Wacky to ground voltages

[tmwallace]

I had a situation in the field come up recently and I am looking for input.
Our local utility provider was going to cut off service to a 2000 amp 480 volt delta system that is approximately 55 years old because of unsafe voltages to ground found during routine maintenance.
This system has 6 circuits leaving the distribution section ranging from 400 amp three phase down to 100 amp three phase.
with all circuits de energized we were reading at the test blocks A phase 280 volts to ground, B phase 280 volts to ground, C phase 265 volts to ground, normal.
When a new 100 amp circuit was added to feed a transformer 600 feet away and energized our voltages at the test block were A 480 to ground, B 480 volts to ground, C zero volts to ground.
The transformer was electrically disconnected from the circuit with these readings.
B phase and C phase were swapped at the 100 amp breaker in the main service and the readings went to normal A 280 to ground, B 280 to ground, C 265 to ground.
Any thoughts out there??

 

[iwire]

Yeah, you have an ungrounded delta service and the new feeder has phase C connected to ground either by mistake of intentionally.

 

[big john]

...When a new 100 amp circuit was added to feed a transformer 600 feet away and energized our voltages at the test block were A 480 to ground, B 480 volts to ground, C zero volts to ground.
The transformer was electrically disconnected from the circuit with these readings...

So the only thing being energized in the circuit was the new cable? I agree with Iwire you have a faulted phase. Try continuity testing with a meter, if that doesn't work get a 1kV megger.

 

[RichB]

Ditto to both--looks like a grounded phase