Transformer Bonding/Grounding

[mcs]

We recently delivered a machine that requires 240V, 3 phase to a customer who's service is 208/3 phase. They had an electrician install a 240/208 delta/wye transformer, using it in reverse as a buck boost. Shortly after the power was applied to the system, the incoming conduit started to get warm. The electrician was called in and when the transformer was opened, I noticed a ground feed was wired to X0 and another ground wire from X0 to our machine ground. No neutral was run from the main service to the transformer. At first the electrician said X0 must be bonded to ground, but later proceeded to remove both grounds from X0 and tied them together, leaving X0 floating. This appeared to solve the problem.
I deal with low voltage control circuits and was looking for input in regards to this situation.

 

[tkb]

When you use the wye side of a delta/wye transformer you only bring the three phases and NOT the neutral on the primary side (wye side).
You don't bond it either.
Just leave it be.

 

[hurk27]

We recently delivered a machine that requires 240V, 3 phase to a customer who's service is 208/3 phase. They had an electrician install a 240/208 delta/wye transformer, using it in reverse as a buck boost. Shortly after the power was applied to the system, the incoming conduit started to get warm. The electrician was called in and when the transformer was opened, I noticed a ground feed was wired to X0 and another ground wire from X0 to our machine ground. No neutral was run from the main service to the transformer. At first the electrician said X0 must be bonded to ground, but later proceeded to remove both grounds from X0 and tied them together, leaving X0 floating. This appeared to solve the problem.
I deal with low voltage control circuits and was looking for input in regards to this situation.

I would say that part of the current was flowing on the grounded X0 connection from the transformer back to the X0 of the service transformer, this is why you leave it floating, but just to connect the grounding from the supply to the grounding to the equipment does not give you a fault path if a phase was to short to the grounding at the equipment, this is because each transformer will isolate the current from the supply, so a phase must be bonded to the grounding at the secondaries of the transformer, this should be the "B" phase of the 240 volt side of the transformer.