Fault isolation problems 1

[SASHmash]

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?

 

[DanDel]

Add GF protection to the individual motor circuit.

 

[dpc]

This is a very common problem. Only solution is to do as DanDel suggested and add GF protection to the motor starters. This could be done by replacing the circuit breaker in the starter or by adding a stand-alone ground fault relay and then adding a shunt trip to the breakers. Probably cheaper to replace the breakers.

This is not necessarily "incorrect", it's just a cheap installation. You get what you pay for.

 

[SASHmash]

Thanks dpc, that sounds like a logical approach. However it alleviates one problem by creating another. The purpose of the stand alone GF relay, or a CB with integrated GF functionality is to introduce more sensitive GF protection. Or at least faster GF protection than the MCC's CB. This solves the problem of grading the local switchgear to trip first, but now reduces the ground fault setting from spec. I run the risk of the motor tripping under normal operating conditions, or at least GF conditions which would ordinarily only cause a flag. The only option I see is introducing a LV feeder protection relay and integrating it with the existing CB.. but am looking for any other viable options to pursue.

 

[DanDel]

There is no normal condition upon which GF current should flow in a circuit, at least not in the range that these protective devices work.
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.

 

[SASHmash]

Sorry DanDel, I wasn't very clear in my above comment. The CB has integral protection set on a definite time setting. Whereas the motor's local protection device is set on an inverse curve (IDMT), as per our protection scheme. Hence, a trip was experienced on the MCC and none on the motor protection relay due to the current-time curve. Nevertheless, the problem has been solved by placing a stand alone GF relay on the MCC panel with IDMT capabilities, and disabling the GF function (definite time) on the CB. A shunt connection has been placed between the GF relay and the CB's tripping coil. But thanx for the help..

 

[Laplacian]

Are you absolutely positive you must have GF protection? Is your 480V fed from a solidly grounded wye transformer?
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.

 

[dpc]

If the motor ground fault protection is set properly, it will only operate for a ground fault. If the relay sees ground current in the motor circuit, it's time to trip. Either you want ground fault protection or you don't. You can't have it both ways.

 

[redha123]

maybe the capacitf current (Ic) due to the earth fault is bigger than the protection rating current(Ir). the normal setting Ir>3xIc.