I had an emergency call yesterday. The homeowner was having trouble with dimming lights (with motor surges---much more than normal)and a gas dryer with electronic controls wouldn't start.
I was getting voltage readings within norm, when the POCO guy came. I told him I suspected a neutral problem. He put the phases under load, which showed an imbalance. He said anytime a voltage reading in single phase varies more than 4 volts (L1-N, L2-N)it's probably a faulty neutral.
The homeowner asked why the lights dimmed and he described the current flow in a sine wave with a faulty neutral (using his finger on the wall). Not exactly something I'd try with a homeowner.
Anyway, can anyone explain this clearly or show a diagram of how this changes the current flow? He seemed to be saying that since the neutral was faulty the current came back on itself causing the lights to dim and brighten.
Sorry for the long delay. It took me a long time to find what I was looking for, and I've been rather busy lately.
The thread listed below contains the best, most comprehensive and easy to understand explanation for what happens when neutral is lost. Makes much more sense than the POCO guy's explanation with a sine wave. It's even better than I have read in most books!
That IS an excellent thread and explanation. I've been involved with this scenario a couple times and learned what NOT to do! Basically, don't tie in a multi-wire circuit or part of one (any circuit sharing a neutral) while more than 1 leg is still on. Better yet, turn the whole group off. I was forced at the time to work for speed, and not allowed to shut anything down. When the neutral splice came apart (Surprise!) office equipment started leaking smoke and there was alot of screaming and pandemonium.
A friend of mine recently got his service upgraded, and the electrician showed him that his utility neutral had been completely unattached, probably for many years! The current must have been flowing through the waterpipe ground, because they never had any noticeable problems with their power.