120V Neutral to Ground

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lillyqpad

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I have 50Amp 120/208V panel fed from a step down transformer. This panel feeds receptacles in a manufacturing area of a plant along with some Metal Halide floodlighting the plant installed for supplemetal task lighting.

The receptacles are fed from 20A GFCI breakers. We were asked to investigate why they would trip intermitently and why when one tripped several would trip at the same time but not all of them and rarely the same ones. In checking the circuits we found that when the task lighting was on there was a 120V between the ground and neutral and that a receptacle tester would change from normal to indicate a lost ground on the receptacles.

We went back to the panel and checked for a lost neutral somewhere in the lighting circuit but found 120V from each phase to both neutral and ground when the circuit was on.

We started gpoing back through the lightng circuit but could not locate any problems with the wiring or how the fixtures were connected.

Any ideas as to what a cause might be?
 
A few ideas.


  • The transformer is not bonded properly.
  • There is an open neutral that you missed.
  • There is an open EGC and a ground fault on the open section.
 
lillyqpad said:
I have 50Amp 120/208V panel fed from a step down transformer...In checking the circuits we found that when the task lighting was on there was a 120V between the ground and neutral and that a receptacle tester would change from normal to indicate a lost ground on the receptacles...Any ideas as to what a cause might be?
Click to expand...

I'm sure you know that you should never have voltage between neutral and equipment grounding because these must be common at the source.

Start at the source and work your way back into the branch circuitry. Make sure the transformer has it's XO bonded to case frame, make sure the sub panel does not common the neutral to equipment grounding. If your feeder is 3 phase it needs to include A, B, C, N, & Ground and 1 phase A, B, N, & Ground (if the XO is bonded the first transformer disconnect must not bond, it must be one or the other and bonding at the transformer is most common). See 250.30(A)(1)
 
iwire said:
A few ideas.


  • The transformer is not bonded properly.
  • There is an open neutral that you missed.
  • There is an open EGC and a ground fault on the open section.
Click to expand...
I would venture to guess that the main bonding jumper wasn't installed.
 
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