PSN-L Email List Message

Subject: Re:ground rods and noisy power supplies
From: "Charles R. Patton" charles.r.patton@........
Date: Thu, 13 Jul 2000 10:36:04 -0700

I wanted to make one additional comment regarding ground rods in
general.  It=92s very important NOT to use multiple rods in an
installation.  Not only does it add ground current noise as has already
been mentioned, but it is a safety issue as well.  In particular if you
have a lightning strike in the vicinity, the ground currents become
enormous, and so the voltage drop across the ground and thereby the
voltage across the ground rods can be very large, and depending on the
distance between them can be thousands of volts.  We=92re not talking
miles here, just feet.  So if your equipment has connected grounds,
suddenly opposite ends of your equipment are connected to a very
powerful, high voltage source which causes large currents to flow,
melting cables and of course, frying the attached electronic equipment.
And the big point is, not only can it fry equipment, it can electrocute
you.  If you used multiple ground rods, but the grounds aren=92t
connected, then the problem becomes the high voltages placed on the AC
isolation means, i.e., the primary of the AC line transformers suddenly
have to withstand huge voltages, again with the same result, breakdown
and fried equipment.  Within a single installation, use only a single
point ground rod, and then provide a ground strapping/connection grid
that acts as a reference =93plane=94 for your system.  Try to insulate th=
outlying sensors and bring their grounds back to your central system or
else use total galvanic isolation which is a whole another subject in
itself.  A good idea on the insulated outlying systems is to provide
lightning strike protection.  A good example is the telephone system.
They use carbon blocks separated by a thin mica sheet with holes in it
or gas tubes from each line to the ground rod at the entrance to the
house.  These are very high impedance devices until they breakdown, at
which point they can carry large discharge currents.  I don=92t remember
now, but I believe Sean-Thomas talked about some seismo equipment inside
PVC pipe that took a lightning hit.  Not a pretty description!

Charles R. Patton


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