I know what happens when you lose the neutral that is feeding the electric panel. But how can you test if there is a bad connection with the neutral? Or maybe one of the feeders?
Whenever a motor starts up (fridge, washer, powertool) in the house either some lights dim or some lights get brighter momentarily.
Also, some voltage drops while the heavy load is on. Maybe a 5 to 7 volt drop.
Any ideas?
Replies
Measure the three voltages A-hot to neutral, B-hot to neutral, and A-hot to B-hot, all while there's minimal load on the panel. Then figure out how to load down, say, the A side of the panel with a coupla thousand watts (toasters, space heaters, electric iron, motors, lights, etc), while leaving the other side unloaded.
If the A to B voltage remains largely unchanged, while the A to neutral voltage drops and the B to neutral voltage climbs (roughly as much as the A voltage dropped), you have a bad (open or high resistance) neutral connection somewhere between the pole transformer and your panel.
If, on the other hand, the voltage variations are just due to normal line resistance (though possibly with an undersize line) then you should see a drop in the hot-to-hot voltage, but any increase of the B to neutral voltage should be comparatively smaller (probably less than 1/4 the A-to-neutral voltage drop).
Note that lights getting brighter when a motor starts is kind of prima-facie evidence of a bad neutral connection, though. Any increase in the B to neutral voltage when the A side is loaded is necessarily due to resistance in the neutral line. It's just hard to tell whether it's bad enough to classify as "bad", though.
Dan, thanks for the info. This sounds familiar; I think I used to know this but couldn't remember.
I figure the same test(s) work for a sub-panel as well?
Should. And at a subpanel you have a supposedly independent ground, so you can measure the neutral-ground voltage as another parameter. The neutral-ground voltage should be zero with no load and should be no more than half the hot-neutral voltage drop (preferably much less) when loaded.
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