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Old 10-27-2010, 02:49 PM   #1
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GFCI trip when ground and neutral touch


I opened up a ceiling box to replace a single bulb with two fluorescent. The box was wired for two switches at opposite ends of the room. There was no ground existing, so I pulled one from another outlet to be able to ground the new lights and the box.

The 15 amp circuit is protected by a GFI breaker, as there is also a bathroom on it. If the ground wire touched the neutral, it tripped. I wired an outlet to the circuit to test it, and the lights indicated that all was well, but it bothers me that something might be amiss.

Any ideas on what might cause this? Should I be thinking about pulling new wires (please, no!)? Help!

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Old 10-27-2010, 02:53 PM   #2
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GFCI trip when ground and neutral touch


you are on the load side of the gfci.thats what they are supposed to do,how they work.when gnd and neutral touched there is a fault..POP.line side don't matter beacause gnd and nuetral are tied togethet in mai panel.

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Old 10-27-2010, 03:58 PM   #3
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GFCI trip when ground and neutral touch


Why are you touching the ground wire to the neutral?

Oleguy is right, that is the GFCI operating as it should. There was a ground fault, so it tripped.
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Old 10-27-2010, 07:34 PM   #4
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GFCI trip when ground and neutral touch


That should have been obvious, but thanks for the reminder! Secutanudu, the first was by accident, subsequent were just to figure out what was going on. I feel safe now!
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Old 10-27-2010, 08:17 PM   #5
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GFCI trip when ground and neutral touch


Is the outlet where you are pulling the ground on the same circuit as the light ?
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Old 10-27-2010, 10:06 PM   #6
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GFCI trip when ground and neutral touch


@Scuba-Dave - no, it's separate. The original circuit is ungrounded. I'm still trying to assess the wiring in this place, which was done in 1957. Grounding is spotty, and often run to the nearest copper pipe! Some outlets have a thin ground wire connected to the outside, some don't. It's a mess.
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Old 10-27-2010, 10:27 PM   #7
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GFCI trip when ground and neutral touch


be extreamly careful.don't want you hurt...
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Old 10-27-2010, 10:37 PM   #8
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GFCI trip when ground and neutral touch


Quote:
Originally Posted by thewoodguy View Post
@Scuba-Dave - no, it's separate. The original circuit is ungrounded. I'm still trying to assess the wiring in this place, which was done in 1957. Grounding is spotty, and often run to the nearest copper pipe! Some outlets have a thin ground wire connected to the outside, some don't. It's a mess.
You need to run grounds back to the panel
Not from another circuit
And not to water pipes
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Old 10-28-2010, 04:53 AM   #9
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GFCI trip when ground and neutral touch


GFCI is a differential amplifier. Both neutral and hot are wrapped around in a coil. So, if the inbound and outbound aren't perfectly symmetrical, it sees it as a leak. When you let the neutral touch the ground, the ground wire will act as a parallel jumper back to the panel and some of the current will be diverted and shared with neutral, which causes asymmetry between neutral & hot within the GFCI. So, it will see it as a ground fault.

Last edited by HVAC_NW; 11-01-2010 at 07:20 PM.
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Old 10-28-2010, 11:09 PM   #10
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GFCI trip when ground and neutral touch


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Originally Posted by HVAC_NW View Post
GFCI is a differential amplifier. Both neutral and ground are wrapped around in a coil. So, if the inbound and outbound aren't perfectly symmetrical, it sees it as a leak. When you let the neutral touch the ground, the ground wire will act as a parallel jumper back to the panel and some of the current will be diverted and shared with neutral, which causes asymmetry between neutral & hot within the GFCI. So, it will see it as a ground fault.
GFCI's compare the current in the Hot conductor and Neutral conductor using a coil. Touching a ground wire to the neutral will cause some of the "return" current to flow in the ground wire and some to flow in the neutral so the GFCI will see an imbalance between the Hot and neutral.
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Old 11-01-2010, 07:21 PM   #11
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GFCI trip when ground and neutral touch


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Originally Posted by a7ecorsair View Post
GFCI's compare the current in the Hot conductor and Neutral conductor using a coil. Touching a ground wire to the neutral will cause some of the "return" current to flow in the ground wire and some to flow in the neutral so the GFCI will see an imbalance between the Hot and neutral.
That was a typo. I fixed it. Even with no load, if you short the neutral to ground, it will cause a small amount of current between ground and neutral, which the GFCI will see as asymmetry between hot & neutral, then trip.

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