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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?
 

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RS-232 and 485 the ground is pin 5 so yes the ground is carried through the cable to both ends, what the equipment does with that internally is up to the manufacturer, some will isolate that ground others not.
 

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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?
Yes, ground is carried through with RS232 and RS485. But it may not cause a problem, depending on how the Crestron system is grounded, and what Crestron has done to avoid the problem. It's certainly something I would have thought the Mighty Crestron would have worked out, being their stuff goes into huge installations all the time, and they're, well, Mighty and all. What did they say when you asked them? How about Lutron?

There's always the fiber RS232/485 approach...
 

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You can use shielded CAT-5 or CAT-6 cable for an ethernet connection, where the shield can serve as a proper ground or drain as long as matching shielded jacks are used on both ends.
 

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You can use shielded CAT-5 or CAT-6 cable for an ethernet connection, where the shield can serve as a proper ground or drain as long as matching shielded jacks are used on both ends.
Yeah...well...not quite the whole story. You can put shielded connectors on either end and still not get the shield grounded because the jacks aren't grounded to anything either. Having shielded connectors is an important part of the picture, but unless they actually grounded somehow, you still don't have it. For example, if the shielded cat5 cable has a shielded RJ45, and plugs into a standard patch panel, it's not grounded. Or plugged into a shielded patch panel, but the ground wire hasn't been attached to a real ground, you aren't grounded.

More importantly, proper installation of shielded Cat5 or Cat6 requires that the shield be grounded only at the patch panel end in the data center/closet, where the patch panel is also shielded, and has it's supplied ground wire bonded to the main ground. You do not bring the shield connection out to the other end, the user/computer/client end with ground carried through. Doing so creates a potential ground loop. Grounding the shield at one end accomplishes the goal and prevents a ground loop.

So, you do not actually want the shield to provide a ground path for anything other than stray fields.
 
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