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From: dan@gacvx2.gac.edu
Newsgroups: comp.dcom.lans
Subject: Re: Possible Ground Loop problem
Message-ID: <1991Jun17.200906.245@gacvx2.gac.edu>
Date: 18 Jun 91 02:09:06 GMT
References: <0094A40A.0DF70DA0@EA.USL.EDU>
Organization: Gustavus Adolphus College, St. Peter, Minnesota
Lines: 54

In article <0094A40A.0DF70DA0@EA.USL.EDU>, johnson@EA.USL.EDU writes:
> I have 2 buildings about 140 feet from each other which I need to run
> ethernet between.  I have decided to run direct bury thinwire and am
> aware that there could be a possible ground loop problem between the
> two buildings.  I have not checked this, but know it can be.  Is this 
> something about which I should be concerned???  Also, why should
> a coax only be grounded on 1 side of a segment?  Why not put a small
> resistance at the other end to drain that small amount of current which
> might be there?
> 
> Any suggestions would be appreciated.
> 
> Lee J.

Under normal circumstances the difference in the grounds between buildings is
small.  However since the cable will also be connected during abnormal
conditions, you must plan for the reasonable extream.  When an event like a
thunderstorm passed over your campus, the ground potential changes rapidly. 
These changes are part of what makes lightning work.  Between two points within
just a few hundred feet of each other there can be hundreds to thousands volts
difference during a storm.  The grounding within a building keeps it at same
level through out a building.  The difference during a storm has the potential
to do damage to electronic ciruits.  This is damage due to a direct hit, just
from a potential change caused by the charges in the ground and the clouds.  If
you tie into the grounds on both ends of your cable, then it must be able to
take the current it could possibily be subjected to, because you would be tying
the grounds in the two buildings together.  Optical isolation is the way to go. 
You could do it with a IR tranmiter and receiver pair, but it is better to go
for fiber.  Fiber read for ground burial is about $2.00 a foot.  It should be
about $150 an end for termination with connectors.  Ground burial coax cable is
expensive too, since it usually has an armor coating, and sometimes a second
sheild.

When I used to run RS-232 all over the place using various concentrators and
line drivers.  I found that sometimes a ground difference of only a few volts
would make the difference between a terminal working and not.  The ground float
would sometimes cause 0s to be read a 1s and 1s also would be read as 1s or
vice versa.  Checking to make sure that the ground on the lines into the MUX
ran all the way back to the computer center where the other MUX was located
would usually turn up a place where I forgot to tie it together.  Balanced line
or differential drivers were virtuall immune to this.

Gustavus is built on a hillside (well, top of a valley side.)  It turns out
that the hillside used to me mostly ravine.  Over time this has been filled
with sand, and whatever else.  The sandy soil makes a very good dielectric. 
Because of this, we attract they type of damage caused by ground differentials,
and some argue that we even attract lightning.  Some soil conditions are better
than others, ours is about the worst.

-- 
Dan Boehlke                    Internet:  dan@gac.edu
Campus Network Manager         BITNET:    dan@gacvax1.bitnet
Gustavus Adolphus College
St. Peter, MN 56082 USA        Phone:     (507)933-7596