Stray Voltage Puzzler

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jeff43222

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Minneapolis, Minnesota
I just got back from a service call where the initial problem was that the homeowner found sparks jumping between a strap connected to a metal drain pipe and some EMT, and he also said that the lights were dimming. He separated the strap and the EMT and wrapped the EMT near the strap with electrical tape, then called me in.

My voltmeter showed about 30V between the drain pipe and the EMT (as well as 30V between the shower faucet and the drain), and I managed to duplicate the sparking when I shorted the two with a jumper wire. When I did that, the lights went from dim to full power. While I was scratching my head, a roommate came home and turned on a light upstairs, making the lights even dimmer, and when I checked the voltage between the drain and the EMT again, this time it was more than 50V.

I searched through the entire house and didn't find any visible connection between the plumbing and the wiring. Only one circuit is involved; the stray voltage disappears when the fuse is pulled.

What puzzles me here is how shorting the drain with the EMT results in full power back to the outlets. If there's a hot wire touching the drain somewhere causing a high-resistance short, wouldn't shorting the stray voltage result in less voltage to the outlets, rather than more?

My working theory at this point is that a neutral is touching the drain pipe somewhere and is preventing some of the current in the circuit from returning to the panel. Shorting the drain and the EMT provides a complete path back to the panel, so all the lights go back to full power. Alternately, I'm thinking that maybe an isolated neutral is inducing voltage on the drain pipe. It wouldn't surprise me if this is the case, as I saw plenty of hack wiring in the house, including at least one case of a lone current-carrying conductor entering a device.

Anyone want to weigh in on my theories and suggest their own?

In the meantime, I'd like to make the situation safer at least temporarily, so I was thinking that bonding the drain pipe to the grounded cold-water pipes or the grounded EMT would work. Long-term, my thought is that it's probably not worth trying to find the source of the problem, as that would likely involve tearing open a lot of walls (the circuit has plenty of outlets in the basement, first floor, and second floor). I told the homeowner it's probably best to cut the circuit at the source and start over with new wiring to replace the stuff that gets cut.
 
Re: Stray Voltage Puzzler

Based on the info I would suspect a loose or open neutral. The drain pipe is providing the neutral currents over the EMT back to the panel completing the circuit for the open neutral.
 
Re: Stray Voltage Puzzler

One last thing, I would fully recommend investigating this. Tell the owner this is a T&M job and that the issue at hand is not safe. My concern is that if you just "bandage" a serious issue and something happens, you may be held liable. If the owner declines a complete repair I would note it on the bill that "customer declined full repair and was advised the issue was unsafe." This may help cover you if something happens.
 
Re: Stray Voltage Puzzler

I'm thinking there must be a loose connection between the neutral and the drain pipe, rather than an open neutral, as the circuit does work, albeit not very well. An open neutral should result in lights not working at all.

I had a long talk with the homeowner, and he is just as concerned about this situation as I am (the homeowner is the father of one of the tenants of the house). He understands that my bonding idea is just a temp fix, and he is interested in making things safe on a more permanent basis. He wants me to work up an estimate to fix the situation properly via rewiring, but his options range between expensive and very expensive. The house has 60A fused service (with the original 1928 vintage fuse box and lots of original K&T still in use), and the offending circuit runs through several bedrooms, thus requiring AFCI protection for anything new I install in them. Swapping out the panel or upgrading the service triggers a "housing maintenance" ordinance that requires the whole house be inspected and brought up to city code (not the current NEC, though). Without even looking very hard, I saw all kinds of stuff that would have to also be done to comply with the city code.

I did tell him I really couldn't give him an estimate for finding the problem and that it would likely be cheaper and more effective to cut the circuit and do a re-wire job.

[ June 14, 2005, 11:57 PM: Message edited by: jeff43222 ]
 
Re: Stray Voltage Puzzler

When it comes to repairs, I tell the owner strait up that its Time and Materials, maybe an approximate time, but NO Guarantee on actual time or cost.

I realize there may be a lot to bring up to code for city purposes so I would just recommend finding the actual problem and correcting it and then offer to fix other things later when the person has the money.
 
Re: Stray Voltage Puzzler

stud doesnt that depend on the violation and the inspector if the single family residence is fed by only a 60a main cant the inspector if he so wanted force you to upgrade the main size?
 
Re: Stray Voltage Puzzler

Sure the inspector might depending on the city ordinances.

I wasn't talking about pulling a permit (yes it might be required). I was talking about finding the problem such as a loose neutral and fixing that problem, not new installations. Tighening a loose terminal or connection doesn't require a permit in most cases. Then after thats fixed if the owner has the money do the other upgrades needed.
 
Re: Stray Voltage Puzzler

I agree with the loose or open neutral. There must be another connection between the EMT and drain that makes the "neutral" connection without your jumper, but it has high resistance, which is why the voltage there goes up when more lights are on. I suspect two problems. The open neutral that is making the neutral current go on the grounding path, and someplace a not great bonding that is causing the large resistance in the grounding path.
 
Re: Stray Voltage Puzzler

Fixing the problem wouldn't require a permit, if I limit the work just to a repair. But the problem is a neutral wire buried in a wall somewhere in the house, so the only way to repair it would involve tearing open a lot of plaster/lath walls and then repairing the walls. If the problem is solved by running a new circuit, it would definitely require a permit and upgrade.

There's plenty of K&T, Romex, and BX in the house. Hard to say which type of wire might be causing the problem, as the Romex I saw was not done to code, the K&T was frayed entering the metal boxes I saw (no grommets, bushings, etc.), and the BX was rusty in some spots.

The homeowner said the problem started when a new roommate plugged in his big air conditioner and blew a fuse. I'm guessing a decaying neutral wire that was touching the drain pipe lost its last bit of insulation between the conductor and the drain pipe as a result of the overload.
 
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