The plant ... this is a major, heavy industrial complex ... was built in the early 1990's, with an emphasis on CHEAP.
The PoCo delivers high voltage; the customer owns the transformers that step it down to the ungrounded 480 delta. There is no compelling reason, apart from they wanted to save money by not pulling a neutral.
FWIW, my input two years ago led, last summer, to a 'pilot program' where a small section of the complex replaced the HV transformers with WYE transformers.
The equipment in this plant is far older than the plant itself; some of the production equipment was used in WW2 to make cannon barrels. probably not an issue then; ungrounded delta works fine when all you have are motor and resistance loads.
Of course, these days we have frequency drives all over the place, and we heat the metal by induction. 1200 amps of Scr's firing will put one hell of a spike on the line.
Ironically, many at the plant still think it's the PoCo that's giving us "bad power."
High impedence ground ... that's exactly what we had when that ballast shorted out, causing bad welds on the line.
I'm not done with my daydreams, with just a power conversion. I figure separating the lights from the production power is just the beginning. (Ambitious, aren't I?)
I'd like to deep-six the MH in favor of T-5's ... which ought to help the summer heat load and cut the power use in half. Get better light in the bargain as well.
Another advantage to switching to 277 is that I can get a 277v multi-tap ballast anywhere; 480v ballasts are harder to find, and thus much more expensive.
I am aware of the grounding issues. Oddly enough, the buildings are extensively grounded. Remember, just because the system is 'ungrounded' it does not mean the building need not be. After all, the system relies upon building bonding to make the fault monitors work. Plus, of course, there is that incidental 120v for the receptacles.
The place has a "grounding guru" who has been kept apart from me ... but it's clear his reputation was built on the results he produced by making sure things are bonded.
As for an 'earth ground' ... well, I submit that the cement slab, with all the bond wires and steel beam anchors, cannot help but be one heck of a big Ufer. Grounding doesn't get much better than that.
