Ground fault in control wiring of 480 volt motor circuit
Ground fault in control wiring of 480 volt motor circuit
(OP)
I am trying to better understand how a ground fault in the 120 volt control portion of a 480 volt, 3 phase motor starter circuit can cause a ground fault alarm in our high resistance ground fault detection relay on our MCC.
We incurred a ground fault alarm on our 480 volt, 3 phase MCC. I looked at the ground fault relay, and it was showing 270 volts at about 3.3 amps. Our electrician determined that the start-stop station of was full of water, and was causing the ground fault. This is a size 5 starter, and the start-stop controls are 120 VAC via a 480/120 .100 KVA CPT (one side grounded) in the starter. The control is a basic 3-wire field start-stop station, operating a 120 volt control relay in the motor starter bucket, which in turn operates the 480 volt coil on the size 5 starter.
I know it happened, because when we cleaned the water out of the start-stop station, the ground fault went away. But I don't see how the ground fault went through (or around) the control relay and caused the high current (3.3 amps @ 270 volts) in the ground fault relay, without blowing the primary or secondary control fuses around the CPT.
We incurred a ground fault alarm on our 480 volt, 3 phase MCC. I looked at the ground fault relay, and it was showing 270 volts at about 3.3 amps. Our electrician determined that the start-stop station of was full of water, and was causing the ground fault. This is a size 5 starter, and the start-stop controls are 120 VAC via a 480/120 .100 KVA CPT (one side grounded) in the starter. The control is a basic 3-wire field start-stop station, operating a 120 volt control relay in the motor starter bucket, which in turn operates the 480 volt coil on the size 5 starter.
I know it happened, because when we cleaned the water out of the start-stop station, the ground fault went away. But I don't see how the ground fault went through (or around) the control relay and caused the high current (3.3 amps @ 270 volts) in the ground fault relay, without blowing the primary or secondary control fuses around the CPT.





RE: Ground fault in control wiring of 480 volt motor circuit
2) Assuming that it is a Zero Sequence GF protection scheme (as opposed to Residual Current method which is typically not used in resistance grounded systems).
3) Assuming that 3.3A is enough current to activate the Alarm level setting of the GFR.
Simple answer based upon these assumptions:
One side of your 3 wire start/stop relay wiring is hot (120V) off of the CPT secondary. This was then flowing to ground in the water, a Ground Fault pulling current which ultimately came from the 480V side of the CPT. That GF current was obviously enough to trip the Alarm in the GF relay AND the relay trip time settings were faster than the thermal curve of the CPT fuses.
"Venditori de oleum-vipera non vigere excordis populi"
RE: Ground fault in control wiring of 480 volt motor circuit
NEMA size 5 would usually indicate an interposing relay, and line-votage coils(s) on the magnetic starter. If you have not yet, megging control wiring is in order.
It almost sounds like someone may have mistakenly wired the wrong side of the interposing relay into the stop-start station. Else, if it is truly grounded 120V, water should not effect the 480V-ground detection, unless also the CPT had its secondary-side ground lifted, and had a primary-to-secondary fault.
RE: Ground fault in control wiring of 480 volt motor circuit
or there is a 480V system wire present in the push-button station.
Besides I am curious what type of GF relay and scheme is this, because you have voltage reading there.
RE: Ground fault in control wiring of 480 volt motor circuit
RE: Ground fault in control wiring of 480 volt motor circuit
I should have had another ASSumption in there. IF the CPT primary leads are connected to the 480V down stream from the ZSCT, a ground fault in the 120V of the CPT secondary will be seen by the ZSCT. I have witnessed this several times, sometimes the GFR trips just on the CPT inrush current. The solution is to tap off the CPT primary power from the UPSTREAM side of the ZSCT. It is a common mistake.
aolalde may have a point, especially since people often will oversize non-time delay CPT fuses to get around the nuisance blowing on inrush. But again, moving the CPT feed to the other side of the ZSCT will solve it.
"Venditori de oleum-vipera non vigere excordis populi"
RE: Ground fault in control wiring of 480 volt motor circuit
jraef — Shouldn’t a CPT-secondary fault be reflected into the primary as phase-to-pahse overcurrent? I wouldn’t think that would trip 480V ground-fault protection.
CPT-fuse types and sizes should be verified. Sometimes, though, "correct" fuses don't always coordinate well with ground-fault protection. There may be non-ideal tradeoff given the need to limit ground-fault damage in controls or motors.
RE: Ground fault in control wiring of 480 volt motor circuit
RE: Ground fault in control wiring of 480 volt motor circuit
jraaf -- I too fail to see any other way that a 120v GF would show up as anything but overcurrent on the primary side, other than this unlikely scenario of a primary-secondary bond. Please explain.
I think busbar's probably got the right idea. In fact, I won't even wait til I find out for sure -- red star for busbar.
It sure seems to me
RE: Ground fault in control wiring of 480 volt motor circuit
"Venditori de oleum-vipera non vigere excordis populi"
RE: Ground fault in control wiring of 480 volt motor circuit
Well, then we get 10 amps in A, 10 in B, and 0 in C. But the net current through the 3-phase circuit will still be zero, no matter if you're detecting this with zero-sequence or residual detection.
Matter of fact, on a motor circuit like this, the currents will generally be very well balanced, when compared to many switchboards or panelboards that are powering extremely unbalanced loads but have no problems with their GF detection.
I still don't buy it jraef. . . .
RE: Ground fault in control wiring of 480 volt motor circuit
We can agree to disagree I suppose.
"Venditori de oleum-vipera non vigere excordis populi"
RE: Ground fault in control wiring of 480 volt motor circuit
The 120V ground current might then find a lower impedance path back to the CPT by flowing on the 480V grounding conductor.
Happens quite regularly on medium voltage systems.