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#130065 - 01/25/06 07:22 PM GFI breaker working across a transformer  
maintenanceguy  Offline
Joined: Dec 2001
Posts: 300
Southern NJ, USA
We've got a 2500A GFI breaker that has tripped several times on a ground fault that we can't find. It happens days apart.

This is one of two main breakers that feeds a 260K sq ft building at 480V. There are several small and large transformers all over the building to provide other voltages.

I called in our electrical contractor since we were stumped and he told us that a GFI can "see" a ground fault even if it's on the secondary side of our transformers.

I'm not sure I believe this and wanted more opinions.

It seems to me that the GFI breaker feeding the transformer can't possible know if there's an imbalance on the secondary side.

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#130066 - 01/25/06 09:17 PM Re: GFI breaker working across a transformer  
Radar  Offline
Joined: Apr 2004
Posts: 349
Los Angeles, CA
The GFI is not so much sensing ground currents as such, but senses the sum of the currents flowing thru the line wires and neutral, and will presume any unballance, or unaccounted for current (sum <> 0) will be leakage current.

A little more info would be helpful. I presume these 2500a breakers are not new, and that the GFI trip settings have been calibrated? We've had problems in the past in new installations when started up. The GFI in the 480V main is factory set to minimum time delay, meaning instantaneous, and minimum current. So sometimes the starting of a large motor would cause it to trip. Having the unit calibrated to specified settings completely eliminated the problem.


There are 10 types of people. Those who know binary, and those who don't.

#130067 - 01/25/06 09:55 PM Re: GFI breaker working across a transformer  
Dave T  Offline
Joined: Jan 2005
Posts: 155
Waukesha, WI, USA
Ok, let's see if we can sort this out.
Whose breaker is it and what are the features? Breakers of this size normally have solid state trip units with a number of bells a whistles on them. Are you sure it's tripping out on GF?
How old is thew installation and is it part of a switchboard.
I had this problem on a large Pick N Save food store that they were just doing the final stocking before the grand opening. The breaker tripped out on GF every time they switched on the 277v parking lot lighting load. Note that it was a 1ph load. The switchboard manufacturer had wired the neutral CT incorrectly and because the breaker couldn't see the neutral current it assumed that the neutral current was GF current.
1) Is the breaker feeding a 3ph 4w load? If so there must be a neutral current sensor that senses the neutral current. Check the wiring and assure that the neutral CT is wired correctly to the breaker.
2) Is the GF pickup setting on the breaker set too low? There is no setting requirement other than the feature must be included. If there is a GF is there any coordination with the downstream devices? It should be able to be set up to 1000a. Also, is there a time delay? If so, set the delay longer.
3) There are normally test kits that you can plug into the breaker to use secondary current injection to test the breaker. Although not cheep, the PC boards can often be replaced in the field if confirmed and proven to be defective. Don’t just replace it as it may be and expensive way to find it not to be the problem.
If you feel comfortable with the integrity of the breaker then try to isolate the external problem for the trip.
1) Is there a specific time of the day that it trips or pattern? I had a breaker that tripped about 5:00am. It was an older 600a breaker with an older technology SS trip that was peak sensing and not RMS at a sewerage lift station. I assumed that there was a PFCC that was switched in that cause a voltage spike that resulted in current spike that tripped the breaker. I changed the breaker to and older technology thermal magnetic breaker and the problem went away.

#130068 - 01/25/06 10:46 PM Re: GFI breaker working across a transformer  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA


I called in our electrical contractor since we were stumped and he told us that a GFI can "see" a ground fault even if it's on the secondary side of our transformers.

Nahh, that's jibberish.

The GFPE (Ground Fault Protection for Equipment) Main Disconnect you are dealing with will only sense a "Leakage Of Current" on the 480Y/277 VAC System ("Side").

At the MSB (Main Switch Board) - where the GFPE Main Disconnet is located, all 4 Conductors of the Load Side Feeder (ØA, ØB, ØC, N) are routed through a single C.T., which is coupled to the trip circuitry. The trip logic reacts to Amperage levels from the C.T., which are produced from Zero-Sequenced Current levels in the Feeder.
GFPE or GFI devices do not "See" Ground Faults - they only react to an imbalanced current flow on the circuit(s) run through their C.T.

As to sensing the Secondary side of an Isolated Transformer, if the Primary Circuit is complete - without some type of L-G leakage, then there would be no imbalanced flow, and no way to have the trip logic sense an imbalance - even if there was a bolted L-G fault on the Secondary side.

A few things to check regarding the GFPE trip would be:

[*] Insulation Breakdown,

[*] Randomly occuring Arc Fault,

[*] N-G connection downstream of the GFPE,

[*] A Motor which is infrequently used, which has a fried Winding - or Windings,

[*] A Transformer with one or more Primary Coils leaking current to Ground - or possibly arcing to ground,

[*] Some 277V Lighting Circuit, which has a Ground Fault on the Load side of a switch - so when someone throws that switch, it causes the GFPE to trip, and the person responsible for the "Major Power Outage" throws the switch back off, then quickly vacates the area,

[*] The GFPE is "trimmed" way too low - meaning the adjustments are set to the extreme minimums on all settings, and the device is simply reacting to normal power quality issues - like hard starting Motors on very long circuits, lots of Fluorescent Fixtures on very long circuit runs, etc.

Good luck shooting trouble on this.
Let us all know what the outcome is.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#130069 - 01/25/06 11:29 PM Re: GFI breaker working across a transformer  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma

I thought that one of the ground fault detection techniques was to use a CT on the ground to neutral bond. Any common mode current on the supply conductors would need to be matched by by current on the ground to neutral bond, problem being that any other current on the ground to neutral bond could also trip the breaker.


#130070 - 01/26/06 09:05 AM Re: GFI breaker working across a transformer  
Zog  Offline
Joined: Sep 2005
Posts: 120
Charlotte, NC
First, get a contractor in there who know what he is talking about to test the unit.

They should verify polarity of the CT's and inject current to check operation.

The injection test should verify the unit does not trip at 90% of the pickup setting and that it does trip at <125% of the pickup setting or 1200A, whichever is less.

Additionally the delay should be measured at 150% of the pickup setting and verified on the TC curve for accuracy.

You can find someone in your area that knows what they are doing at

MV/HV Testing Specialist, "BKRMAN"

#130071 - 01/26/06 09:56 AM Re: GFI breaker working across a transformer  
JBD  Offline
Joined: Jul 2001
Posts: 599
We once found an outdoor wall pack luminaire with a bad gasket. When the wind blew the wrong way, the ballast would short to ground when the photoeye turned on. Took months to find it. The problem was found by properly coordinating the GF settings on the main which let the branch circuit trip first.

#130072 - 01/31/06 12:41 PM Re: GFI breaker working across a transformer  
maintenanceguy  Offline
Joined: Dec 2001
Posts: 300
Southern NJ, USA
Thank you all. Excellent information.

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