# Strange Shared Neutral Problem



## zenmurphy (Dec 2, 2012)

I deal mostly with medium voltage solar power, and it has been a while since I have seen a really tricky problem with household electricity. This afternoon however, I found a problem that has me baffled.

My friend has an arc fault breaker in his new house that has never worked. He says it very rarely holds, sometimes, but very rarely. 

I went to his house today and took apart the receptacle where the homerun comes in. There's two wires coming in, one going to the next receptacle and one heading down to the homerun. After I disconnected all of the wires in the box, I was getting 120 volts from the positive to both the neutral coming in from the homerun and the neutral going out to the next receptacle. This led me to conclude there is a shared neutral and that was what was tripping the arc fault breaker. 

So I came up with a test to figure out which circuit the affected circuit was sharing a neutral with. I isolated the neutral on the affected circuit, and then started disconnecting the neutrals on the neutral bus one by one, figuring that as soon as I pulled the one that was sharing the neutral, then I would get 0 volts between the positive and the isolated neutral feed. 

After disconnecting each neutral one by one, I am still getting 120 volts between the isolated neutral (which should be a completely open wire with no potential as its disconnected from the panel during the whole test) and the hot wire. 

Is my test flawed? It seems to make perfect sense to me electrically. With the affected circuit isolated, I should have had zero volts between the isolated neutral and the hot wire as soon as I disconnect whatever neutral is shared with the circuit.

Can anyone recommend a better way to figure out which circuit is sharing a neutral or tell me how my test could have possibly isolated every neutral in the panel and not found the shared neutral?


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## hardworkingstiff (Jan 22, 2007)

Did you separate the grounds between the 2 cables? If not, how do you know you don't have a neutral to ground connection somewhere downstream? 

How did you determine the neutral was coming from another circuit and not the ground?


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## Bbsound (Dec 16, 2011)

and it would have been easier to turn the breakers off one at a time instead of pulling all the neutrals apart............


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## McClary’s Electrical (Feb 21, 2009)

Lou is right, and that is where your test is flawed. There is a N-G bond downstream, that is not only causing the tripping, but is the reason your still reading 120 volts with your neutral lifted.


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## zenmurphy (Dec 2, 2012)

BBsound... I thought of that but turning off the breakers is not going to isolate the potential from the individual neutrals one of which is shared with the affected circuit. 

I isolated the ground from the affected circuit. With the ground isolated I also tested for continuity between the neutral bus and the isolated neutral and got continuity. Seems like this might mean the ground and the neutral of the homerun wire are connected somewhere in the wire?

I need to do some more testing and then I will ask more questions. It seems like a bond between the ground and the neutral somewhere, but to find out where it seems like I should isolate everything in the circuit and maybe in multiple circuits.


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## EBFD6 (Aug 17, 2008)

hardworkingstiff said:


> Did you separate the grounds between the 2 cables? If not, how do you know you don't have a neutral to ground connection somewhere downstream?
> 
> How did you determine the neutral was coming from another circuit and not the ground?





mcclary's electrical said:


> Lou is right, and that is where your test is flawed. There is a N-G bond downstream, that is not only causing the tripping, but is the reason your still reading 120 volts with your neutral lifted.


Bingo!


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