Why does voltage still exists when a circuit is broken by a 3-way switch?

Question

My parents got a ceiling light for my parents for Christmas, which I was tasked with installing. I don't know why I bothered to check the voltage before putting the new fixture on (and after taking the old one off,) but I did, and it read 123V when on (good, obviously,) but it read 38V when turned off.

I disconnected all of the wires and checked the switches, and they worked perfectly fine (complete discontinuity when broken). I did break one of the switches, however, when releasing the wires that they'd stabbed in the back (20 year-old plastic is sorta brittle, I guess,) so I got new 3-way switches and not surprisingly, this did not fix the problem.

I removed the wires from the switches, except I kept the line attached to the first switch and tested the switched terminals vs. ground. I got 123V on the active terminal and <.5V on the inactive terminal -- checked out just fine. As soon as I connected the wires that lead to the box with the second switch (which was not connected,) I got 37V on the wires that were supposed to be disconnected from the line. (Actually, one reads 55V when disconnected, and the other reads 37V when disconnected.)

Unfortunately, I do not have an insulation tester, but it would seem as though the insulation has failed on those wires (assuming no stupid mistake was made by a previous installer.) However, I am open to comments about why this might be happening. Any thoughts? Anyone ever had this happen before? Etc.

Answer 1

Do some research on capacitive coupling, and Inductive coupling. This should explain why you are seeing voltage on the "dead" traveler.

If you load up the traveler that is not carrying current; with a test light for example, you'll likely see this voltage go away.

Working in a residential setting, this voltage is likely harmless. Phantom voltages, however, can be lethal when working with distribution lines and other high voltage situations.

You may still want to have a licensed electrician come out and take a look, just to be sure this is actually the case.

Answer 2

As was mentioned above, check your neutral source. I've seen the same situation and what cured it for me was tightening down the screw holding the neutral to the bus bar in the breaker box (after turning the box off of course). While I was at it I found a couple other screws holding down neutrals that were loose by a screw turn or less. It is always a good practice to check voltages before installing and/or replacing - one never knows who has been working on the electrical. Or as in the case of an older home, what condition the wiring, outlets, etc. are in. An odd voltage is usually a clue to something amiss.

Answer 3

I have experienced the same thing where when off certain brand of led bulb would glow on a hall light controlled by 2 3 way switches. I did all the outlined tests and everything checked out ok.

I tried an experiment with ~30’ of 14-3 rolled out straight. Connected the black and neutral only with no load and then measured a voltage on the third (red)conductor while both it’s ends were disconnected.

I believe this is simply due to induction of the changing electrostatic field. Any load, a second led, high ohmic resistor or neon bulb pulls down this voltage.