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Mystery voltage at light receptacle
Old 03-10-2010, 10:06 AM   #1
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Mystery voltage at light receptacle

Here's another home repair mystery for the consideration of the electrically inclined:

Yesterday I installed a replacement light fixture above our garage door. When I checked the voltage (to see if it was "cold" with the switch turned off) it read 122 V with the switch on and 55V with the switch off. I turned off the breaker and completed the installation of the new fixture.

As before, the light works fine, and an incandescent bulb doesn't illuminate (even very dimly) with the switch turned "off." There's no voltage between either wire and ground with the switch turned off.

Any suggestions/common causes for this that I should investigate? I just can't imagine where 55 volts could be coming from, unless it is 120V through a really high-resistance path. And why doesn't the lamp illuminate with the switch off?
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Old 03-10-2010, 10:16 AM   #2
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What happens if you disconnect the switch (rather than just shut it off)?
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Old 03-10-2010, 11:26 AM   #3
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Originally Posted by TromboneAl View Post
What happens if you disconnect the switch (rather than just shut it off)?
I haven't tried that yet. Everything does go to zero volts with the circuit breaker turned off.

There were just two wires coming in to the fixture box, so I'm assuming this is a "light at the end of the circuit" situation, not a "switch loop."
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Old 03-10-2010, 11:59 AM   #4
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My guess is the switch is probably dirty, therefore resistive. When you connect the meter on it the loading of the meter makes the voltage drop down.
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Old 03-10-2010, 12:10 PM   #5
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My guess is the switch is probably dirty, therefore resistive. When you connect the meter on it the loading of the meter makes the voltage drop down.
This could also be a case of a 'switch in the neutral leg', which leaves the hot side voltage always present to 'return'. be careful!
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Old 03-10-2010, 01:36 PM   #6
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Like programming the time on the VCR, or hooking up your Wii, I'm sending a 10-year-old kid kid over to help:
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Old 03-10-2010, 06:37 PM   #7
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Sounds like bad house wiring or perhaps a naughty device slowly bleeding capacitance on the same circuit. Just a couple of wild guesses.
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Old 03-10-2010, 06:59 PM   #8
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There's no voltage between either wire and ground with the switch turned off.

Any suggestions/common causes for this that I should investigate? I just can't imagine where 55 volts could be coming from, unless it is 120V through a really high-resistance path. And why doesn't the lamp illuminate with the switch off?
What was your ground reference? It doesn't make sense to have voltage between Hot & Neutral, without any voltage from either to ground. Was the bulb removed when you made those measurements? The bulb is low R, and it will overwhelm any hi-R path.

IOW, your ground may be floating as well.

-ERD50
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Old 03-10-2010, 07:09 PM   #9
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unless it is 120V through a really high-resistance path. And why doesn't the lamp illuminate with the switch off?
That is most likely. I assume that you use a digital voltmeter (DVM). These things typically have a high impedance of 10 Megohm. Even the capacitance between the wires may induce a voltage that is read by the meter. If so, the condition would be safe.

It could also be a high-resistance path caused by humidity. In that case, it is not safe, as the resistance can drop with increasing humidity.

The bulb would not illuminate with the switch off because the current is very much limited by the high-resistance path. Look at it this way: If your DVM has an infinite impedance, it would read 110V. At 10 Megohm, the voltage has dropped down to 55V already. Now, the light bulb has a cold resistance of only a few Ohm. If you try to measure the voltage with the bulb screwed in, you will see that it is not measurable.
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Old 03-10-2010, 08:06 PM   #10
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It is legal to run the hot wire to the fixture then loop to the switch. I hate this type of wiring but it is the way they wired my house too. You have to read up on the wiring and be sure to turn off the circuit breaker. Of course with this type of wiring, you cannot tap the switch for additional receptacles.
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Old 03-10-2010, 08:41 PM   #11
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It is legal to run the hot wire to the fixture then loop to the switch. I hate this type of wiring but it is the way they wired my house too. You have to read up on the wiring and be sure to turn off the circuit breaker. Of course with this type of wiring, you cannot tap the switch for additional receptacles.
I don't think that's the situation in this case. Just two conductors enter the metal box, both connect to the fixture. As far as I can tell, that should mean that the light fixture is at the "end" of the circuit. If there were a "switch loop" I should (!?!) see at least one more set of wires in the box, connected to each other. I think . . .

I'll get out the VOM and do more investigating if the weather is good tomorrow. Thanks to all for the tips and ideas.
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