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I have two buildings (A + B) each with their own mains electrical supply. The buildings are about 20m from one another. Between the two buildings are the Cat6 cables (one active and two spares).

The Cat6 cable is terminated in a patching / data rack in each building.

Both buildings have their own patch panel and network switch. Building A houses the modem and router.

My concern is that I've heard having connected electrical devices / equipment, across two or more mains electrical supplies can cause issues / damage to the electrical devices / equipment due to different mains electrical supplies having different grounds / earths.

Is this a valid concern? And what can I do to rectify the issue?

  • If the supplies are properly earthed there should be no issue. In any case, Ethernet cables do not normally carry power, unless you are running PoE, which I don't think you will be. – DavidPostill Dec 9 '18 at 20:37
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It won't be a problem. The Ethernet standards for twisted-pair have always required isolation of the signal wires from all local power supplies and grounds. In most equipment using twisted-pair Ethernet this is achieved simply by having all of the pairs run through transformers at each end. There is therefore no metallic connection between the switches or whatever the TP cable is plugged into, and therefore complete isolation, between the equipments' grounds and power supplies.

If you were using shielded twisted pair, you would want to connect its shield to building frame ground at one and only one end. This was a concern back in the days of 10Base2 and 10Base5 (Ethernet over coax).

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    So what about PoE? – DavidPostill Dec 9 '18 at 21:20
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    Ethernet uses differential signalling. Thus the interface looks at difference between two wires, not absolute voltage levels. No common zero needs to be identified, as the difference will be equal no matter what gnd is with reference to the other end. – vidarlo Dec 9 '18 at 21:21
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    PoE is always transformer-isolated from the local AC supply. – Jamie Hanrahan Dec 9 '18 at 21:23
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    @grawity Ethernet over TP has always used differential signaling. You get it basically free with transformers at each end. – Jamie Hanrahan Dec 9 '18 at 21:25
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    @grawity it does. That's basically the reason behind twisted pair; noise will impact each wire almost equally, and thus the difference will be preserved. – vidarlo Dec 9 '18 at 21:26
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If you're at all concerned about it, you can fairly cheaply use a fiber-optic connection instead. That completely avoids the issue by being non-conductive.

Gigabit media converters run for $50–100 a piece, allowing you to fairly cheaply user fiber instead. (Search Amazon or wherever for something like "fiber media converter" and you'll find plenty). This does require running fiber — and that's only easy if you can pull a pre-terminated fiber through existing conduit; that'll add around another $20 for the fiber. If anything is going to fail, it's going to be a media converter (or its power supply); therefor, keep a spare on hand. Total of under $200 including spares easily doable.

Note that there are multiple fiber connectors, so you need to make sure the pre-terminated fiber you buy matches the media converters. At the distance you're running (meters, not kilometers), everything will be multimode fiber and fairly low-power.

[Of course, if you have the tools and know-how to terminate fiber, that makes it easier, you can pull unterminated fiber through a much smaller or more tightly packed conduit. But then I suspect you'd have already considered this option.]

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