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From: rsd@sei.cmu.edu (Richard S D'Ippolito)
Newsgroups: comp.misc,sci.electronics
Subject: Lightning rods. Was: Protecting computer equipment
Message-ID: <6179@aw.sei.cmu.edu>
Date: 11 Jul 88 14:45:49 GMT
References: <1988Jul8.045005.17867@utzoo.uucp>
Sender: netnews@sei.cmu.edu
Reply-To: rsd@ae.sei.cmu.edu.UUCP (Rich D'Ippolito)
Organization: Carnegie-Mellon University, SEI, Pgh, Pa
Lines: 27

In article <1988Jul8.045005.17867@utzoo.uucp> Henry Spencer writes:

> [...] Lightning rods and the like are based on giving
>it a more attractive place to go, not on stopping it from doing what it
>wants. 

Ligthning rods work by _preventing_ the buildup of static charges in the
vicinity of the pointed end(*). This reduces the formation of streamers
radiating from the ground, which become the return path for the
cloud-to-ground bolt when they reach the charged cloud. They do _not_
attract the bolt (God forbid!). Look at the size of that skinny pole and
wire and imagine hundreds of thousands of amperes flowing through it...

The normal path to the computer equipment is the same as it is for all other
appliances -- through the power lines (and phone lines for a MODEM). Those
utilities strive mightily to protect their investments with voltage-limiting
sparkgaps and suppressors, but the resulting surges can still do in an
electronic appliance. In many cases, an extra fifty-cents of protection is
worth just about that, so it isn't worth the bother. I suggest commercial
surge protectors on the lines, not in the equipment.



Rich


* Electric field theory will explain the reason for the sharp point.