I'm trying to troubleshoot some electrical issues myself before I hire an Electrician. Partly out of curiosity about what the #$@%! the previous owners of the house did, but also hopefully to save the Electrician a few hours, and myself a few $$$ :)

Multimeter usage - Am I doing this correctly? When I attach the black probe of the multimeter to the ground in a 3-wire cable, and touch the red probe to the other wires in the SAME cable, I get these voltages: black 30V, red ~6v, white ~6v. But, when I attach the black probe to a completely SEPARATE ground (like from an extension cord coming from a completely different circuit on the other side of the house), I get: black 120v, red 0v, white 0v -- which was closer to what I was expecting. Are you not supposed to use a ground in the same 3-wire cable when testing with a multimeter? Does this mean there's something wrong in the cable??

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    Sounds more like you have an open ground in your cable, not a short. – Ecnerwal Feb 28 at 2:30
  • @Ecnerwal -- So, connecting the multimeter to an "open" ground (instead of a properly grounded wire) would cause the reading to go from 120v to 30v? – Dura Feb 28 at 3:41
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    The open ground is "floating" - it's not connected to ground, it's functionally not connected to anything. So it may float to 6V with respect to neutral, and then float to 90V with respect to neutral (and thus 30V with respect to hot) just from the combination of capacitive coupling in the cable, and having the meter connected to hot rather than neutral. The particular voltages are actually somewhat meaningless here, because it IS "phantom." Have you made the other checks that DoxyLover suggests in their answer? – Ecnerwal Feb 28 at 4:20
  • Ah, okay, I see. And, no I haven't, yet -- waiting until daylight tomorrow so I'm not working in the dark after turning off the power. – Dura Feb 28 at 4:28
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    Worth mentioning that the problem (ground wire not properly connected, it appears most likely) is almost always (like 99.9% of the time) at a junction box or fixture box, not "broken" in the middle of the cable. – Ecnerwal Feb 28 at 4:35

You are doing correctly. All grounds should be connected together and should all read the same. The readings you are getting indicate that the ground in your cable is disconnected (“floating”). You are getting non-zero reading due to phantom voltage. That is to say, your volt meter is high-impedance and so picks up stay energy.

To verify this, you can do the following with the power turned off (I’d suggest turning off the main breaker): switch your meter to resistance and connect across ground and neutral in your cable. These should be tied together back in your main panel and should read close to 0 ohms. I suspect you will see a high resistance or open (display just shows “1”).

  • Thanks, @DoxyLover -- I'll give that a try. So, I kind of understand the phantom voltage thing (I was assuming that explained the low ~6V readings I was getting). However, why does the reading on the hot 120v black wire drop to 30v when connected to the "floating" ground (if it is floating)? Is this just an "Electricity 101" thing? It needs a proper ground to throughput the full 120v? – Dura Feb 28 at 1:46
  • why does the reading on the hot 120v black wire drop to 30v when connected to the "floating" ground - If you connect one probe of your voltmeter to a completely isolated wire, in theory it's the same as not connecting that probe to anything, and the meter should show zero volts. As you have seen, stray capacitance and induction can make a sensitive meter show almost anything. – A. I. Breveleri Feb 28 at 5:26
  • @DoxyLover -- I had to set my multimeter to "2000kΩ" to get any reading on the ground-to-neutral on the bundle in question, but it did end up settling on about 1250. So, I imagine that's equivalent to "1", and means an "open" ground... which isn't all that surprising, I guess, in an older house (with some obviously DIY renovations haha) Thanks! – Dura Feb 28 at 18:25

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