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-   -   GFCI trip when ground and neutral touch (http://www.diychatroom.com/f18/gfci-trip-when-ground-neutral-touch-85062/)

thewoodguy 10-27-2010 01:49 PM

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!

oleguy74 10-27-2010 01:53 PM

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.

secutanudu 10-27-2010 02:58 PM

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.

thewoodguy 10-27-2010 06:34 PM

Thanks
 
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!:)

Scuba_Dave 10-27-2010 07:17 PM

Is the outlet where you are pulling the ground on the same circuit as the light ?

thewoodguy 10-27-2010 09:06 PM

@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.

oleguy74 10-27-2010 09:27 PM

be extreamly careful.don't want you hurt...

Scuba_Dave 10-27-2010 09:37 PM

Quote:

Originally Posted by thewoodguy (Post 524198)
@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

HVAC_NW 10-28-2010 03:53 AM

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.

a7ecorsair 10-28-2010 10:09 PM

Quote:

Originally Posted by HVAC_NW (Post 524305)
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.

HVAC_NW 11-01-2010 06:21 PM

Quote:

Originally Posted by a7ecorsair (Post 524656)
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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