Old switch wiring strange behavior

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Old 01-01-18, 08:38 AM
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Question Old switch wiring strange behavior

I have this very old switch with BX cable coming in. 2 wires in only.

The issue I am having is when the switch on, neither my multimeter nor voltage tester is picking up anything on either of the cables, even on the bare copper side. When I turn off the switch, I can easily pick the hot cable, both the multimeter and voltage tester are picking up values.

To test my sanity, I opened up two other switches and I dont observe this behavior. I understand when the switch is on position (lights are on), both wires are hot. How can they not be in this case?
 
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Old 01-01-18, 09:10 AM
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The reason what you are not picking anything up is because the two wires will have the same potential.

When measuring voltage you are measuring the difference in potential. When the switch is on, the two wires are essentially connected together by the switch so you will get no reading between the two wires. When the switch is off the connection is broken by the switch and you are reading between the hot wire and through the light fixture on the other end which will go through the bulb and connect to neutral.

When measuring voltage I always use a grounded reference point such as ground or neutral. If one is not available I will take a piece of wire and connect it to a known ground.
 
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Old 01-01-18, 09:17 AM
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The voltmeter is going to show 0 volts when the switch is on if you're placing the probes on the two screws because the screws are at the same voltage. That's normal.

By voltage tester you mean your non contact tester? It should chirp on both wires when the switch is on. I can't explain that one unless you have a poltergeist. I'm trying to think of how a cheap NC tester would behave if someone wired up a switched neutral. I can't say I've ever tried it.
 
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Old 01-01-18, 09:23 AM
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Yes, I am talking about non contact tester. It is a pretty decent one (I think I spent $30 on it - it was the most expensive in homedepot) and it doesnt do this anywhere else. It doesnt beep on either of the cables, the screws on the switch, or bare copper around the screws.

It does beep only on the hot wire when the switch is in the off position.
 
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Old 01-01-18, 09:26 AM
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All non-contact testers are only good for a quick check to see if there is a hot wire in the box, they are not reliable for real troubleshooting.

Perhaps the switch is switching a neutral instead of a hot wire. While a violation, and dangerous, it was done back in the day.
 
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Old 01-01-18, 09:26 AM
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Switched neutral is the only thing I can think of, and that's grasping at straws. Maybe unscrew all the bulbs and see if the NC tester stops chirping even when the switch is off.
 
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Old 01-01-18, 09:55 AM
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Could this be why my occupancy sensor did not work here even after getting a ground wire from a known receptacle that I had verified to have ground connection?

This thing is driving me crazy. I spent 2 hours and I cant get the darn thing to work.
 
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Old 01-01-18, 10:05 AM
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A non contact tester will read power on an open neutral if voltage is returning from a load.
 
 

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