Breaker sets circuit to weird voltage

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  #1  
Old 07-29-08, 01:52 AM
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Breaker sets circuit to weird voltage

Hello folks,

I pulled out a dead dishwasher (won't switch on) and measured the voltage from black to white. It was 120 volts as expected. I turned off the breaker in the main panel, measured again and now it's 36 volts from white to black. White to ground is zero volts, and black to ground is 36 volts. I changed the batteries in my voltmeter. I flipped some related breakers to see if that made a difference in the 120v/36v circuit, but found no effect. I can't find anything else wrong.

Any ideas on how to track down the rogue 36 volts?

Thanks!
 
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Old 07-29-08, 07:44 AM
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Are you using a digital volt meter? I suspect that the answer is yes, and you need to get an analog meter. I think they call what your meter reads, a phantom voltage. Nothing wrong with the circuit. Does sound like a dishwasher problem.
 
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Old 07-29-08, 08:49 AM
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Just a little further explanation. A digital meter put far less load on the circuit then does an analog meter. This is a good thing in electronics when you do not want to load an extremely low current circuit but in electrical work where you deal with higher currents this can be a problem. It is not the meter but the loading of the meter. A digital meter would read the same as an analog meter if it had the same loading on the circuit.

The reason you see a voltage, or at least the probable reason, is that AC can induce a voltage into another wire running adjacent to the wire under test. It also could be that there is a very small leakage in the circuit breaker. It does not take much when you have a very high impedance (resistance) digital meter.

You could make a digital meter work by adding some resistance across the leads. A 47K ohm 1 watt resister would work fine but unless you understand this you probably don't want to be messing around. Just an explanation.
 
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Old 07-29-08, 10:56 PM
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Thanks very much to the both of you. It was gratifying to wake up after a frustrating night and to have the answer waiting for me.

I was indeed using a digital voltmeter. I spent quite a lot on it, now I wonder if I shouldn't have gone analog instead. I have a $5 analog one somewhere in my mounds of junk.
 
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