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06-12-2013, 11:21 AM   #16

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Quote:
 Originally Posted by jeffnc Why? A GFCI circuit doesn't know where the current goes after it leaves the circuit. It couldn't possibly tell if the ground and neutral are connected after the circuit. A GFCI circuit only knows if the current going into it equals the current going out of it.
If you are talking about on the load side of the GFCI, it most certainly does "know" if the ground and neutral are connected. It will trip immediately, no load required.

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06-13-2013, 04:58 AM   #17
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Quote:
 Originally Posted by jeffnc It couldn't possibly tell if the ground and neutral are connected after the circuit.
As the earth line is in parallel to the neutral line,
some of the current that should return via the neutral line
will end up going via the earth system.
this will cause an imbalance tripping the gfci.

06-13-2013, 07:15 AM   #18
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Quote:
 Originally Posted by dmxtothemax As the earth line is in parallel to the neutral line, some of the current that should return via the neutral line will end up going via the earth system. this will cause an imbalance tripping the gfci.
The "ground" in GFCI is a bit of a misnomer. It doesn't detects leaks to the ground wire. It only detects leaks to "ground" external to the circuit. So if the current coming into the outlet equals current leaving (by whatever wire), there is no imbalance. This is why GFCI works without even having a ground wire attached.

06-13-2013, 07:46 AM   #19
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Quote:
 Originally Posted by jeffnc The "ground" in GFCI is a bit of a misnomer. It doesn't detects leaks to the ground wire. It only detects leaks to "ground" external to the circuit. So if the current coming into the outlet equals current leaving (by whatever wire), there is no imbalance. This is why GFCI works without even having a ground wire attached.

Correct, but if the ground wire is touching the grounded wire, on the load side of a GFCI, will it not trip?
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 06-13-2013, 07:49 AM #20 Member   Join Date: Apr 2011 Location: North Carolina, USA Posts: 4,273 Rewards Points: 4,796 Not sure what you mean by "ground wire" and "grounded wire".
06-13-2013, 08:05 AM   #21
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Quote:
 Originally Posted by jeffnc Not sure what you mean by "ground wire" and "grounded wire".

yeah I missed that, should have been 'grounding wire' not ground wire.

Make sence now?
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06-13-2013, 01:00 PM   #22

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Quote:
 Originally Posted by Gac66610 Correct, but if the ground wire is touching the grounded wire, on the load side of a GFCI, will it not trip?
Yes it will. Modern GFCIs can detect a low resistance connection between the grounded and grounding conductors on the load side that will result in immediate tripping with no load required.
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06-13-2013, 01:10 PM   #23
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Quote:
 Originally Posted by HouseHelper Yes it will. Modern GFCIs can detect a low resistance connection between the grounded and grounding conductors on the load side that will result in immediate tripping with no load required.
House Helper.... Boy I thought I sorta understood GFI (not litterly its circuitry) but now I'm mixed up.

I thought the GFI just looked for exact current comming "in" and exact current going "out" on the neutral.... and if unequal it trips..... so how can it trip if there is no load (ie current flowing)

Thanks

Peter
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06-13-2013, 01:29 PM   #24

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Quote:
 Originally Posted by MTN REMODEL LLC House Helper.... Boy I thought I sorta understood GFI (not litterly its circuitry) but now I'm mixed up. I thought the GFI just looked for exact current comming "in" and exact current going "out" on the neutral.... and if unequal it trips..... so how can it trip if there is no load (ie current flowing) Thanks Peter
On a neutral (grounded) - ground (grounding) conductor fault, yes. There is a small induced current applied to the load neutral at 120Hz. If there is a low resistance connection between the neutral and ground on the load side, current flow results through the neutral via the service panel N-G connection. Since there is no concurrent flow on the hot, the GFI sensor causes the device to trip.
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06-13-2013, 03:12 PM   #25
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Quote:
 Originally Posted by HouseHelper On a neutral (grounded) - ground (grounding) conductor fault, yes. There is a small induced current applied to the load neutral at 120Hz. If there is a low resistance connection between the neutral and ground on the load side, current flow results through the neutral via the service panel N-G connection. Since there is no concurrent flow on the hot, the GFI sensor causes the device to trip.
Wow..... never knew that at all....THANKS!

Is that a relatively new function in GFI's or has it always been around?

And if I am understanding correctly, with noload, that GFI would just keep tripping when you try to reset it.

And it's just an extra check of a GFI to make sure you don't have a potential hot or bootleg ground???

Best

Peter
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06-13-2013, 05:55 PM   #26
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Quote:
 Originally Posted by MTN REMODEL LLC House Helper.... Boy I thought I sorta understood GFI (not litterly its circuitry) but now I'm mixed up. Thanks Peter
I can understand your confusion !
It is a complex subject, and to understand it requires
understanding of mains wiring amd how the neutral and ground
systems function.
no short order.
But to be put simplist, for gfci's to function reliably and
underall circumstances, it is best practice and code in some area's
that the neutral line and the earth lines are both seperate,
Only one ground point should be used (at the main panel)
and only one bond between neutral and earth (again at main panel).

A short breif explanation - Because the earth circuit is in parrallel
with the neutral line, if a secondary earth comes into use
some of the current that should return via the neutral line ends
up returning via the secondary earth, this will trip the gfci's.

Now it doesnt happen under all circumstances but it can under some.

So best practice / and sometimes code as well requires seperate neutral,
seperate earth, and only one bond, as well as only one main earth.

Last edited by dmxtothemax; 06-13-2013 at 08:57 PM.

06-13-2013, 08:31 PM   #27

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Quote:
 Originally Posted by MTN REMODEL LLC Wow..... never knew that at all....THANKS! Is that a relatively new function in GFI's or has it always been around? And if I am understanding correctly, with noload, that GFI would just keep tripping when you try to reset it. And it's just an extra check of a GFI to make sure you don't have a potential hot or bootleg ground??? Best Peter
Since this tripping mode relies on the connection of the grounding wire back to the service panel, it will not work on a bootleg ground where there is no true ground.
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06-14-2013, 04:10 AM   #28
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Quote:
 Originally Posted by MTN REMODEL LLC Boy I thought I sorta understood GFI
Me too. I guess we can disregard what I wrote if it contradicts any of this, I didn't realize it.

06-14-2013, 05:37 AM   #29

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Quote:
 Originally Posted by HouseHelper Since this tripping mode relies on the connection of the grounding wire back to the service panel, it will not work on a bootleg ground where there is no true ground.
There is alot of misinformation in this thread. I was going to respond to DMX's post, but it is so full of wrong I didn't know where to start. Just ignore his post.

HH, a GFI will function (trip) without a grounding conductor (ground). It will not function without a groundED conductor (neutral). This is the reason one of the acceptable methods of replacing a receptacle on an ungrounded circuit is to use a GFCI.
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06-14-2013, 08:03 AM   #30
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I would like to know how you induce a small current on the neutral without using any current from the hot.

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