On Wed, 23 Jan 2008 08:07:45 -0800 (PST), miso@sushi.com wrote:

>Isn't there an issue with the potential difference between the grounds
>of the two buildings?

I think ethernet is guaranteed to have 1500 volt (common mode)
isolation between any of the signal wires and ground. I'm too lazy to
look it up.

The only way that a hot ground is gonna cause a problem is if you:
1. Ground one or both ends of the data lines. (won't happen)
2. If the CAT5 is shielded and you ground both ends.
3. If the tiny isolation xformers on the ethernet interfaces decide
to arc over or short in some weird way.

Even if you were running 10base2 (RG-58a/u coax) between buildings,
the coax is suppose to be grounded at one end only. Now, if you
grounded both ends of the coax, then you've got a real problem. At
best, you'll have a ground loop which couples 60Hz crud onto the data
stream. At worst, you'll have a smoking mess as the current heats up
the coax and eventually melts it. That's happened to me when I ran
RG-58a/u between building for a WWV and stabilized clock distribution
system for the test equipment clocks. It was not suppose to be
grounded at either end, but someone decided I was in error and
grounded everything. It took a few minutes to start a fire and fill
the building with smog as several hundred feet of coax literally
melted. I think the potential difference was only about 10-20VAC, but
it had full PG&E power behind it.

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