voltage drops out

[jamhunnell]

I'm new here so please bear with me. I'm currently working as an outside contractor for a manufacturing facility. We are installing around 30 machines which are all 240 volt 3 phase. The company insisted on purchasing a used 225 kva transformer and a used 400 amp bussway. The transformer and bussway voltage all test fine until a load is applied, then one phase drops to 0 volts to ground while the other 2 read 240 volts to ground. When you test across the phases they all read 240 volts. Approx. 6 machines of the 30 are up and running and they all run fine. One of the machines has been running parts for 3 days and they're the type of company that if a machine wasn't working they would be on the phone to let me know about it. The transformer has a grounding conductor from the panel which it is fed from and also a ground to building steel. It is a delta to delta transformer 480 volts to 240 volts with no XO tap. I was fully expecting to have one high leg, but not to have one with no voltage to ground. I appreciate your time and thoughts.

 

[rbalex]

You won't get a high leg with only a delta-delta; you have to have one side center tapped and grounded. You have a ground-fault on one or more of the loads. The ground fault is on the same phase if there is more than one. Add each load separately to isolate the circuit(s).

 

[Smart $]

... You have a ground-fault on one or more of the loads. The ground fault is on the same phase if there is more than one. Add each load separately to isolate the circuit(s).

Only if an ungrounded system. It may be a corner grounded system, which would exhibit the conditions noted by the OP during nominal operation. Additionally, if the system is ungrounded, Code requires ground detectors.

PS: Welcome, jamhunnell...!!!!

 

[rbalex]

Only if an ungrounded system. It may be a corner grounded system.

It wouldn't "... test fine until a load is applied, then one phase drops to 0 volts to ground." He should know whether he grounded one of the corners.

 

[Smart $]

It wouldn't "... test fine until a load is applied, then one phase drops to 0 volts to ground." He should know whether he grounded one of the corners.

Errrr... I read that part, but the brain didn't assimilate it hmy:

Anyway, the part about the ground detector(s) being required does apply