Contactors for motor starters are usually supplied with fuses for short circuit protection. The engineer needs to verify that the fuse will clear faults above contactor rating before the contactor drops open. If the motor starter or contactor is not protected by a correctly sized fuse, the circuit protection relay should trip an upstream device to clear the fault, including ground faults .
Compare the fuse curve with the contactor's interrupting rating. For larger loads, there usually is a small triangle below 100 mSec between the fuse curve and the vertical line for the contactor interrupting rating. It looks like for fault currents in that triangle area, the contactor will try to clear a fault above its rating. But since the contactor takes some time to drop open, the actual contact opening occurs after the fuse has cleared the fault.
Many MV systems are high resistance grounded with ground faults limited to a low level (200-400 A) well within the contactor rating. But a ground fault can quickly escalate into a phase-phase fault. Dropping the contactor open during an escalating ground fault may cause a violent contact bottle failure, unless the fuse protection is adequate.
I know of one fatality from an MV contactor that opened under fault above its rating. It blew open the MV starter door and killed the electrician who just happened to be in front of it.
Any time a fuse blows, or the contactors trip on a fault, the contact integrity should be checked, or at the very least verify the contacts are not welded shut. A major equipment burn down occurred when electricians repaired the faulted cables but just replaced the fuses in the motor starter. Two contacts were still welded closed. When they closed the disconnect switch, the 750 HP motor was single phased. The relay tripped but the contacts remained closed and the motor and starter melted down. Lucky no one was hurt.
Most modern motor protection relays can be set with a backup protection output to trip an upstream device if something like that occurs.
