[ale] wiring two buildings

[John M. Mills]

FWIW -


On Tue, 13 Jun 2000, Jeff Hubbs wrote:

> > 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 once taught in a lab where wall sockets on opposite walls were so
miswired that grounding the signal lead of an oscilloscope to one set of
plugs' grounds and the scope itself to the other, would burn out either
the test lead or the item being tested. Folklore was that the two sets
were fed from different distribution transformers, but I never verified
that. We were talking large voltage and substantial current:
short-circuiting the wall power, essentially. (Obviously this was a code
and safety problem! Fortunately, to my knowledge no student or teacher got
in the way of the current!!) [_No_ -- this was not at Georgia Tech.]

In the same building we ran data lines from a lab on the first floor to a
computer in the basement, and connecting signal grounds resulted in
visible (and audible) sparks. I traced that one to power-supply bypassing
capacitors in the computer. A/D's were not comfortable under those
conditions. Here we were talking substantial open-circuit voltage, but
small (10's of ma.) current once the circuit was made. Result was
unacceptably noisy instrument readings as ground-loop currents varied.

That said, there are surely isolation boxes you can put in-line: optical,
transformer-coupled, or something. This must be a common problem in
distributed networks.

   John Mills
   Sr. Software Engineer
   TGA Technologies, Inc.
   100 Pinnacle Way, Suite 140
   Norcross, GA 30071-3633
   e-mail: jmills at tga.com
   Phone: 770-441-2100 ext.124 (voice)
          770-449-7740 (FAX)

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