It sounds like a center tap(on 1 pot) grounded 480 delta. The voltages read like a fuse was out, or as he stated, a bad pot.
I thought of 480V "Hi-Leg". I've never seen one and have only head of one. But, I thought could be. So I had sketched the vectors figuring both a Y primary and a D primary, with a D secondary, one center tapped and grounded.
I didn't see any failure modes that would give 510, or 484 to neutral/ground. 240V, 415V - yes. A bad transformer if it were not completely out, maybe it could. - that I don't know. A transformer that were completly out would look like an open delta. So help me out here. How does an open primary fuse give 510V to ground on a 480D, centertapped? How does an open transformer give 510V or 484 to neutral/ground?
An un-grounded delta can - but this ain't that.
I'll chalk this up to another one of the many things I have not seen. But if you have time to explain - I'm listening.
ice