K/T - Voltage on ground


Old 10-02-11, 07:11 PM
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Join Date: Aug 2011
Location: USA
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K/T - Voltage on ground

I'm ridding my house of the last of the knob and tube, and came across something that made me curious.

I have a light fixture hanging in the hallway that is fed by knob and tube. When I took the fixture down, I found a separate piece of green #12 tied to the ground screw of the fixture. When I opened the wall, I found the hot and the neutral from the K/T, and also the green #12. The green #12 had a wire nut and was just dangling in the wall attached to nothing (obviously the fixture was not grounded).

I tested the green #12 and it read 38volts AC.

My questions are:

1. Why is this green #12 reading 38volts?

2. If I wasn't getting rid of the wire soon, would this be a concern, as those 38volts currently have no where to go? Wouldn't this make the fixture plate "hot"?

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Old 10-02-11, 07:36 PM
Join Date: Mar 2006
Location: Wet side of Washington state.
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Where were you measuring the green wire to and were you using a digital voltmeter? If yes, then what you were reading was a "phantom" voltage. Do a Google for more information.
Old 10-02-11, 07:37 PM
ray2047's Avatar
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If you were using a digital volt meter it was probably ghost voltage. That is voltage induced from near by wires. Test with an analog tester and you will probably get 0 volts.

Dang! Furd types faster.
Old 10-03-11, 10:17 AM
Join Date: Mar 2010
Location: USA
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Another way to see if voltage is not phantom is to connect a small incandescent lamp (7 to 25 watts) between the wire in question and neutral and repeating the voltage measurement. The lamp socket might have alligator clips at the ends of its two leads to make quick connections with.

If the voltage goes to zero then it is phantom.
Old 10-03-11, 03:31 PM
Justin Smith's Avatar
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If it's bootlegged off something there could also be a switched neutral.
Old 10-03-11, 04:19 PM
SD515's Avatar
Join Date: Aug 2011
Location: Michigan, USA
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"I tested the green #12 and it read 38volts AC. "

Tested it how? What was the other reference point?

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