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I've been having some flickering issues when I tried to install LED lights bulbs. This led me to do some testing, and I'm getting some concerning results. It's a 3-way switch with some fairly old wiring. The only place I've found a neutral wire is in the light fixture. Measuring AC volts between hot and ground in the lower light switch box gives me ~108V, while in the upper light switch it's ~140-145. There is continuity between the two grounds, and I also measured volts between the two grounds and saw ~30V. Is this normal? In the upper light fixture, I'm getting a range depending on when I measure (this is the one I've measured the most. I've seen as low as 125 between hot and either neutral or ground, all the way up to 170 or hitting the limit of the 200V setting on the multimeter. Once I saw this, I shut the breaker down (hoping that makes things better, not worse). I've checked for continuity between neutral and ground in the fixture and it's there. What is causing this? What else should I check for or measure?

Additional info: I do have a cable tracer, but I'm not entirely sure what I'm looking for. I've used an outlet tester on the outlets coming from this breaker and they all look normal. I've also tested the voltage on them and they're normal too (~120-122V). However, there is another light circuit on this same breaker with 130-140V.

Edit: The problem disappeared as I took more things apart and tested them. Now everything is back together and acting normally. So, maybe it was just a loose connection somewhere? If so, I'm still curious what kind of loose connection could cause this.

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    That sounds like a classic Lost Neutral, either on a MWBC circuit or the entire service. Does your microwave pulse on/off when you select Defrost? Put a mug of water in there and run "defrost" and see if the lights wildly brighten and dim. That's a Lost Neutral for the whole house, and it's usually a utility problem. Look close at your overhead service drop (at the ends) and you might see it.
    – Harper - Reinstate Monica
    Commented Dec 2, 2023 at 22:37
  • @Harper-ReinstateMonica I haven't seen any other problems throughout the house, including while using the microwave. What is the overhead service drop?
    – Reese
    Commented Dec 2, 2023 at 23:11
  • Continuity tests on ground and neutral systems are usually not useful. You should focus on troubleshooting the voltage between ground wires. This indicates a problem with the GECs inside the house, even if there are other issues.
    – Robert Chapin
    Commented Dec 2, 2023 at 23:39
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    @Reese sorry, got busy. The overhead service drop is the electric wires from the top of the pole down to your house. Very prone to damage. If you've walked round your house and not found one, it might be underground. Those are far less likely to fail.
    – Harper - Reinstate Monica
    Commented Dec 9, 2023 at 5:48

2 Answers 2

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There is at least one problem here, so I would start with stabilizing the grounding system.

As a general rule, there should never be any potential between ground wires (EGC) within the same home. Even 5 volts would be highly unusual.

I'm getting a range depending on when I measure

Hopefully this is due to a simple reason such as a loose wire or bootleg neutral connection.

The best thing to try, that I can think of, is to get a spool of cheap wire, such as 24 ga, and attach one end to a faceplate screw near the panelboard. Roll out the wire to the most distant receptacle, and use your multimeter to check the voltage against the receptacle ground. This should give you a fixed reference to use at different receptacles around the house. Try to determine whether the unexpected voltage is only on that one circuit.

If it's just one circuit, then you can proceed to check which points on that circuit aren't grounded. Hopefully you have at least one light switch on the circuit that's grounded. Beyond that, you've got a bad connection somewhere. There's no way to know if the problem is in a light box or a switch box just based on the description in the question.

After repairing the bad EGCs, you'll have to go back and check hot-to-neutral and neutral-to-ground indications to see if there's a separate problem with those. When the EGCs are working and the neutrals are not working, it will appear as though some circuits have higher voltage and some have lower.

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  • What do I do once I find which receptacles/circuits are affected? There is already continuity between the two grounds, how do I eliminate the potential?
    – Reese
    Commented Dec 3, 2023 at 6:23
  • Continuing my ponderings... What could cause this potential in the first place? My only wild guess is a ground-neutral short somewhere along the line, and the difference is caused by a load in some other part of the house going through that ground. (I don't know if I'm using the right words or if this even makes sense conceptually)
    – Reese
    Commented Dec 3, 2023 at 6:38
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    @Reese Again, continuity is not the problem. They are supposed to be continuous. If the neutral and ground wires were working, then the multimeter would not read anything on them. When you find an EGC that's not bonded to the system, you have to repair the problem whatever you find.
    – Robert Chapin
    Commented Dec 3, 2023 at 10:19
  • Robert, the ECG conductors/connections are not involved with these issues. The problem lies most commonly in the neutral bonding/grounding electrode systems. If there is only one or maybe two Branch circuits having issues then the problem might just be in bad neutral connections in those particular Branch circuits. But the equipment grounding conductors will not create voltage imbalances like these.
    – Keith
    Commented Dec 3, 2023 at 17:49
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Generally this is caused by a failure at your main bonding jumpers and or grounding electrode conductor connections. Go to your service entrance, either wear your meter is if there's a main breaker with the meter, or wherever the first main breaker is to shut off your whole house. It should be labeled as a service disconnect.

On the ground neutral bars in that panel, there should be a 4 gauge bear copper wire running out of that box to your grounding electrode(s). Make sure that wire is secure in the ground buses and then turn your main breaker off and check the connection to the ground rod. Get right by your ground rod, turn your main breaker back on, and check voltage between the actual wire a couple inches away from the ground rod and the ground rod itself. Your checking for voltage drop across the connection between the ground rod and the grounding electrode conductor. It used to be very common for people to use water pipe bonding clamps on those ground rods and they do not maintain a solid connection. To be very careful with this connection and don't wiggle it around while your main breaker is on. You can receive a very very nasty shock touching that ground wire in the ground rod if there's a bad connection there

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  • The panel has a big copper wire connected to 2 pipes in the basement as well as running outside to 2 ground rods. I'm getting no voltage reading between any two points I've checked so far, including the ones you mentioned, and the connections all seem secure
    – Reese
    Commented Dec 4, 2023 at 6:29

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