This is for a huge home theater.

I have a power transformer that converts 380VAC between two legs of a 3-phase power system down to 230VAC, with a center-tap of the secondary being our "neutral" for this secondary. (FYI, there are three such transformers, connected between phases A and B; B and C; and C and A. All this is needed.)

A hundred feet away we have a Lutron lighting panel that operates at 230 VAC from one leg of the same 3-phase power, on the same breaker panel, to neutral.

The lighting system is controlled via a Crestron system powered at the above-mentioned transformer, with control from the Crestron to the Lutron via RS-485.

If the RS-485 cabling makes a connection from the chassis ground of the Crestron system to the chassis ground of the Lutron, it looks like we have a HUGE ground loop. Hence the question, which I'd like to generalize:

Do ethernet, RS-232, and/or RS-485 essentially isolate grounds at the two ends of their cables? Do they carry a ground through? Does it depend on the equipment?