GF nuisance tripping-480v system-troubleshooting
GF nuisance tripping-480v system-troubleshooting
(OP)
I have a 480 volt switchboard with a 1600 amp main circuit breaker with a GE type SS circuit breaker with ground fault trip, and feeder breakers of GE type SGH, SGL, SFH downstream of the main. These breakers feed lighting panels and rooftop units.
Some of the feeder breakers and the main are experiencing nuisance trips (I think I was told that it was at least some of the rooftop feeders). The nuisance trips have subsided since the GE rep came out and cranked the trip settings up, I think.
I figured the best way to troubleshoot is to turn on all the rooftops and lights/loads I can find, and read the current in the main switchboard from neutral to ground. If there is more than "leakage" type current reading there, I could then 1) shut down hvac units 2)shut down lights/loads 3)trip breakers if necessary to identify which circuits have an effect on the amount of neutral-ground current. Once the offending circuits are found, they can be traced down to figure out where the problem is, fix it and reset the GF sensors to some level that might kind of coordinate (I didn't include a zone-selective scheme on the GF - mea culpa).
I am assuming the contractor somehow made a downstream connection between neutral and ground - otherwise we would have had an equipment burndown by now.
Any thoughts?
Some of the feeder breakers and the main are experiencing nuisance trips (I think I was told that it was at least some of the rooftop feeders). The nuisance trips have subsided since the GE rep came out and cranked the trip settings up, I think.
I figured the best way to troubleshoot is to turn on all the rooftops and lights/loads I can find, and read the current in the main switchboard from neutral to ground. If there is more than "leakage" type current reading there, I could then 1) shut down hvac units 2)shut down lights/loads 3)trip breakers if necessary to identify which circuits have an effect on the amount of neutral-ground current. Once the offending circuits are found, they can be traced down to figure out where the problem is, fix it and reset the GF sensors to some level that might kind of coordinate (I didn't include a zone-selective scheme on the GF - mea culpa).
I am assuming the contractor somehow made a downstream connection between neutral and ground - otherwise we would have had an equipment burndown by now.
Any thoughts?






RE: GF nuisance tripping-480v system-troubleshooting
I think the exercise you mentioned to trace where the fault is coming from is the best policy. Once you have tracked it down and you know the cause, then you can remedy it.
RE: GF nuisance tripping-480v system-troubleshooting
Gunnar Englund
www.gke.org
RE: GF nuisance tripping-480v system-troubleshooting
Are all trips on ground fault or do they inlcude overcurrent trips?
Does the feeders have GFP as well?
If over current trips, have someone verify that all breakers are sized correctly. It is not uncommon to find motor feeder breakers undersized.
RE: GF nuisance tripping-480v system-troubleshooting
RE: GF nuisance tripping-480v system-troubleshooting
Possible problems include incorrect ratio, incorrect polarity, incorrect placement, and accidental shorting of the neutral CT wiring. All will cause nuisance tripping on GF only when unbalanced line-to-neutral loads are present.
Because the neutral conductor does not pass through the CB frame, the CT is added to the switchgear and wired into the breaker. Lots of room for error.
RE: GF nuisance tripping-480v system-troubleshooting
RE: GF nuisance tripping-480v system-troubleshooting
However, since there are apparently more than one unit which is tripping, that would seem to indicate the wiring problem (assuming there is one, and I think it reasonable to do so) is out on the system and not at the unit.
I ALWAYS suspect shorted, malfunctioning or backwards-wired CT's on this sort of problem but it would appear in this case the problem is elsewhere if the information is being transferred to me correctly - that is, that more than one breaker is tripping. However, it's possible they're ALL wired wrong.
I will look at the coordination, but GF on low-voltage breakers is notoriously difficult to coordinate without zone-selection. Further, even with bad coordination, that should only cause a problem if the breaker trips are set WAY too low (a possibility, ie that the GF trip unit is reading CT error current), or if there is some other reason there is current between ground and neutral. But lack of coordination doesn't cause trips, although very low settings might.
THis is a commercial building - there are VFD's but they are only like 15 HP, maybe 3-4 of them.
RE: GF nuisance tripping-480v system-troubleshooting
What are your GF trip settiings?
Gunnar Englund
www.gke.org
RE: GF nuisance tripping-480v system-troubleshooting
The system we're talking about is power ground fault - not personnel protection, if I'm understanding Gunnar's question correctly.
RE: GF nuisance tripping-480v system-troubleshooting
RE: GF nuisance tripping-480v system-troubleshooting
I need to think about it.
Gunnar Englund
www.gke.org
RE: GF nuisance tripping-480v system-troubleshooting
RE: GF nuisance tripping-480v system-troubleshooting
Further confusing the problem was that during World War 2 the War Materials Board changed the grounding of electric ranges and dryers to TNC using the neutral as an equipment grounding conductor. Took us more than 50 years to get everybody to stop doing that with new installations but now we have a Connector Conspiracy Problem.
What is a real laugher is that Duquesne Light Company in Pittsburgh PA uses form 2s meters on 120 volt 2 wire services with 1 current coil in the neutral. Since some downstream fuseboxes have the neutral permanently bonded to the box the water pipes and equipment ground wires tend to bypass 1/2 of the meter!
Could be worse. In one building hillbilly electricians used 3-wire range outlets on 240 volts corner grounded only it was a C phase grounded system. When a tenant rented some 240 volt portable heaters he got a surprise when 1 of them contacted grounded metal.