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Before drilling a wall (drywall at the surface; not sure what's beneath) separating the bathroom from the corridor, I took a multimeter with a non-contact voltage (NCV) detector to check that the location where I want to drill doesn't have wires underneath.

To my surprise, the NCV beeped for... the entire wall. (Note: for other walls, the NCV beeps only in locations where there are wires inside, i.e. above the outlets or switches up to the ceiling, or sometimes to the left or right of them.)

If I cut the power for the whole apartment, NCV stops beeping. It also stops beeping when I turn off at the service panel just the items labeled “bathroom” and “lights,” however, if I turn off just one of the two, beeping continues.

What could be a possible explanation for that? What could it mean in terms of safety issues?

Note: there was a practically identical question asked before, but the answer which was provided doesn't apply in my case. The multimeter is quite new (bought a few months ago), the batteries should be quite new as well (haven't used the multimeter that much since I bought it).

Comments are not helpful either: I doubt there are metal profiles in the wall, given the location of the wall and how thin it is. If there is water, this could be, I suppose, a troublesome thing. The detector is indeed triggered when in direct contact with the skin, but not within a few millimeters. When I check the wall, I keep the multimeter at a few millimeters as well.

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  • 3
    Then we won’t comment...
    – Solar Mike
    Commented Jul 6, 2019 at 19:08
  • Plaster with steel grid somehow shorting to a live wire?
    – MadHatter
    Commented Jul 8, 2019 at 11:55
  • What happens if you lay a piece of sheet rock, plywood, other material (even thick cardboard) over the wall, and then try? Sometimes if the detectors are too sensitive, giving them a "shim" off the wall like that will reduce sensitivity enough to get only the positive hits where there's actually a wire in the wall.
    – dwizum
    Commented Jul 8, 2019 at 14:52
  • @dwizum: if the plywood is narrow, then the detector beeps for the whole wall. If I put two layers of plywood, then it doesn't beep any longer, neither for the wall, nor for the locations around switches and outlets, and the only location where it beeps is around the service panel.
    – Arseni Mourzenko
    Commented Jul 13, 2019 at 14:15
  • Sounds kinda like your outlets are grounded to the metal electrical boxes themselves instead of having a dedicated ground wire. If you had PVC/plastic electrical boxes behind the outlets it shouldn't conduct the electricity around the outlet like your describing even on a really sensitive digital noncontact voltage pen and those ones typically don't let you attach a ground wire to them because the material is an insulator unlike metal ones which just ground it to the box which could maybe send it through the wall. You might want to look into the cause of the excess voltage for health & safety.
    – David Smith
    Commented Mar 7, 2021 at 2:23

2 Answers 2

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You have an extremely sensitive digital non-contact voltage detector. Buy an analog one and you'll reduce the amount of false positives.

Your current one is obviously detecting AC voltage in the wall, it's just too sensitive.

If you don't know the difference, basically just get one that doesn't beep.

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Sometimes you just get a wall like that. I have similar spot in my house, where detector beeps over several foot of wall. Electrician just laughed at it, but didn't suggest any fixes, so it is probably ok. It may be connected to moisture, grounding wires grounding in wall itself, induction from cables and so on.

If you have a neon voltage probe (the "glowy" thing), you can press it over random points of the wall to see if it detects any voltage (that would be bad).

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