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#196546 - 10/12/10 11:40 AM GFCI or AFCI operation
Niko Offline
Member

Registered: 08/17/06
Posts: 358
Loc: Campbell, CA
Will a GFCI or AFCI circuit breaker trip if it only sees a return current on the neutral that is from another circuit due to crossed neutrals in a Jbox.

For example, the GFCI circuit feeds bathroom receptacles but does not have any load on it, however the neutral of this circuit has been crossed with a non protected circuit in the same junction box, with out any load on the GFCI will it trip if it sees a return current on the neutral that is from another circuit?

Thanks
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#196549 - 10/12/10 12:42 PM Re: GFCI or AFCI operation [Re: Niko]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9012
Loc: Estero,Fl,usa
Yes it will trip.

In fact one of the the most common source of mystery AFCI trips is a short from neutral to ground, essentially the same thing. You have some phase current going out that is not matched by the neutral current coming back.
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#196551 - 10/12/10 01:26 PM Re: GFCI or AFCI operation [Re: gfretwell]
Niko Offline
Member

Registered: 08/17/06
Posts: 358
Loc: Campbell, CA
Greg, i agree with you 100% on what you said.

but in this hypothetical situation there is no phase current going out. there is only return current coming back from another circuit.
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#196552 - 10/12/10 02:09 PM Re: GFCI or AFCI operation [Re: Niko]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
Quote:
You have some phase current going out that is not matched by the neutral current coming back.


Zero going out, and non zero coming back. Trip.

Zero going out, zero coming back. NO trip.

Non zero going out. SAME non zero coming back. NO trip.

Non zero going out, different coming back. TRIP

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#196553 - 10/12/10 02:25 PM Re: GFCI or AFCI operation [Re: LarryC]
Niko Offline
Member

Registered: 08/17/06
Posts: 358
Loc: Campbell, CA
Got it thanks a lot.

good explanation i will use it.
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#196558 - 10/12/10 11:27 PM Re: GFCI or AFCI operation [Re: Niko]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9012
Loc: Estero,Fl,usa
A GFCI is really a pretty simple device. There is a small transformer in there (sensing coil) with the "hot" wound one way(top pair of green wires) and the "neutral" wound the other way (bottom pair of green wires. That is the primary. If the current is exactly the same they cancel, there is no flux in the core and the secondary winding has zero volts. Any difference shows up as a secondary voltage and the device trips.




Edited by gfretwell (10/12/10 11:31 PM)
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#196561 - 10/13/10 10:57 AM Re: GFCI or AFCI operation [Re: gfretwell]
Niko Offline
Member

Registered: 08/17/06
Posts: 358
Loc: Campbell, CA
Very cool, thanks Greg.
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