Hi,
How long does it take for a typical 120V-15A circuit to de-energize ? I was using a voltage detector (the type that detects an electrical field when you run it over a wire) to check if a wire was still energized before i did some work and it was detecting a field, even though the breaker was off. The tester beeped for about 20s – 30s before it stopped. I assume that the residual electricity is minimal once the breaker is tripped but enough to still detect an electrical field ?
Thanks
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Replies
The field from that circuit is gone immediately. But that is the nature of those. You have to get a fell for their sensitivity. But also don't rely only on that indicator. Back it up with a voltage measurement or something connected to that circuit (if was working before starting the work).
I replaced a ceiling fan. From the CB labels it was not clear which one it was on. I tried using one of the cheap circuit breaker identifiers, but the light was on a dimmer and it don't work through a dimmer. I finally found it by flipping breakers until the lights went out and running up and down the stairs MANY TIMES. However, my sensor still indicated that it was hot. And there where no other circuits in the box.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
maybe it was detecting the magnetic field from the magnets in the fan motor. Or maybe the motor was turning a little and creating an electric current.
Anyways, thanks for your input. I don't actually use it to test if a circuit is live. I usually use it to see if i have any wires close by before drilling or screwing into a stud.
My 9-year-old daughter and i were watching a reno show this morning and the contractor was using one of these testers. She was wondering why it was beeping.
I showed her my tester and put it against a switch activated light. It started to beep. I told her if I flick off the switch, there would be no more electricity to the lamp, and it would stop beeping. It didn't :-(. She said i didn't know what i was doing :-)
It did stop beeping about 20s later so i was wondering if there were any residual fields in the wire.
It's very unlikely that there would be magnets in a ceiling fan motor -- or any other AC motor. Induction/repulsion (squirrel cage) motors are much cheaper to make. If there were, they wouldn't affect the detector. Even big DC motors don't use permanent magnets, it's only small DC motors.
Those detectors are very sensitive to static electricity, though. Rub it on your sleeve, pull it away, and then touch it back to your sleeve. It should give you a beep. I do that when I use one, as a test to be sure it's working. If it passes that test, it's much more likely to give you a false positive than a false negative.
The other thing it'll detect is capacitance coupling from a real hot wire to a floating wire - like a three way runner.
As for the lamp, if you were upstream from the switch, you'd have both hot and neutral. On a zip cord, my Greenlee will detect which side is which. On the downstream (lamp) side of an in-cord switch, you should detect cold with the light off. If it tests hot, the switch is on the neutral side. Reversing the plug in the wall will fix that. (But it's probably too late to convince the kid.... ;-) )
-- J.S.
They also detect neutrals carrying current. If neutrals are combined it'll never stop beeping.
..and yes, my tester is sensitive to static electricity...it beeps when i rub my hair and put it close to my head.
..and there is also a live circuit running parallel to this one so it might have been picking up the electric field from that circuit.
..and it did not pick up any current on my DC cordless drill.
The "sniffers" can be fooled easily. I've had them register a positive when the arm holding it was next to a live circuit.
If a live circuit runs parallel to the one you're checking, that can also cause a false positive.
I'm surprised nobody responded that you should check to see if the wires are running uphill, since it takes longer for the electrons to drain out that way. Humor quotient seems lower than usual. ;-)
Mike Hennessy
Pittsburgh, PA
Well actually, there is a dip in the wiring at that location so the electrons are probably just pooling at the bottom. Can I add a drain of some kind :-)
good thot.
I usually unscrew a light in the basement for the system to drain ...
sucks when ya gotta run back up to the attic to crack a bulb up there to bleed the air outta the system after recharging it though.
Jeff Buck Construction
Artistry In Carpentry
Pittsburgh Pa
"I usually unscrew a light in the basement for the system to drain ..."Don't forget to put in a vacuum breaker or you might be siphoning contaminated electrons in from the branch lines. :)
BruceT
Any chance you were reading back voltage from a dimmer circuit?
Jeff
I had one the other day.
The house is little more then a shell at this point.
Needed to take an outlet out of the way.
pulled the fuse- still juice.
pulled all four fuses still says I've got juice.
Creative framing around it, to get the wall done.
But it got me wondering.
no dimmers around.
I'm fairly sure it was picking up the current from the live wire running nearby. if i ran my meter further away on my cable, it stopped beeping
Edited 5/15/2008 10:28 pm ET by Chucky