Strange voltage measurements?

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  #1  
Old 11-05-05, 03:08 PM
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Strange voltage measurements?

I have a ceiling receptable box with the light fixture removed. There are two pairs of wires going into the box, currently unattached. One pair has 120v on one side and 0 v on the other. But the second pair has 14v on one side and 14.6v on the other. The wall switch that controls this fixture has 14v on one side and 14.6 on the other.

Can someone suggest what this means...is there a short in the wall somewhere or is there a normal explanation for these voltages.

John
 
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  #2  
Old 11-05-05, 03:18 PM
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It suggests that you are using a digital voltmeter. Ignore the readings. You are reading phantom voltage. Put away your digital voltmeter. Use an analog meter or none at all.
 
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Old 11-05-05, 04:12 PM
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Is this true with all digital meters? I'm using a good one, a Fluke. I'll scout around and see if I have an analog meter that still works, but this is a surprise.

John
 
  #4  
Old 11-05-05, 04:12 PM
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Digital voltmeters are pretty-much useless for home wiring. Read this if you're interested in more on phantom voltage. For the most part, any voltage reading less than 100 volts should be treated as if it was zero. A simple $2 neon circuit tester is a more useful instrument than a voltmeter.
 
  #5  
Old 11-05-05, 05:12 PM
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It suggests you have a switch loop.
Connect the hot black the 14 volt white.
Connect the fixture to the other black and the white that is in the same cable as the hot black.
The switch should have a black and white connected to it.
 
  #6  
Old 11-06-05, 04:56 AM
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The better the digital meter, the worse the problem with 'phantom voltage'.

The reason is that 'phantom voltage' is very real; it is actual voltage imposed on the wire by various inductive, capacitive, and leakage effects.

All voltage sources are characterized by how the voltage changes when current flows. When you are looking at a properly connected circuit, the 'voltage drop' is slight, and many amps have to flow before the voltage drops much.

But with a 'phantom voltage' source, there are only microamps available, or less. The current flow is so small that as soon as you place any sort of load on it, the voltage will drop right to zero.

The better the digital meter, the less the current it draws to make a measurement. Old analog meters draw significant current to make a measurement, and thus 'drain' away the phantom voltage. But a _very_ good analog meter would show phantom voltage, just like a digital meter.

In industrial environments with high voltages and long control lines, 'phantom voltage' can be enough to cause a noticeable shock.

-Jon
 
  #7  
Old 11-06-05, 07:17 AM
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I work on center pivots and use a fluke meter (does 187 sound right). Common problem solving for these systems is continuity testing. I have found that in a quarter mile machine you canít trouble shoot the system looking for power because you will get 120 volts of induced or phantom voltage. I have never tried an analog meter and wonít because my trouble shooting is better done with the power off anyway.

However, we often bury a guidance wire that is a couple of miles long and a five dollar analog meter works better to tests its continuity than my three hundred and fifty dollar fluke.

Anyway, not really on subject other than I have also seen induced or phantom voltage.

Keep in mind though that a cheap meter probably does not carry the same safety rating as the fluke. I have seen videos of people who have tested live circuits and had a meter blow up in their faces. These people where severely disfigured and some even lost limbs to the burns. This is pretty important to me as most of my work in on 460 volts, but I would bet that is important on any voltage. My meter and leads are cat three and 1000 volt certified.
 
  #8  
Old 11-06-05, 02:28 PM
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Thanks everyone for the explanations, I understand what phantom voltage is now. Unfortunately I have two brand new looking analog meters(Radio Shack) and neither of them work.

Joed, talking about the switch loop...I think you left out a crucial word in your second line but if I understand what you said its to: 1) connect the 120v black to the 14v white, 2) connect the fixture to the 14v black and the 0v white, and 3) make sure the switch has a black and a white going to it. This being a very old house I hadn't noticed anything but black wires in these pairs, but I'll look closely and see if I can distinguish one from the other.

John
 
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