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Thread: Class II Ground Fault Protection

  1. #1
    Join Date
    Jul 2016
    Saudi Arabia

    Class II Ground Fault Protection

    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.

  2. #2
    Join Date
    Jul 2006
    San Francisco Bay Area, CA, USA
    I think you have it mostly right, except for the magnetic pick-up of the FH100 (assuming you meant FH36100) MCCB being 2kA minimum, and that's where I think the issue lies.

    The mag trips on an FH36100 are adjustable from 900-1700A (900-1700% of the thermal trips). So technically you COULD HAVE set the mag trips on that breaker to 900A and that would have picked it up before the Multilin relay would. But you didn't say what the actual motor current is. I can extrapolate though, and therein lies the problem in spite of the lack of details.

    The OLR GF setting is adjustable between 10% and 20% of the FLA setting, so because the 25A inhibit is associated with the GF being adjustable from 1.5 to 2.0A, that must mean your motor current is below 10A. So even if it's 10A FLC, even the lowest setting of 900A on the mag trips would end up WAY too high per 430.52 for a motor that size. I think what's wrong with this picture is that your circuit breaker is extremely over sized if it is the only SCPD in the circuit. So if you had the proper size breaker in place, i.e. the max I setting is 170A, that would have tripped LONG before your Multilin began to act on the GF.
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  3. #3
    Join Date
    Jul 2016
    Saudi Arabia
    Thanks for the detailed response Jraef. However I am still a bit confused....

    The MCCB in our MCC is not adjustable. The MCC bucket is a standard square D issue with a NEMA 3 starter. The Motor FLA is 55A. I have attached the Trip Characteristics MCCB ( rating is 100A).

    Is the MCCB too large for this application? From What I know, we cannot replace the breaker with another type since the combination starter is UL listed for the fault level in our system. Does this mean that square D standard design for a NEMA 3 starter is incorrect?

    I rechecked the SSOLR - it is a Motor Logic Plus type SP36 and GF is set at 0.18 x OC (~ 9.9A). Based on your feedback the correct selection for our application would be SSOLR type SP46 or SP56 both of which can be set for 9A GF pickup and have inhibit at 75 and 150A respectively. However how can it be ensured that this level of trip inhibit would be safe for the contactor?

    Even if the MCCB were adjustable, and I set it for 900A Magnetic trip and use SSOLR SP56 , still there would be problems and risk of fire for GF currents ranging from 150A to 900A right? I am unable to understand wow can 100% protection be guaranteed in this case?

    Appreciate your help!
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    Last edited by khamoshie; 10-13-17 at 02:05 AM.

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