We could just be misunderstanding each other. [Linked Image]

Sticking with the 480 delta primary 208/120 Wye secondary transformer.

If the primary windings fault to the case what is the fault clearing path back to the 480 supply?

A house service includes a grounded conductor, the delta 480 supply has no grounded conductor.

If you do not provide a primary side EGC the only path back to the 480 supply neutral will be through the transformer GEC and most likely building steel back to the service main bonding jumper.

It also could be that I am misunderstanding the function of the bonding jumper at the transformer. [Linked Image]

I have to go to bed now, I will talk to you later. [Linked Image]

Bob

Bob Badger
Construction & Maintenance Electrician
Massachusetts