35 Vac ????

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  #1  
Old 07-31-07, 02:45 PM
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35 Vac ????

I was starting to replace a ceiling light with a fan when I encountered a problem that I have not seen before. There is a 3 wire romex cable running between the ceiling box and the wall switch. With the switch off, I used a Greenlee non contact tester to check for voltage and it registered voltage on the red(switched) wire. I then tested the red (switched) wire and it has approximately 35 VAC with the switch off and 120 VAC with the switch on. If I turn the breaker for this circuit off, the voltage goes away, so there does not appear to be another circuit involved. I have turned off all lights, and disconnected everything on this circuit but the 35 VAC remains. Also there is 0.00 amps registering at the service panel for this circuit with everything turned off/disconnected.

What could cause a low voltage reading like this?
 
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Old 07-31-07, 02:48 PM
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Put away your digital multimeter and use a neon bulb type tester. This reading is called phantom voltage.
 
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Old 07-31-07, 02:52 PM
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Read this:

http://forum.doityourself.com/showthread.php?t=308699&highlight=stray+voltage

It's phantom voltage due too your digital meter. Everything is fine.


++Oops, Ben beat me to it..
 
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Old 07-31-07, 02:54 PM
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This will also cause the non-contact Greenlee to beep?
 
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Old 07-31-07, 03:05 PM
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Never attempt to take measurements of a cable that is not connected. This practice is a waste of time.

Do not use your digital meter for home wiring until and unless you learn how to use it.

Always turn the circuit breaker off for a circuit before working on it. Never trust that a switch turn s the power off. You could be dead wrong.
 
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Old 07-31-07, 03:27 PM
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Originally Posted by racraft View Post
Never attempt to take measurements of a cable that is not connected. This practice is a waste of time. ----- I disagree, suppose you want to know which is the line feed into a junction box. How else could you do it without disconnecting the wires?

Do not use your digital meter for home wiring until and unless you learn how to use it. --- Your stating that since I wasn't familiar with phantom voltage I'm somehow incompetent to use a meter?

Always turn the circuit breaker off for a circuit before working on it. Never trust that a switch turns the power off. You could be dead wrong. --- exactly why I used the non-contact and digital meter to check for voltage.
Your response isn't helpful at all. I thought the purpose of user forums is to assist, not berate.
 
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Old 07-31-07, 03:36 PM
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I am sorry you were offended by my remarks.

When I stated not connected I was referring to the fact that the switch was turned off, not that one end of the cable was loose. I think you misinterpreted what I wrote.

Most people do not know about phantom voltage, until they encounter it and either read about it or someone explains it to them. However, it is explained in the directions for your meter and in just about every book on home wiring.
 
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Old 07-31-07, 03:38 PM
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You asked, "This will also cause the non-contact Greenlee to beep?"

The answer is maybe. Depends on how strong the capacitive coupling is between the wires. Non-contact testers are fun and quite useful in certain situations, but never trust your life to one.

In residential wiring, a digital multimeter is useful for distinguishing between 120 volts and 240 volts, but it's not really good for much else. A reading of approximatly 0 will probably be 0. A reading of approximately 120 will probably be 120. A reading of approximately 240 will probably be 240. All other readings are probably bogus--phantom voltage caused by capacitive coupling. A google search of "phantom voltage" will tell you more than you want to know.

Those $2 neon circuit tester are most useful, assuming you have a known good ground to test against. A non-contact voltage tick tester is very useful when you do not have a known good ground.

Are all your questions now answered. Anything else?
 
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Old 07-31-07, 03:52 PM
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John - thanks for the explanation on phantom voltage. I do have a neon tester and I did use it when I first encountered this problem. It did not light, but since the Greenlee non-contact and the meter showed voltage I wanted a better understanding of what was going on.
 

Last edited by racraft; 07-31-07 at 04:28 PM.
  #10  
Old 08-01-07, 09:12 AM
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Non-contact testers often give false positive readings as they are sensitive enough to be tripped by most any voltage including phantom voltage. It can also be tripped by adjacent wiring in the walls if another circuit is nearby or even completely benign sources like a wristwatch, jewelry or the operator's body. For a quick example, turn your non-contact on and hold the tip between your thumb and forefinger. Pull the tester away quickly, and I bet it will alarm.
 
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