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I just replaced a pair of single-pole switches in a metal two-gang box and when I went over it with a non-contact voltage tester, they alarmed despite being off.

I have a Fluke 1AC II Non-Contact Voltage Tester that I use to check for live voltage and it started beeping whenever I touched the lower portion of either of the switches despite being in the "Off" position.

Turning the switch "On", causes the detector to detect voltage on the upper half as well and moving it around the faceplate also causes it to alarm as well as the entire body of the other switch (still Off).

I didn't check before I replaced these particular switches, but I tried the same on a few other switches in my apartment and none of them alarmed.

What did I do wrong? Bad ground? Loose connection somewhere?

The apartment in question was built in the 1960s, so there is no dedicated ground wire but the box itself (should be) grounded. The switches are brand-new standard rocker switches from a big box store.

EDIT: Doh! As someone pointed out, when the switch is on, it has current flowing through so this is expected behavior. Why it doesn't happen to any other switch still confounds me. Perhaps a different brand of switch was used with different materials.

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    because there is voltage on half the switch even when off? seems to be working just fine...
    – dandavis
    Dec 15, 2021 at 19:39
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    This is a light switch? If there wasn't voltage on one side of the switch, it couldn't switch anything on. The switch itself is always live; its job is to break the circuit.
    – Tetsujin
    Dec 15, 2021 at 19:41
  • The worry for me is none of the other switches in my home exhibit this behavior. Dec 15, 2021 at 19:58
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    Not a rental, I own the condo. Anything involving moving outlets, new circuits etc does require a number of steps but replacing like for like does not. Dec 15, 2021 at 20:08
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    Get a multi meter.
    – JACK
    Dec 15, 2021 at 21:46

1 Answer 1

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That is 100% correct and normal. The funny thing about AC is that there is current flowing even when the regular circuit is not complete. This has good and bad aspects:

  • Good - You can detect the hot wire on a switch (or just loose, or connected to a broken device, or whatever) even when the switch is off. Which is what you found. That is really good because if you are replacing a dumb switch with a smart switch and the smart switch cares about "line" (hot) vs. "load" (switched hot) then you can turn off the old switch and use a non-contact tester to figure out which wire is which.
  • Bad - The hot wires are dangerous, because if you touch them you could complete a circuit through the ground. Which is why GFCI is a thing.

As to why, my take on it (but an electrical engineer or a physicist (or a mathematician could confirm/deny and explain better) is that the current is going back and forth 60 (Europe 50) times a second and essentially trying to complete a circuit but never quite gets there. Or something like that. I think of it that way.

In any case, what you have reported is normal. The only time you should not get any positive readings with a non-contact tester is if all circuits in a particular location are turned off at the circuit breakers.

Range of a non-contact tester can vary considerably. Touch the tester to a hot wire or screw and it should always show a positive result. Half an inch away? Varies a lot. Through a faceplate? Sometimes yes, sometimes no. Etc. Generally they should be designed to fail safe - report positive even if not quite 100% - rather than the other way around.

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  • I replaced a dumb switch with another dumb switch. Like-for-like except for the color. Dec 15, 2021 at 20:00
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    Voltage doesn’t flow. Voltage is a potential difference between two points. Current and power flow.
    – nobody
    Dec 15, 2021 at 21:51
  • afaik, no current flows in an incomplete circuit. there's voltage, like on battery terminals, but no current to speak of, other than miniscule leakage and coupling.
    – dandavis
    Dec 16, 2021 at 7:34

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