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Old 03-05-2013, 01:32 AM   #1
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one more gfi question


I've just read a ton of gfi-related posts, and don't know if I am more educated or more confused. Either way, I did not see exactly what my question is. Situation: My new shop is wired, and I think properly. All receptacles and switches are working just fine. I removed a receptacle on the lid to install a fixture instead. As usual, I shut the light off, then disconnected the neutral wire (white) and slapped it against the sometimes-hot wire (black) just for giggles. Check. All is fine; no sparks, no zapping. The cheap receptacle is one of those two-wire, single light bulb deals, with no ground screw. Like the electrically impaired individual I am, I figgered I'd hook the (bare) ground wire that used to be on the receptacle to the second neutral screw. Bang. There went the GFI breaker. WTH? Is this how things should be? After reading, I think I am seeing that it is. However, I don't really know WTH is going on. Please advise or send a link to a previous discussion. Thanks. BTW: This is the only circuit that had any activity; another light on the same circuit was on.

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Old 03-05-2013, 02:30 AM   #2
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one more gfi question


Your gfi worked perfectly. You created a second return path. A parallel path for electricity would split the amperage, half on each path. That's a big no-no and more than enough to trip a gfi.

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Old 03-05-2013, 03:10 AM   #3
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one more gfi question


Glenn: Thanks. That is what I was vaguely thinking after reading here and bringing up very old memories of an AC/DC (not the band, either) class I once took. I really need to sit down and study a few of these no-so-obvious concepts of electricity.The whole concept of AC is not entirely obvious when you think of what is actually happening w/ electron and/or energy flows.
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Old 03-05-2013, 03:47 PM   #4
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one more gfi question


It would do the same thing with DC. It's just a circuit-thing, not an AC-thing. That neutral wire is connected to the rest of the neutral wires on that circuit, and those have current flowing through them from other loads that are not turned off. When you connected the grounding conductor to the neutral terminal, you created an alternate return path for some of that current from the other loads. The proportion of the total current that will take the alternate path instead of the preexisting neutral path depends on the relative resistances of the grounding conductor versus the neutral conductors - that's Ohm's law. Since it only takes 5mA to trip the GFCI, only a very small amount of the load current needs to flow through the grounding conductor to trip the GFCI. If the total load on the circuit is 5A, then only 1/1000th of that current would need to be diverted through an alternate path in order to trip the breaker.
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Old 03-05-2013, 04:05 PM   #5
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mp: thanks. good to know.
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