Troubleshoot neutral problems.

[dekeller719]

Hello all. Looking for some help.
I have an outlet powered from a 3ph 208/120. It's a 120 volt, 20 amp receptacle. If I have the wires disconnected from the GFCI receptacle, I have 120 volts hot to neutral and hot to ground. I have nearly zero colts neutral to ground. All ok. When I attach the GFCI it will not reset and now I have near zero volts got to neutral and 120 volts neutral to ground and hot to ground.
Once I disconnect the GFCI, I'm back to normal voltages hot to neutral, hot to ground.
Changed to a new GFCI, same result.
If I put a non GFCI receptacle on the circuit, voltages are normal until I plug something in. Then I'm back to zero volts neutral to hot, but 120 volts neutral to ground.
Its as if the neutral becomes the hot as soon as something is trying to utilize the voltage.
I will add that the neutral is a shared neutral with two other circuits.

Any help is is greatly appreciated.

 

[Dennis Alwon]

I believe you have an open neutral

 

[dekeller719]

If it was an open neutral, would I still get 120 volts hot to neutral when it isn't connected to the receptacle?

 

[ptonsparky]

The GFCI, if newer, will be looking for a neutral for it to reset. The GFCI internal parts will be a load causing the Neutral to EG to be at line voltage, if the neutral is open.

We just duplicated your problem in the shop.

Open Neutral as Dennis indicated.

 

[rlundsrud]

I agree that it's an open neutral, you're seeing the 120 volts neutral to ground from the same phase being fed through the neutral that's open, possibly across a light bulb element elsewhere in the circuit. If it was a multiwire branch circuit you would potentially have 220 volts with the open neutral. Was there any electrical work done recently that might have caused the neutral to become disconnected?

 

[dekeller719]

Thanks all. Now I know a direction.