I started writing this while waiting at my house, with the electrician, for the power company to pull my meter. The problem was solved before I could post this, so I will spoil the ending.
This is in the US, 240/120V center tap, two hot lines and a neutral. Early symptom was flickering lights. I bring out my voltmeter, and one leg of the house is at 125VAC, the other leg at 116 VAC. Sounds like a neutral problem.
We have underground electric lines with the transformer in the neighbor’s yard. The lines run from the transformer to the meter panel in a non-conductive conduit. The meter panel is connected to the breaker panel by** conductive** conduit. A heavy ground wire goes from the breaker box to the ground rod. Also connected to the ground rod are the cable and phone ground wires.
Outside, I put my clamp-on AC Ammeter on the breaker’s ground wire - it showed 4 amps - very bad.
I look closely at the short piece of metal conduit between the meter and breaker panels. There is arcing between the metal conduit and the meter box - very bad. I don’t want to touch the breaker panel to open it. I grab a piece of wood and push on the conduit, hoping to create a better ground connection between the two panels. The current in the ground wire drops to 1 amp. I fiddle with the conduit some more, and the current drops to zero. Back in the house, both legs are now at 120V. So far so good.
I open the breaker and flip off the mains. I check continuity between the incoming neutral wire and the breaker case. Looks good. I reckon the problem is most likely with the neutral wire inside the meter panel. I call the electrician.
Electrician arrives and he confirms the neutral connection in the breaker is good. Now comes the debate. The electrician says the poor connection could be anywhere between the breaker box and the transformer. I argue that there must be a poor connection, almost certainly in the meter panel, of the neutral wire that runs from the meter to the breaker, otherwise there would be no arcing. I assert that there would be no arcing if the poor or open connection solely occurred in the neutral wire running between the transformer and the meter. In other words, there was arcing because that piece of metal conduit was trying to take the place of the breaker-to-panel neutral wire.
We called the electric company and asked them to come out and pull the meter so we could check the neutrals. Electrician and I continue our lively discussion, complete with hand drawn electrical schematics. I think he hopes to run a re-wire from the transformer to the meter, which is my responsibility and not that of the power company. At this point, I started writing this post. The electrician is giving my house a free inspection while we wait. Power company arrives and pulls the meter before I can post this. Here is the result:
I was right, the connector of the neutral wire from Breaker to Meter box was somewhat discolored from over heating and slightly loose. Jiggling the conduit earlier with the stick had either improved the connection of the conduit, or else it wiggled the poor connection of the wire back in place enough that the symptoms disappeared. The electrician put some grease on the wire and fitting, and tightened it back up. This was all copper wiring BTW. Aluminum might have melted and created a floating neutral. Power company guy put the meter back on and all is well. I am not dead and the house did not burn down.
So was my guess just a lucky guess, or did the problem almost have to be where I said it would be? The only other thing I can think of that would cause the arcing would be if somehow one of the two hot wires was shorted to the conduit, which would seem to be a rarity.