Re: [SI-LIST] : FCAL DB9 cable shield

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From: Christian S. Rode (csrode@mediaone.net)
Date: Tue Nov 16 1999 - 16:56:44 PST


Certainly any difference in ground potential between locations could
cause
current to flow preferentially through the shield. I don't think being
on
different phases is as much the issue as unbalanced use. If one phase
of
your home triplex service is used more heavily or a large motor blows a
phase on three-phase industrial service I think the center-ground taps
will create a voltage differential.

Capacitively or inductively coupling shields and ground at one end
solves the ground current problem but you'll have to choose a
coding scheme without a DC component...

Isn't this why optocouplers were invented?
Aren't there safety issues, too? Seems I remember that it's not to
(current?) code to run Ethernet cabling between buildings. Not that
people don't do it anyway...

> >
> Wouldn't this cause nasty galvanic currents through the shield if the two
> pieces of equipment, attached by the cable with the DB9s, were, say, on two
> different phases of a three phase power feed to a building.
>
> Am I missing something?
>
> thanks
>
> Tom Gandy
> Industrial Catalyst
>
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