GFCI outlet trips, no load, bonding issue?

jamesfjamesfJanuary 8, 2010

I've installed a new circuit in the garage. First outlet in the run is a GFCI, which trips if the load wires are connected, and works fine otherwise. All outlets are grounded.

Reading old posts it looks like ground and neutral can't touch on the load side otherwise it will trip. They don't, however, the neutral and ground are bonded at the service panel, and once the GFCI is active, the load neutral has a path to the ground system, no?

How do I rectify this? Aren't neutral and ground bonded at the panel in normal practice?

thanks,

jim

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jamesfjamesf

Er, nevermind. After posting I thought I'd disconnect everything at the GFCI box to make sure there was no neutral-ground connectivity downstream, so as not to look like an idiot.

There is. Looks like an afternoon of fun opening up 9 boxes...

jim

    Bookmark   January 8, 2010 at 12:06PM
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terribletom

Looks like you've already gotten this solved.

FWIW, the fact that the neutral and equipment grounds are bonded at the point of the first service disconnect (usually the main panel) has nothing to do with the functioning of a GFCI'd sub-circuit. The GFCI measures what passes through it -- both hot and neutral -- and detects any unexpected imbalance between the two.

It is when an alternative grounding path downstream "siphons off" (for want of a better layman's expression) a portion of the neutral current that the GFCI will trip. This can be any alternative path to ground -- including YOU!

Cheers and happy hunting.

    Bookmark   January 8, 2010 at 12:17PM
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Ron Natalie

But in addition TT, the GFCI puts a small amount of signal equally on both the hot and neutral wires. If the neutral touches ground (even with nothing else connected to the load side), it will trip. If your GFCI trips with nothing connected this is almost certainly why.

    Bookmark   January 8, 2010 at 6:20PM
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terribletom

"If the neutral touches ground (even with nothing else connected to the load side), it will trip."

True, and entirely consistent with the statement "[i]t is when an alternative grounding path downstream 'siphons off' a portion of the neutral current that the GFCI will trip." Whether the current is a GFCI-managed signal or an actual through-load, it passes through the GFCI and is downstream of the line terminals. (This could include otherwise unused load terminals themselves. If either is grounded, a **modern** GFCI will trip.)

But yeah, it's a point worth making because it does cause confusion at times (e.g., "I turned off the light switch but the GFCI still trips. How can that be?"). ;-)

[Revisiting that statement now, the part that is really too narrow is that a ground fault "siphoning off" a portion of the hot feed is equally capable of tripping a GFCI. Oops! :-) ]

In either case, though, tripping has nothing to do with the neutral being bonded with ground at or prior to the line-side GFCI terminals.

    Bookmark   January 8, 2010 at 7:56PM
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