GFCI Main Circuit Breaker Tripping

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Jimmy4645

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Hello, wondering if you guys can lead me to the correct direction. I'm working on a project that has a 3000 amp 120/208 volt 3 phase electrical service with a bolted pressure switch and protected by GFCI at the main switchboard. During an inspection, the inspector activated both elevators at the same time (1) 50HP and (1) 40HP and it trips the mainswitch and shuts off the power for the entire building. I believe the GFCI settings are set too sensitive and needs to be adjusted. What is the proper way to correctly set the GFCI settings, so that the in-rush current from the elevators do not trip the mainswitch?

Thanks for your help
 
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rcwilson

Senior Member
Location
Redmond, WA
I think your system probably has a Ground Fault Protection relay (GFP), not a GFCI. A GFCI trips at 5 milliamps for personnel protection. A GFP can be set to trip from about 30 amps to 1200 amps.

But the setting is not the problem. As long as there is no ground fault on the elevator circuits the GFP should not trip at any load. If there is a fault there would be a lot of smoke, so we could assume there is no ground fault.

More likely there is a wiring problem with the ground fault sensor or a faulty sensor. Maybe the neutral CT is connected backwards, or not connected at all. Maybe all the phase wires are not centered in the sensing CT so the high inrush to the elevators is not accurately measured. Or maybe the GFP relay is one of the bad ones that sometimes trips due to saturation on high currents or harmonics.

If the elevators have modern VFD based controls, the harmonics might affect the GFP relay. This usually is a combination of 3rd harmonic currents in the neutral and a reversed neutral CT connection giving a false trip signal.

Another possible problem is improper connection of a neutral bonding jumper downstream of the GF sensor. An incorrectly placed jumper can shunt neutral currents around the sensors so the phase and neutral currents do not add up to zero, causing a false trip.

Bottom line, thoroughly inspect the ground fault installation and find the problem, or that main will continue to trip. Just raising the setting is dangerous.

Excessive ground fault trips on a Bolted Pressure Switch can wear it out, leading to a switchboard burn down when the switch contacts no longer fully make up. (This happened some years ago on two projects I was hired to repair.)
 
zog

zog

Senior Member
Location
Charlotte, NC
Jimmy4645 said:
Hello, wondering if you guys can lead me to the correct direction. I'm working on a project that has a 3000 amp 120/208 volt 3 phase electrical service with a bolted pressure switch and protected by GFCI at the main switchboard. During an inspection, the inspector activated both elevators at the same time (1) 50HP and (1) 40HP and it trips the mainswitch and shuts off the power for the entire building. I believe the GFCI settings are set too sensitive and needs to be adjusted. What is the proper way to correctly set the GFCI settings, so that the in-rush current from the elevators do not trip the mainswitch?

Thanks for your help
Click to expand...

rcwilson made some good points, but there could be other factors depending on the method of current sensing this switch uses, there are several different configurations for a GFP (Not GFCI, apples and oranges). A qualified testing company should have inspected and tested this switch for proper wiring, routing of the conductors through the CT's and operation of the GFP. You seem top be in over your head and may want to contact a testing firm who is familar with these, Brian John should pipe in here with some comments too, he is as good as they get with these.
 
J

Jimmy4645

Member
i'm not sure what setting were adjusted on the GFP, but it did not trip this time when the elevators were activated. I will find out what was actually performed to correct this issue.
 
zog

zog

Senior Member
Location
Charlotte, NC
Jimmy4645 said:
i'm not sure what setting were adjusted on the GFP, but it did not trip this time when the elevators were activated. I will find out what was actually performed to correct this issue.
Click to expand...

Turning up the setting is not the correct solution. you need to do some testing.
 
brian john

brian john

Senior Member
Location
Leesburg, VA
1. What are the settings?
2. As noted was it ever tested?
3. Was there a coordination study?
4. I have seen cases where inrush resulted in a trip, it was explained to be by a factory engineer that inrush currents are not always zero sequence. I can not support this with what little I know. BUT the inrush was from a 750 KVA 480 Delta to 208y/120 transformer.
5. Are you sure it was the GFP rela, not some other component in particular the phase failure relay
5. The NEC does not mandate GFP for 208y/120 VAC distribution systems was this specified or a miss understanding in ordering the equipment. I have seen the later several times. GFP was NOT specified, no biggy nothing wrong with installing it were not mandated just a thought.


Number one cause of tripping with what you described is a defective relay.

In no particular order of what causes a GFP trips

1. Human interface, an electrician tripping circuits to locate a circuit breaker or a laborer during demo.
2. Defective equipment or conductors , A ground fault.
3. Defective GFP relay and less often a sensor.
4. Improper grounding at the switchboard or downstream neutral bonding.
5. Cross wiring of distribution systems.
 
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