Class II Ground Fault Protection 1

[nauas]

We recently had an incident at our company in which a Grounded Low Voltage motor (30kW, 480VAC, 60Hz) led to the tripping of entire MCC bus. The motor is fed through a Square D Model 6 MCC. Our System is solidly grounded and the maximum available GF current at the Motor terminals is around 20kA (calculated by ETAP)

The Motor is protected through a Solid State overload relay (SQ-d SSOLR Motor Logic Plus), with GF trip set at 10A, however the SSOLR did not take any action, and the upstream protection tripped on Ground Fault. Event recorder from the Protection relay installed at the feeder breaker of the upstream switch gear (Multilin 750) indicates that the actual magnitude of the GF current was around 970 A.

I read through the instruction manual of the Motor Protection relay and it states that the relay has ground fault protection of Class II, and the inhibit current is 25A. I understand that Class II GF protection relays are used with devices that have low interrupting capability, and therefore they must inhibit tripping above a certain level of fault current, however I do not understand why would you inhibit tripping of a contactor that can carry the 55A motor current and can safely interrupt up to 330A (Locked rotor current) when the Ground fault current exceeds 25A only.

Another question is how would one guarantee protection coordination for ground faults in such scenarios. From what I see:

1) GF current up to 25A will be interrupted by the contactor (SSOLR willtake action).
2) The Circuit breaker in the motor starter is Square-D FH100 MCCB. Magnetic pickup for the same starts at around 2kA. The GE relay installed upstream is set for GF pickup at 900A with 100ms time delay. Therefore GF current above 25A but less than 900A will not be interrupted immediately. Depending on the magnitude of the current, the SSOLR will inhibit tripping of the contactor and the MCCB will see the fault in its inverse time region and will take too long to trip, resulting in a possible risk of fire.
3)For GF current greater than 900A bus less than 2kA, the upstream protective relay with take action resulting in outage of the entire MCC bus.
4)For GF current exceeding 2kA the MCCB will interrupt the fault instantaneously.

Since the actual GF current can take any value depending on the impedance to the ground path, I feel that such a protection scheme can never achieve 100% coordination based on the above? Is my understanding correct? Also how to overcome these limitations?

Any help from the members of this forum will be highly appreciated.

 

[LionelHutz]

25A is likely a number they picked because resistive grounded systems are typically limited to less than 25A.

It doesn't sound like your system is co-ordinated. Why does a 55A motor have a MCCB with a 2kA instantaneous trip?

 

[jraef]

Yeah, as I said in another forum, to have that OLR with a 25A GF limit, that means the motor FLA is less than 10A, so a 100A circuit breaker is WAY too big for that.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[nauas]

LionelHutz... thats exactly what baffles me... the motor starter is of NEMA size 3. The MCCB inside the starter is FH100 thermal magnetic type (see attached characteristic curve) Although the magnetic trip of the MCCB starts at around 2kA, the thermal pickup is at 100A. I have been told that even though the Magnetic pickup is too high for the motor rating, the breaker size still complies with NEC 430.52 (max CB rating = 250% of FLC and 100A ~ 180% FLC). I just cannot seem to get my head around this. BTW the starter bucket is a standard square D issue and is apparently UL listed for 65kA as required by our system. So replacing the breaker with a different type is also out of the question. Plus we have 100's of similar motor starters withe the exact same arrangement!

Regarding the SSOLR, from the manual of the relay, I can see that it is available in different models, the one installed in our bucket is SP36. Apparently there are other types (SP46 and SP56) which can be set to 10A GF trip and have higher inhibit currents (75A and 150A respectively).I contacted the local square D office and when I asked them why they used the SP36 in the first place, they had no answer. I also asked them to confirm if it had got anything to do with the contactor and they were unable to respond. They told me that they would get back to me with an answer soon!