacts.

There is a write up about this in ABB site ,please go thru ."contactor control Motor app guide 756085E"