So i am trying to change my regular 3 way switches to smart ones and before i started taking them out, I took a multi meter to find hot wires and label them.

So with the upstairs 3 way switch ON ( aka light is ON), using the multimeter probes:

Black probe to green ground, red probe to bronze screw with red traveler wire: 120v

Black probe to green ground screw, red probe to black screw ( which i believe is the common) as soon i touched it with the probe, POOOOOOOOOOOF! smoke, bright spark, and the tip of probe got black. Breaker didn't trip tho, the light was still on.... Is there something funky with my wiring? I would assume i should be able to measure voltage this way without any issues.

Or maybe the multi meter is bad? Don't think so because it measures the red wire to 120v just fine without firewoks..

Any ideas?

I can get pictures of both upstairs and downstairs switches later when i get home if that helps, but the wiring seems pretty standard.

  • 4
    Normal measurement will not do that. You accidentally touched two things at once, and they had power between them. Does the meter still work? Is it a Cheese cheapie? Jul 19, 2017 at 21:37
  • Yeah meter still works, its not a expensive Fluke meter, but it isn't a Harbor Freight freebie either, Mastech $30 from Amazon. <br/> Its a 3 gang receptacle, of the switches controls which is on another breaker, controls an outlet across the room. Did the same thing ( probed the black screw ) and poooof again, this time the breaker did trip. Jul 19, 2017 at 21:51

1 Answer 1


Well it's possible you're unrealizingly touching something else, like the shell of the metal junction box, but that would cause arcing damage to your test lead.

Yes. The switch you are probing is defective.

Cracked switches happen all the time. When you lean on it a little bit with the probe, you are causing something internally to make contact and POOF. That's why it's not causing arc damage to your test lead.

Needless to say, stop doing that.

I would first replace that 3-way with a plain 3-way. Since you seem to be replacing several 3-way sets with smart switches, I would do this one second, so you can use the 3-way liberated from your first repair to fix this one.

Do not try to short-circuit (heh) the process and just upgrade the broken switch. It's possible something else is broken and you want to remove that possibility before you proceed, otherwise you'll have a miserable time troubleshooting.

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