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Old 05-06-2010, 06:41 PM   #1
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Metal roof energized (showing 29 volts to ground)


My uncle recently had a metal roof installed. I was doing some minor electrical work and cut the breaker to the fixture. My non-contact meter kept on picking up voltage from somewhere. I quickly realized that it was coming from the roof. My meter could sense the voltage under the sheathing and where the screws penetrated the sheathing. I checked the voltage going from the bottom of a screw to ground, and my meter read 29 volts. The only thing I can think of is that a screw from the roof penetrated a wire. I isolated the breaker that is feeding voltage to the roof. Unfortunately, there is no attic access to the area it feeds. Has anyone ever seen this before? Does anyone have tips on fixing the problem or finding the damaged wire? I'm gong to start by finding out what circuits the breaker feeds. I will then try to see if I can isolate the wire and possibly just disconnect it and run a new one.

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Old 05-06-2010, 06:56 PM   #2
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Metal roof energized (showing 29 volts to ground)


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My non-contact meter kept on picking up voltage from somewhere.
my meter read 29 volts.
Load the voltage down with an incand. bulb; if it stays at 29 this needs to be fixed. Otherwise it is a phantom voltage but it would still be good to find out how this is happening.

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Old 05-07-2010, 02:40 PM   #3
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Metal roof energized (showing 29 volts to ground)


Does he happen to have a satalite dish or power antenna on the roof? You can always bond the roofing to the grounding electrode and rid it of stray voltages.

Last edited by codeone; 05-07-2010 at 09:28 PM. Reason: spelling
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Old 05-07-2010, 09:17 PM   #4
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Metal roof energized (showing 29 volts to ground)


Thanks. I put a load between one of the screws and ground, and the voltage went away.
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Old 05-07-2010, 10:15 PM   #5
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Metal roof energized (showing 29 volts to ground)


Modern voltmeters have a very high input resistance, which means very little charge on something can translate to high voltages. You always have to put a load on the voltage to see if there is any power behind the voltage. And in your case there is not.
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Old 05-07-2010, 10:33 PM   #6
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Metal roof energized (showing 29 volts to ground)


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Thanks. I put a load between one of the screws and ground, and the voltage went away.
I would still bond the metal roof to ground (at the panel). Large areas of ground-isolated metal make me nervous. There are too many ways they can become energized. If there was one specific breaker that was responsible for the "phantom" voltage, then there may be a screw partially penetrating some insulation somewhere, and it may wear through in the future. That makes it even more important to bond the roof.

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