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I just moved into a new home. The living room has a covered up switch and a covered up receptacle nearby. Since there is no ceiling light, I presume this used to be a switch-controlled receptacle.

It was wired as shown below, with all of the hot wires capped off, and the neutral wires joined at the switch, but capped at the receptacle. All ground wires were properly screwed to the boxes.

wiring diagram

I verified this by hooking up a switch to the two hot wires in the switch box, and finding that the hot wire in the receptacle would carry current if and only if the switch is on.

Now things get weird. If I turn off the breaker, all wires in the switch die. The hot & neutral in the receptacle also die, but the receptacle box itself beeps using a non-contact voltage tester! I would use a multi-meter to test for a false-positive, but I'm scared of uncapping the wires to make contact if there is still current. Also see there are nails going through the receptacle box that make contact with ground.

receptacle

But here's the weirdest part. If I turn off a completely separate breaker for the basement lights, the receptacle's hot & neutral start beeping consistently, regardless of the switch! I have looked closely at all of the wires running through the basement for this separate breaker, and at no point should they be mixing (although they do run close by to one another).

How can it be that turning a different breaker off causes the wires to consistently turn on? And why was the receptacle box beeping in the first place? Is there a safe test I can do with my multi-meter?

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    Voltage detectors may be responding to low level voltages present everywhere, such as from a transmitter (WiFi, radio, baby monitor, etc.) Do you have access to a voltmeter? – wallyk Oct 2 '16 at 20:58
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    Well this is embarrassing. It turns out the real difference between the two conditions I was testing was leaning against a nearby sofa. The static discharge from all the dog hair etc was enough to set the thing if I was leaning against it. Moving the couch out of the way removed the weird behaviour. Thanks for the help! – Aaron Voelker Oct 2 '16 at 21:35

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