Fault isolation problems
Fault isolation problems
(OP)
I am currently having the problem that an earth fault on my motor (LV) is tripping out my MCC rather than isolating the fault to that particular motor, by locally tripping the motor. The obvious cause is that the fault grading is incorrect. However, at this particular sub station we use a CB with a very sensitive earth fault protection relay (integral protection). The protection on the motor cannot be adjusted to trip on a lower fault as this causes the motor to trip under normal operating conditions. A suggested approach is to replace the integral protection on the CB and integrate these relays with the old CB. This is quite a costly and laborsum approach, as is replacing the circuit breakers. Any ideas?






RE: Fault isolation problems
RE: Fault isolation problems
This is not necessarily "incorrect", it's just a cheap installation. You get what you pay for.
RE: Fault isolation problems
RE: Fault isolation problems
The reason you are having a trip now is because you have a ground fault. Adding GF protection to the motor circuit will locally trip the motor instead of tripping the Main device, as you requested. The fix is to find the ground fault and repair it.
RE: Fault isolation problems
RE: Fault isolation problems
Are you in an industrial plant with critical process where unplanned interruptions increase hazard risk?
Is the breaker trip unit less than 1000A?
Please reference 215.10 and 230.95 of the NEC for more information.
RE: Fault isolation problems
RE: Fault isolation problems