Getting 12 volts at light fixture with switch off

[tsherrod]

I have a customer who has had recurring issues with a light in her living room. The only erratic behavior I can find after addressing a few potential issues is continual voltage (12v) coming from the switch when it is turned off. The light is wired on a switch loop. Power comes to the fixture and a 3-wire cable (flex) is run from fixture to switch. With the switch on, I get 120v at fixture. With it off I am still getting 12v even after changing the switches. I assume the living room originally had a fan because of the 3-wire run and the double gang box. She only has a light fixture. Although I don't get continuity between the red and black conductors, when one of them is connected to power the other is getting some power even though the red wire is capped at both ends. Would this mean there is likely a knick in black and red wires somewhere between lighting outlet and switch box that is causing power to jump between them? I don't suppose it would cause a short because the unused wiring isn't connected to anything. If so, would the inability of the switch to completely cut voltage affect the operation of the light after an extended period of time, even it if it only 12 volts? Hope that made sense, it was a little harder to explain than I was expecting.

 

[Little Bill]

I would say you are reading a "phantom" voltage. Try checking with a "Wiggy" or solenoid tester. If you are testing the voltage at the switch and the neutral is at the fixture, then most likely you are reading a "phantom" voltage.

Is the switch a lighted switch? That might explain some voltage present with the light off.

As far as the red conductor reading voltage when not connected, it is probably capacitive coupling due to the wire being part of a cable.

In any case, I don't think you have a problem with the light or switch. Unless there are problems you haven't mentioned.

 

[tsherrod]

I would say you are reading a "phantom" voltage. Try checking with a "Wiggy" or solenoid tester. If you are testing the voltage at the switch and the neutral is at the fixture, then most likely you are reading a "phantom" voltage.

Is the switch a lighted switch? That might explain some voltage present with the light off.

As far as the red conductor reading voltage when not connected, it is probably capacitive coupling due to the wire being part of a cable.

In any case, I don't think you have a problem with the light or switch. Unless there are problems you haven't mentioned.

It was enough to set off my NCVT at the light fixture, which is what originally got my attention. My NCVT would not detect voltage under like situations. I was also getting the 12v at the light fixture with it uninstalled. That is a good thought about the capacity coupling. Thanks, hopefully some of the other adjustments I made are sufficient, just wanted to make sure that the voltage I was getting with switch off didn't have anything to do with her issues.

 

[templdl]

It was enough to set off my NCVT at the light fixture, which is what originally got my attention. My NCVT would not detect voltage under like situations. I was also getting the 12v at the light fixture with it uninstalled. That is a good thought about the capacity coupling. Thanks, hopefully some of the other adjustments I made are sufficient, just wanted to make sure that the voltage I was getting with switch off didn't have anything to do with her issues.

If there are any concerns just get a 12v light bulb for a car such as a 1056 an put it across the 2 points you are testing measuring the voltage at the same time to see if the light goes on and if you still read a voltage.

 

[ActionDave]

If there are any concerns just get a 12v light bulb for a car such as a 1056 an put it across the 2 points you are testing measuring the voltage at the same time to see if the light goes on and if you still read a voltage.

I know the reason this would work, but how do you actually get the circuit conductors, light bulb, and meter leads all to come together without having four hands?

I'm not being nit picky. I would like to be able to show people phantom voltage disappearing before their eyes.

 

[GoldDigger]

I know the reason this would work, but how do you actually get the circuit conductors, light bulb, and meter leads all to come together without having four hands?

I'm not being nit picky. I would like to be able to show people phantom voltage disappearing before their eyes.

Put alligator clips on the lamp leads and attach to the voltmeter probes?

 

[templdl]

I know the reason this would work, but how do you actually get the circuit conductors, light bulb, and meter leads all to come together without having four hands?

I'm not being nit picky. I would like to be able to show people phantom voltage disappearing before their eyes.

Excellent point! I have an older Fluke26 that has a complete set of test leads that has removable probes where alligator clips can be plugged on to the ends.. Thus with my meter I can free up my hands when needed.
Since this is something that may run across from time to time from time to time you may want to get an inexpensive socket for the bulb, solder a couple of leads on the socket with alligator clips attached to each lead.
You make this assembly once and would use it when you question if there is a phantom voltage available or not. Then throw it in you tool box. (This is where the old school analog meter with the need display would come in handy. With one of those when using them you would not be able to measure the common phantom voltage because they are low impedance. Using the old wiggy is good at this also.
Shouild there be a phantom voltage by placing the bulb across the circuit the bulb will load the circuit and not light and the voltage will to zero.

 

[GoldDigger]

But for best safety, use a 110V bulb, not a 12V bulb.
It may not light up, but it will provide a fine low impedance load.