[ale] wiring two buildings

[Jeff Hubbs]

> I have 2 buildings in a small strip shopping center. I need to tie the
> networks in the two buildings together. GA Power rep. tells 
> me to go fiber
> optic because the buildings are grounded separately. If I 
> hard wire he says
> I will blow nic's because of different ground currents.

He could be right.  Even though both building's grounds *should* be at the
same potential, in practice, that is probably not the case.  I'd speculate
that under ordinary conditions, a few volts' difference would waver one way
or the other between the two, but a passing storm cloud could make the
difference shoot up - and at the NICs' repsective grounds, that could
translate to tens or hundreds of volts.  It's not as though that kind of
potential would express itself across the NICs' cable connector pins
themselves (the Ethernet cable's conductors "float," IIRC), but if the
potential difference between grounds got high enough, you've got a
conductive path bridging things that are PHYSICALLY CLOSE to grounded things
on both ends, with air gaps, insulator gaps, and semiconductor gaps between
them, and I'd imagine the semiconductor gaps would break down first.  You
wouldn't hear as much as a "pop."

If you want to be an absolute stickler, it's not a matter of ground
"currents" but ground "voltages."  Currents go "through" but voltages go
"across."

So, basically, anything that doesn't involve a conductor will do.

- Jeff (ex-EE)

--
To unsubscribe: mail majordomo at ale.org with "unsubscribe ale" in message body.