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I swapped out a dimmer switch for another one. This was fairly standard, only two wires to connect, no ground (old condo). Turned the breaker back on, tried things out, the on/off switch worked, the dimmer worked, but playing with the dimmer, there was a short, a brief flash of light, then the breaker tripped. I swapped out the light bulb but I still have no light, so I'm inclined to conclude that the dimmer is fried.

I'm about to get a voltage tester to confirm that the wire is still live, then will try putting on a new dimmer switch.

question: what should I check to make sure this doesn't happen again? could it be debris? I'm really puzzled as to why this happened and would love to make sure I'm not overlooking something as I rewire this.

my wiring: enter image description here enter image description here enter image description here enter image description here enter image description here enter image description here

  • There is a 3rd screw, which you didn't use. Is that for ground or something else? What model # is the new switch? (so we can see the full installation instructions) – manassehkatz Dec 5 '18 at 2:56
  • Can you give us a photo looking squarely into the back of the box? – ThreePhaseEel Dec 5 '18 at 3:32
  • @manassehkatz, many modern dimmers are sold as for three-way also. Saves the cost of packaging and marketing two different products, i guess. – isherwood Dec 5 '18 at 4:01
  • Looks like your neutral is looped backward, btw. It might also go beyond 180 degrees to overlap itself, which isn't great either. – isherwood Dec 5 '18 at 4:01
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    I have confirmed from installation instructions that the empty screw is (as @isherwood suspected) for a 3-way connection. So it looks like you hooked it up correctly, which I expected since it worked for a little while. My guess is that old cloth insulation. Whenever I find that (which is a lot in my house), I wrap it up well with electrical tape as a precaution. It only takes a little bit of exposed hot wire touching the metal box to zap. – manassehkatz Dec 5 '18 at 5:18
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Looking at that old cloth insulation I would guess a short in the box developed when you pushed everything back in, it worked so you know you wired it correctly.

With the breaker off, pull the switch back out, you can wrap the cloth with electrical tape but I prefer heat shrink tubing, I usually carry it to make repairs for cases like this, slip it on and heat and it is much easier to work with than tape wrapped wire. But you probably have a roll of electrical tape, and that will insulate the wire.

  • I very carefully wrapped the wires and the contacts in electrical tape, and made sure the wires weren't getting close to each other as I pushed them back in. Everything has been fine since. Thanks for the help! – Jonathan Protzenko Dec 6 '18 at 2:23
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My first guess would be the switched hot terminal on the switch was in contact with the metal box. If the wiring is cloth covered BX, the metal shield of the BX is a ground path.

I'd make two or three wraps of high quality electrical tape around the switch, covering the terminals. I'd also carefully remove some of the excess plaster, being careful not to crack the visible plaster. It may be the excess plaster is pushing the switch into the box.

My second guess would be with @EdBeal, tape the wires up in case there's a bare spot or nick on those that's coming in contact with the box when you close things up.

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