I noticed that the ground prong of the only 5-15 receptacle in my laundry room appears to be floating.
Findings:
- The ground wire behind the receptacle measures 0 ohm (i.e. continuous) with the ground prong in the receptacle.
- The ground wire/prong measure 0L ohm (i.e. not continuous) with the neutral wire/prong in the same receptacle, 0L ohm (i.e. not continuous) with ground prongs from other receptacles.
- The ground wire/prong measures 60V with hot in the same receptacle, 60V with neutral in the same receptacle, 60V with ground prongs from other receptacles.
- The ground wire behind the receptacle appears to come from a 12/2 cable.
- There is a 1-pole 20amp breaker in the subpanel that serves this receptacle. I believe this receptacle is the only thing that the breaker serves.
- In the subpanel, there is a corresponding 12/2 cable.
- In the subpanel, the ground wire from the 12/2 cable measures as expected. ("as expected": 0 ohm with neutral, 0 ohm with other ground, 120V with hot, 0V with neutral.)
- I checked all other receptacles throughout my home. All of them measures as expected. ("as expected": same as above)
- I opened up every blank wall plate that I could find in areas between the subpanel and the outlet, and used a non-contact voltage detector to see if it’s from the same breaker.
- I found one blank plate that opens into a hole in the wall (instead of a box). In that hole, I can see 3 cables. One of them is from the breaker of interest. The cables do not appear to be protected (definitely not in a conduit).
- I did not find any box with wires from the breaker of interest.
Background:
- The receptacle is a 5-15 GFCI receptacle.
- I recently had a contractor replace my subpanel. The subpanel serves everything in my home, except the air conditioner.
- I believe the grounding of the subpanel is good:
- After installation (and fixing a wiring error behind an electric oven), I verified that ground and neutral are not continuous when neutral feed is disconnected. I also verified that ground and neutral are continuous when neutral feed is connected.
- A UPS (a small one for network and storage devices) is plugged into the outlet of interest, and its "building wire fault" light is on.
- I do not know how long the light has been on.
- I found the issue because I was replacing converting a nearby 10-30 into a 14-30, and used the problematic to sanity check the previously unused ground wire behind the 10-30. (On hindsight, I could have used the neutral of the 10-30, but anyways.)
What are possible causes? How do I diagnose it? (The first thing that popped into my mind is that the ground wire is severed. But is it likely that one wire in a 12/2 is severed, but not the other two?)
(I have a workaround. I found a box 1 ft (30 cm) from the problematic outlet in the same room, which holds wires appropriate for a 5-15 outlet. That box was fed from a different breaker, and its ground appears to behave as intended. But I’m still interested in understanding what’s going on)