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Hot wire reads 110VAC when switch is off. when switch is turned on, light turns on, but hot wire in and out read 0 when measured to ground. Why is this happing?

Edit: I wanted to put in a smart switch. It would not work. So I started to test the switch that was in place, when I found this problem. I did not install the original switch. I was thinking maybe it is a bad neutral. I will recheck to see if there is voltage across the terminals when the switch is off.

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    A 'dumb' switch doesn't have a neutral - so if you're measuring from one terminal of the switch to the other then you're measuring from hot to switched-hot, and your observation that there's 0V between them is perfectly normal and expected for a closed switch.
    – brhans
    Commented Sep 10, 2022 at 14:27
  • @brhans OP stated they were measuring to ground, which as far as I know should be 120 for one if switch is off, and 120 for both if switch is on.
    – crip659
    Commented Sep 10, 2022 at 14:54
  • The connection is maybe hot > light > neutral > switch > neutral. It should be hot > switch > hot > light > neutral. Test the wires in the light fixture. The incoming hot should go to the switch first, then back to the light.
    – user19565
    Commented Sep 10, 2022 at 18:50
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    Does this answer your question? Light turns on with no hot wires, but has hot wires after turning light switch off
    – user19565
    Commented Sep 10, 2022 at 22:02

1 Answer 1

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The only way for this to happen is if what you are actually measuring on neutral and the switch is interrupting the neutral. With the switch off, you’ll measure full mains voltage on one side, as conducted through the light. The other side will measure (near) zero volts as it’s connected to ground.

With the switch on, both sides will measure (near) zero volts as they are the neutral.

If you remove the light bulb, the voltage on the switch should disappear as there is no longer a path to hot.

This is obviously dangerous, as one wire to the light is always hot. This is also against any modern electrical code.

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  • @FreeMan There is some weird stuff going on here. It seems like a couple of my comments just disappeared. I've been here about 2 1/2 years and something has changed. I can't even find the comment you are referring to. Maybe I just mis-posted, we'll see. Commented Sep 10, 2022 at 16:01
  • I found your wording confusing. I didn't realize what you were saying until after I made my comment on the question, which I'm pretty sure says the same thing. Maybe you can make a clarity edit.
    – user19565
    Commented Sep 10, 2022 at 18:55
  • @GeorgeAnderson comments can be deleted by mods. Sometimes, they may be flagged by another user to bring them to mod attention. Don't recall what it was - obviously not that critical. Nothing offensive or anything, I think just misplaced, now really misplaced! :)
    – FreeMan
    Commented Sep 10, 2022 at 22:28
  • @FreeMan thanks, the comment I made was just a generic suggestion, I couldn't see anything controversial about it at all. It would just help me to improve my answers and comments if I knew why they were taken down. But maybe it was just bc someone deleted a question.??? who knows? Commented Sep 10, 2022 at 22:34

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