Improper neutral to neutral connections can cause harmonics. For example two neutrals tied together where one is a normal power circuit and the other from an emergency power circuit and each are derived from different transformers.
Improper neutral to ground connections can cause harmonics.
I've seen where these improper connections will shorten the life span of lighting ballasts & sensitive electronic equipment.
There are two ways that I know of how to trace the improper connection.
1. If power can not be turned off use the "Net Zero Current Method" by taking an amp clamp around all the wires and you should read zero. There are some good articles on the internet that go into detail. This can be a long arduous process.
2. Turn power off feeding the panel. Disconnect each branch neutral from the neutral bus bar and ohm it to ground. There should be on open circuit reading between the disconnected neutral and ground. If you your meter detects a connection then you know you have an improper neutral to ground or neutral to neutral. This is a quick way to check out an electrical system.
