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In my old, mostly ungrounded house I have found a 3-prong outlet housed in a metal electrical box that behaves strangely. It operates fine, but when inspected with a digital multi-meter I observe:

  • Hot to Neutral: 120V
  • Hot to Ground: 80V (same for Hot to electrical box)
  • Neutral to Ground: 30V (same for Neutral to electrical box)
  • When I use a GFCI tester it reports "Open Ground"
  • Looking at the wiring, there is a ground wire coming into the box in the romex cable but it is not connected to the outlet
  • There are no outlets connected downstream from this one

My questions:

  • Why am I observing these voltages to what looks like a disconnected ground?
  • Is there any safety risk (e.g. electrical fire) in using this outlet as is beyond the standard ungrounded shock risks?
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Your GFCI Tester tells it all. The ground conductor needs to be connected at the device and all the way back to the panel ground. Otherwise it is just a loose wire and the readings you are getting off of it is a "ghost" reading or a voltage reading of a very low amperage output. So it just means it is not connected properly.

The NEC Is specific about changing out a 2 prong outlet to a three prong. You must install a GFCI outlet instead, or install a GFCI device and feed the entire circuit through it, and all receptacles then must be labeled as ungrounded. If you purchase a GFCI receptacle these labels will come with it.

I did an edit on this answer to install two examples of what I am talking about which might help you understand.

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Just remember your installation does not have a ground.

Stay safe and good luck.

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  • In the installations without a separate ground (old 2-wire receptacles) was the neutral bar grounded near the panel by connection to a water pipe or to a ground rod?
    – Jim Stewart
    Commented Oct 29, 2018 at 17:05
  • In installations that were originally without a ground at the receptacles and where new circuits were later added in 3-wire NM (12/2 + gnd or 14/2 + gnd), what is normally done in the panel with the ground wire? Would it be correct to connect the gnd wire to the neutral bar so at least the new circuit would be bonded to the neutral (and also grounded if the neutral was grounded)?
    – Jim Stewart
    Commented Oct 29, 2018 at 17:07
  • Thanks, that's a very helpful explanation. After I have the outlet replaced with GFCI, is there any risk of fire or other damage from the loose ground or the phantom voltage?
    – spicecat
    Commented Oct 29, 2018 at 17:17
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If the OP measures 30 V between the neutral and the ground wire in the box (not the ground in the receptacle which is not connected to anything), then it is clear that the ground wire is not bonded to the neutral in the panel, but could it be that the ground wire goes to a water pipe or to a ground rod?

To test whether the ground wire goes to ground in the panel and is bonded to the neutral put the digital meter in resistance mode and measure the resistance between the loose ground and the neutral (Not the hot!!! If you mistakenly connect to a hot in resistance mode, you will at the least blow a fuse in the meter and might damage the meter.) A low resistance between the ground wire (~0.1 - 0.3 ohm) means the ground wire goes to the panel and is bonded to the neutral there. That is what you hope for.

If the resistance is higher (say 1 ohm to 500 ohm), then that would mean the wire goes to a ground rod or to a water pipe, but is not bonded to the neutral in the panel. I think this wire would still be usable as a ground for a GFCI receptacle, but expert opinion will be needed.

If the resistance is high kilo- or mega-ohms, that would mean the ground wire is not connected to anything and is useless as a ground.

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