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Path: utzoo!watmath!clyde!burl!ulysses!mhuxl!ihnp4!zehntel!hplabs!tektronix!uw-beaver!cornell!scharf
From: scharf@cornell.UUCP (Gerald Scharf)
Newsgroups: net.lan
Subject: inter-building ethernet grounding
Message-ID: <433@cornell.UUCP>
Date: Thu, 30-Aug-84 00:13:14 EDT
Article-I.D.: cornell.433
Posted: Thu Aug 30 00:13:14 1984
Date-Received: Sat, 25-Aug-84 05:54:32 EDT
Organization: Cornell Univ. CS Dept.
Lines: 37

<>

     There must be people out there who have ethernets, or for that matter
any coax based net that spans several buildings. As one of the technical
people within Cornell designing the net, I am asking for the input of other
net designers.

     In particular, an environmental engineer from DEC has proposed one
catastrophic failure syndrome. If there is a electrical storm with a strike
near one end of the coax, there will be a temporary induced ground potential
in the order of millions of volts relative to the cable ground. If there is
sufficient voltage, the insulating jacket of the cable will break down and
arc to ground. This fries the cable instantly, probably takes several
transceivers with it, and could even fry the repeater logic boards. Wow.

     We decided that trying to tie all the building grounds together with
00 or 000 wire (read this as size of a thumb copper wire 1300 feet long,
read get a major NSF grant to pay for ground connection) was unacceptable. So,
the other proposed alternative is to put the coax backbone in one building,
just long enough to connect 10 transceivers, then use remote (fiber optic)
repeaters to connect to each building coax segment. DEC says that they can and
have connected several remote repeaters in one net, despite Blue Book
restrictions. The unfortunate side effect (for us, not the vendors) is that
it doubles the total cost of the network before building segments to almost
$60000. I am also having trouble locating a supplier who can supply almost
2000m of 2 conductor 100/140u fiber able to withstand freezing, in less than
6 weeks.

     Has anyone ever had a net fried by lightning? How can I determine the
number of times in a ten year period that this is possible? How does one factor
net availability into cost justifications for insurance/safety of the net?
Do other people have uneasy feelings about using a bunch of remote repeaters?

Jerry Scharf

no longer ignorant, I seek knowledge not enlightenment