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I had a bulb burn out, and when I went to replace it, the new bulb didn't work. I put a multimeter on the socket to test it and I'm getting 20V instead of 120V.

There are no other electrical problems that I know of in the house.

Any suggestions about why this could be?

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Was the light that "burned out" an incandescent bulb, or some other type of bulb? Is it possible this is "low voltage" lighting? –  Tester101 Apr 15 '13 at 15:37
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2 Answers

BMitch's answer is most likely correct, but there is another possibility as well, although it's unlikely. If the breaker tripped and the line feeding the socket follows parallel to another live line, then you can get a small amount of voltage on the line. They basically act like a very badly designed transformer. Although it won't have any real power behind it. I've seen this happen a few times before and with voltages as high as 70 volts. But again it's more of an odd occurrence than anything though, but something to think about if you rule out the usual suspects.

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How can I determine if this is the problem, is there a test I can perform? –  Tester101 Apr 15 '13 at 11:40
    
Well the easiest way is to make sure that your circuit is working. If other plugs/devices on the circuit work, then it's not the problem I mentioned. The problem I mentioned will only happen on a circuit that has been shut off, not one that is on. –  Zipper Apr 15 '13 at 15:25
    
I understand that, but there must be a way to verify if this is taking place. If I measure 20V on a dead line, how can I figure out if it's induced or not? –  Tester101 Apr 15 '13 at 15:35
    
Well the fact it's on a dead line basically proves it's being induced. There is always some very remote chance you have something else happening, but assuming the line is actually dead, if you see voltage on it, it's almost guaranteed that it's being induced. The only other things you could do, would be to put a small load across it and see what happens. If it really is induced then it won't have any power behind it so basically nothing will happen, but if it's coming from something else that does have power behind it, it will heat up the load. –  Zipper Apr 15 '13 at 15:46
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You've got a bad connection or short somewhere. Turn off the power to that socket until you locate the issue since this can be a fire hazard. To locate the bad connection, you'll need to open up the switch and light fixture and test the voltage at various points. You need to follow proper safety procedures, like shutting off the power so you aren't working around live electrical lines, except for when you're testing the voltage.

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If any switch or outlets along the path of supplied power uses push-in type connections, this is the likely cause of the bad connection. Moving the wire to the screw binding posts will solve such a problem. –  bcworkz Apr 15 '13 at 23:59
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