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I was called to a home today that the ac guys said the ac duct was shocking them as they were installing it. I read the duct to ground and found 5 volts on the system. I turned the power off to the home by pulling the meter, and was still getting 5 volts. Checked it with another volt meter and the same thing. The AC guys say's it was real bad during a storm that came up. Could this electricty be coming from the Earth?
- Danny Smith (age 57)
You've actually got me stumped. It's pretty easy to get some voltage between different metals due to chemical reactions. That's how ordinary batteries work. However, a single-cell battery typically has 1-2 V, so 5 sounds oddly high. Could there be multiple junction between different metals, effectively making some series voltages? Also, 5V wouldn't be enough to feel a shock. Was that a dc voltage? Maybe there's a larger ac voltage.
As a total amateur, not to be trusted in any way, I'd make the following recommendation.
1. Check the current from the ducts to the ground with an ammeter. Any of the standard sorts of voltage sources couldn't supply much current. If you've got more than a few microamps, I'd be pretty worried about where that voltage is coming from and hire an electrician.
2. Assuming there's no alarming current, ground those ac ducts in several places. That should leave everybody much safer.
(published on 07/27/12)
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