Path: utzoo!attcan!uunet!mcvax!ukc!warwick!cuacw
From: cuacw@warwick.ac.uk (Malcolm Barker)
Newsgroups: comp.dcom.lans
Subject: Re: Maximum Ethernet length (actually lightning)
Message-ID: <1358@sol.warwick.ac.uk>
Date: 18 Jan 89 10:27:19 GMT
Reply-To: cuacw@warwick.ac.uk (Malcolm Barker)
Organization: Computing Services, Warwick University, UK
Lines: 21


In article <164@oregan.uoregon.edu>, J Q Johnson writes:
>Kent and I agree with Phil in principal.  In practice, you might be lucky.
>At Cornell we installed a thickwire Ethernet between 5 buildings in 1984.
>Ithaca NY has lots of lightning storms, but I don't think they've blown
>a transceiver yet (I left before they realized the hole I'd helped dig for
>them :-).

It's my understanding that lightning is not the only hazard to be considered
when LAN cables are routed between buildings.
If buildings A and B have different high voltage transformers to supply mains
power, and an earth fault develops on the high voltage feed to building B,
a fault current flows which can result in a high potential difference between
the two local building earths.
If an Ethernet cable is earthed in building B and is routed between A and B
then the high voltage between the building earths also exists between the
cable screen and the local earth in building A. Any personnel touching the
cable screen and local earth in building A during the fault clearance period
could be subjected to a serious hazard.

- Malcolm Barker