Switched Circuit "lightly" energized?

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Old 01-25-10, 12:04 PM
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Switched Circuit "lightly" energized?

I was changing out an outdoor circuit recepticle that was non-GFCI to a GFCI recp. When I put a circuit "sniffer" on it, it light up. When I put a circuit tester on it I got nothing so I assumed the tester was bad (confirmed it wasn't in another recep)...Turned off sub panel and off course the sniffer got nothing nor did tester. I found out later that that leg was on a switch and when I turned on the switch on the sniffer and tester both went off. When I turned switch off, sniffer still went off but again tester didnt.

So my question, It appears there is enough current in a switch leg to set off a sniffer but not a tester? Is that consistant? Why would a switch leg have anythign going through it, is it the nuetral is carrying something all the time? Or is my sniffer to sensitive (does not go off anywhere else...).
 
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Old 01-25-10, 03:57 PM
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When in doubt, use a real meter. Sniffers can pick up induced voltages that are actually ghosts from elsewhere.
 
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Old 01-25-10, 04:23 PM
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What you found is one of the reasons that some people do not like the non-contact detector. It can read voltage from nearby cables and give a false positive.
 
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Old 01-26-10, 12:36 PM
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Originally Posted by Just Bill View Post
When in doubt, use a real meter. Sniffers can pick up induced voltages that are actually ghosts from elsewhere.
I figured it was a "ghost" as the cable, the box and the conduit all showed energized too. But when I turned off the breaker for the switch leg the Ghost went away? How is it that the voltage can be induced if in romex?? Is that from induction through the nuetral that is part of the switch?
 
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Old 01-26-10, 11:25 PM
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Those sensors are good for a quick survey but as Bill said a real tester is the only way to be sure.

The sniffers simply sense electromagnetic interference - the same stuff that causes a hum on your radio or unwanted flickering on your antenna TV when it's too close to other electric equipment. Radio waves themselves are more or less a controlled form of EMI, generated by modulated AC waveforms at the transmitter. Any wire with AC current on it generates a certain level of EMI, and the sniffers are designed to "tune in" to that EMI and alert you when it is detected.

Your wiring may provide some shielding, but not much - the circuit sniffers are strong enough to pick up the signal through the wire insulation. You might be picking up a residual interference signal from the neutral or ground wire, or as pcboss said, near by energized cables.

Either way, if you have a real voltmeter that shows voltage on a known live circuit, you can most likely trust that tester to let you know when the voltage is safely off.

fm
 
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Old 01-27-10, 04:02 AM
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I've had mine go off while in my pocket due to static.
 
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Old 01-27-10, 07:07 AM
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You can also make them go off my rubbing them in your hair.
Or standing under some high power lines. Now that is a BIG electromagnetic field.

False positives are not really a big deal unless you are troubleshooting. False negatives, on the other hand, can be deadly.
 
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Old 01-31-10, 09:23 PM
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The neutral is a current carrying conductor. You could have been detecting a voltage on the neutral conductor caused by something else on the line side of the outlet you were working on. If you have a meter, try checking for voltage between neutral and ground. I saw this once before and read about 70 volts to ground.
 
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Old 02-05-10, 09:57 AM
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At what point do you "feel it" or get bit? If it is picking up something small, I wouldnt feel it? I know how 120v feels
 
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Old 02-05-10, 10:27 AM
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It depends on the person. I can feel telephone voltage which I think is about 48v. Others can feel even less. To me 120 volts does not hurt but is just uncomfortable. 277 volts? Yeah, that hurts!
 
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