I have a hallway light fixture controlled by three switches (panel -> 3 way -> 4 way -> 3 way -> fixture). Lights in this fixture routinely burn out within 6 months (incandescent, cfl and led -- all name brand).

The other night I noticed the LED lights had a very faint glow even though the power to the lights had been turned off for hours. I've removed the bulbs from the fixture and measured around 40 volts of stray power at the socket with the power turned off. Flipping a switch returns the voltage to 120. I tried all 8 combinations of switch positions and had the same results.

All three switches are non-illuminated "dumb" switches.

I then removed each of the switches and tested resistance with my multimeter. These are regular n-way switches, not smart or illuminated. The wiring in the boxes is textbook. They each show near 0 and '--' for resistance depending on the switch position.

I'm nearly certain each of the switches are still good but don't know where to go next in troubleshooting. What would cause the stray voltage? Is this likely what is killing light bulbs? Is this dangerous? How can I fix the extra voltage?

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    Put 1 bulb back in the fixture. Does that make the mystery voltage go away? If it does, it's phantom voltage. – Harper - Reinstate Monica Jul 30 '20 at 6:03
  • Are any of the switches illuminated? Other than that I agree with phantom voltages being observed. – Ed Beal Jul 30 '20 at 15:05
  • @EdBeal All three switches are non-illuminated "dumb" switches. Updated question. – psaxton Jul 30 '20 at 15:11
  • @Harper-ReinstateMonica With a single LED bulb in a socket, the voltage drops to near 20V on the other socket. – psaxton Jul 30 '20 at 15:11
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    @EdBeal, I am in a rural suburb in Utah, United States. There are only residential homes on the substation grid my home connects to. The phantom voltage of 40V is measured with no bulbs in the circuit, with one LED bulb, the phantom voltage is 20V. – psaxton Jul 30 '20 at 20:16

The 40V you are measuring is a typical value for a "phantom voltage". It is caused by capacitive coupling between wires, through the insulation, and usually in long runs of parallel wires AFTER the switch.

These couplings are high impedance and usually when connecting any load, the voltage will read 0. However, LED units (bulb & regulator) are high impedance loads at voltages below their operating voltage (e.g. 40V), and so it is possible they consume a bit of power even when the source is a phantom voltage. Regulators are notoriously short lived if they don't have enough voltage to work properly. Ironically, the transistors can overheat.

You can confirm this effect as follows: replace one of the lights with an incandescent (or add an incandescent to the load), and the voltage with the switch OFF should drop to 0.

Also, what voltage do you measure at the socket, when you disconnect the 3-way switch? And the other 3-way switch?

Your other point about the 6 month longevity with all bulbs usually suggests they are running at 240V rather than 120V due to mis-wiring somewhere. So to be sure: you are measuring 120V at the socket, between live and neutral (not live and ground), right?

And another check: you don't have weird issues when switching a 240V appliance on that suddenly lights get brighter, or circuits that don't work start to work? (This means that one of your two phases is not working correctly)

Update based on your 20V measurement: Yes by adding a load the voltage would drop, and since it drops by half (which is a lot), the phantom voltage remains suspect number 1 for the "faint glow". If you screw in something of higher wattage (20W...40W) like an old inc. bulb, I would suspect it to drop to 0V.

Are your LED bulbs dimmable? I appreciate that with your simple on/off switching the bulbs don't have to be dimmable. But, dimmable ones deal better with under voltage, they deliberately switch entirely off below a certain threshold, and that might help you here.

Update based on the LEDs being dimmable: The last thing I can think of is to try a different bulb / unit. Without cutting new holes for recessed/regulated systems, you could try a replacement just by wiring it up without installing it just yet. Not all LED bulbs and drivers are made the same, and you could get lucky (electrically, I mean).

The next solution would be to add a dummy load to the circuit after the switch. This is silly if you are trying to save power, but inc. bulbs are impossible to find. This is why we want to check the voltage (n OFF position) with a regular bulb (if you have one), to make the load a bit higher than with just under-driven LED lights.

Can you measure the voltage between the neutral and the ground, or the neutral and a neutral elsewhere in the house (use an extension cord). Let's eliminate a poor neutral as a possible cause too, since you have mixed problems.

The phantom voltage seems to be coupled after the switch, and so far we have the following possible solutions:

  1. Move the wiring after the switch: this is lots of work, likely involves drywall repair, so definitely a last resort
  2. Replace the lighting: this is a bit of a draw of luck, but some LED bulb & box regulators or integrated LED bulbs may have a threshold shutoff below which the LED switches itself off, rather than leaving it on, "leaking" and leaving a glow.
  3. Insert a relay in the circuit right at the socket.

The relay has 120V switching poles (say 5A) and a 120V coil, and must be placed inside an electrical box. It will be wired so that the switch side of the relay simply connects the relay's feed to the bulb. It works as follows: if the voltage is too low, the relay is off, and the LED is cut off from the low voltage. If the voltage is high enough, the relay is on, and the bulb is fed the hight voltage.

There is the possibility that the relay too will switch on as a result of the phantom voltage. Should this be the case, we can experiment with a resistor placed before the relay, so that the relay's feed voltage (but not the LED's) is lowered enough to not trigger on the phantom voltage. It requires a bit of experimentation. Should you like this option, I can sketch you a diagram.

  • Thank you for the prompt response. I did not try measuring the voltage at the socket with the switches removed, assuming it would be 0 with the circuit physically broken (I was assuming a bridged contact in one of the switches at the time). I definitely measured 120V from hot to neutral (not ground) at the socket and don't have any problems with 240V appliances starting or stopping. I will try measuring the socket voltage with a switch removed tomorrow as well as pairing an incandescent with an LED bulb. – psaxton Jul 30 '20 at 5:50
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    I doubt any 120v incandescent bulb would last more than a second at 240v. LED drivers MIGHT be more adaptable if designed for it, but I doubt it. And while I agree with most of your answer, technically phantom voltages in long, parallel runs are caused by induction, not capacitance. – George Anderson Jul 30 '20 at 13:19
  • @GeorgeAnderson capacitive coupling is not uncommon, and induction could be part of it too. Cause doesn't matter here. Also 120V bulbs (inc. & CFL) don't die immediately, and depending on their usage and the casual mentioning of 6 months I cannot dismiss this possible cause. However, the OP's double checking now confirmed this is not an issue. – P2000 Jul 30 '20 at 14:48
  • I agree with George that residential voltage and currents it is inductive coupling. Doubtful it is 240v If a light works more than a second it is probably a universal voltage 90-277 with the LED’s I purchase. Power spikes in the area are more likely the cause of short life electronics where a hole house suppression system added at the main panel has saved my customers thousands compared to what they were spending. – Ed Beal Jul 30 '20 at 15:10
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    @EdBeal I've had a mis-wiring and the CFL bulb lasted days on 240V as a work light before the issue was resolved, and the bulb still survived. As for the coupling, unless you are at the OP's home, you can't tell which dominates. Capacitive is my first suspect, but in his case the type of coupling does not matter. Not sure why all this nitpicking that is not helping the OP's cause. Why not submit a useful answer instead and help him along with his issue? – P2000 Jul 30 '20 at 17:42

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