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I'm in the US. While repairing a light fitting I noticed that with the breaker on I was seeing only 30V AC, instead of the expected 120-ish. However, the now-repaired light fitting is working as expected. Why would I see only 30V?

Possibly relevant details:

  1. I read the 30V unloaded -- i.e. my voltmeter probes across the two mains wires coming out of the ceiling, with no connection yet made to the fixture

That probably renders the remaining points irrelevant -- esp. 3 and 4 -- but here they are anyway;

  1. The fixture is on the same circuit as our garage lights, as well as some lights in the main part of the house. (It's in our utility room, where our washer and dryer are.)
  2. It's a common-or-garden T12 dual tube fluorescent deal, with ballast.†
  3. It's dual-switched -- i.e. a switch at the door into the house, and one at the door into the garage.

thanks!

† For anyone who read the earlier question, I ended up just going back to a ballast set up this time.

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    Alter the wiring temporarily to wire a receptacle across your test points and plug an incandescent nightlight into the receptacle. This can be hokeydokey, you'll only use it for 10 seconds. Measure the voltage, turn the nightlight on. Bet the voltage goes away and the light does not light. It's phantom voltage, the wire is picking up AM radio or waves from an adjacent conductor and your very sensitive DVM is picking that up. We get this all the time. – Harper Jan 28 '18 at 17:53
  • Thanks Harper, but I may have been unclear in my description. The thing is the final fixture does light. Your description makes sense for when the breaker is OFF, and to be sure there is some residual voltage in that state -- under a volt. And that may well be some kind of induction as you describe. But what I'm seeing is 30V when the breaker is ON. So the curious thing is not the presence of voltage where I'd expect none, but rather the presence of only 30V I'd expect to see something up towards 120V rms. – tkp Jan 28 '18 at 18:28
  • Flip one of the 3-way switches and see what changes. – Harper Jan 28 '18 at 20:14
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It is phantom voltage, which is a small voltage induced (capacitively coupled actually) from adjacent power lines. (needless to say, those power lines need to be on.) It's sorta like picking up radio stations except the adjacent power line is much closer, so the voltage is much higher.

On a 3-way switch, you have two parallel traveler wires in the same cable or conduit. The 3/4-way switch complex changes which of those travelers is energized, and whether the energized traveler connects to the light. Ideal conditions for phantom voltage being induced in the un-energized traveler. So if the switch is off, you will see this voltage at the lamp socket.

That must be what you are seeing.

The voltage has no real energy behind it. It can't do useful work, and if you connect even the smallest load, the voltage will disappear. It can only be seen by very sensitive voltmeters such as the common commodity DVM.

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