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I while ago I asked this question while trying to install a smart switch. What's going on with this switch wiring?

The previous dimmer is a standard dimmer and it works fine. Lights have been incandescent and LED with no issues on that dimmer. New dimmer doesn't require neutral but does require ground. Would work at that location but worked in other spots.

Older home, old armored cable with 2 conductors and ground is through armor.

When I used a multi-meter to check the wires I got 120v between the two switch wires.

When I checked between each wire to ground (box) I got 70V from one switch wire to ground and 40V on the other.

What could be possibly be going on with this light/switch wiring and how can it be fixed?

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OrganicLawnDIY
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    Possible duplicate of What's going on with this switch wiring? - If you want the earlier question answered, it is usually best to edit the original question and supply information requested in comments there. – RedGrittyBrick Mar 18 '17 at 08:36
  • Don't consider a duplicate. The first question I asked about the switch and the comments suggested the wiring was bad. This question I'm asking about fixing the wiring. – OrganicLawnDIY Mar 18 '17 at 15:52
  • Do you mean that the dimmer would not work at that location? I suspect the armor on the cable is losing its ability to provide a ground... – ThreePhaseEel Mar 18 '17 at 16:06
  • @ThreePhaseEel yes just that location switch worked in other locations in the house. I don't think it's a problem with the cable not being grounded. Since the voltage across the two switch wires is 120V and the voltage from each wire to ground adds up to 120v I think that somehow some of the voltage is getting from the hot wire to the other leg of the switch wire. Maybe it has something to do with the ground but I think something is going on at the ceiling box for the light. Need some advice on what else I can check. – OrganicLawnDIY Mar 18 '17 at 17:04
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    When you say that you "got 70V from one switch and 40V on another", do you mean you're getting 70V from the line wire to ground, and 40V from the load wire to ground? Are you taking the measurements with the wires connected to the switch, and if so, is the switch in the on position? If you have a solid ground, you should always measure ~120 volts from "hot" to ground (minus any voltage drop). – Tester101 Mar 20 '17 at 10:59
  • I'm almost certain that your "ground is through armor" is not working nearly as well as you think it is. – brhans Mar 20 '17 at 16:54
  • @Tester101 yes I edited the question so it's more clear. 70v from one switch wire to ground 40 from the other switch wire to ground. I measured with the switch disconnected so directly on the wires. I've never seen this before. – OrganicLawnDIY Mar 20 '17 at 20:56
  • @OrganicLawnDIY can you check from the "ground" in the box, to a known good ground? Sounds like the "ground" is floating. – Tester101 Mar 20 '17 at 21:49
  • @Tester101 That will take some time but I'll give it a shot when I can. In the meantime can you help me better understand how this might be a floating ground? It's a new concept for me. If the ground was floating wouldn't I still read 120v between one switch wire to ground? – OrganicLawnDIY Mar 21 '17 at 17:27
  • @OrganicLawnDIY A floating ground just means that the ground is not connected to anything. When you measure voltage between the hot and a floating ground (or any floating conductor), the voltage could be anything. This is because the voltage on the floating conductor could be anything. You're measuring the voltage potential between the hot conductor, and some random hunk of metal. – Tester101 Mar 21 '17 at 17:46

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