I used a voltmeter to test for power on my electrical switch and I was surprised by a blast of electricity. You can see the damage in the picture. Why did I get a reaction? Did I test it incorrectly? Or was it that I seemed to have touched the faceplate?

I want to be sure I can test these for voltage properly in the future and without frying my voltmeter wires, causing a fire or hurting myself.enter image description here


2 Answers 2


You did this while the switch was still in the box, didn't you.

What you didn't know was that the box itself, and thus the metal mounting frame of the switch, us connected to safety ground, and bumping against that while touching hot created a short circuit.

One solution is to use an insulated probe, so it can't make that accidental second connection. Another is to use a non contact voltage sensor, which can pick up house current near its tip without needing any exposed metal; thats what I would recommend for most beginners.

  • Well if the ground wire is connected to the switch, this will ground the metal parts of the switch (including the cover screws), and it looks like your voltmeter probe got too near that. Aug 21, 2016 at 15:32

I've used a cheap voltmeter to do this and blown the back off it. Even some quite pricey meters aren't up to the surges you get on mains. There was a discussion on the last few days at electronics.stackexchange.com regarding regarding the ratings that are needed to do it properly.

The mains testing screwdrivers are better in many ways (a neon lamp in the handle lights up when the tip touches live, but it requires a path to ground through you).

I tend to use both the screwdriver and a decent meter l, plus checking whatever is on the end, if the circuit is unfamiliar (but I know my fuse box was labelled wrong by the electrician who installed it).

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