Xref: utzoo comp.misc:2798 sci.electronics:3316
Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ucbvax!unisoft!gethen!abostick
From: abostick@gethen.UUCP (Alan Bostick)
Newsgroups: comp.misc,sci.electronics
Subject: Re: Lightning rods. Was: Protecting computer equipment
Message-ID: <1041@gethen.UUCP>
Date: 13 Jul 88 18:03:55 GMT
References: <1988Jul8.045005.17867@utzoo.uucp> <6179@aw.sei.cmu.edu>
Reply-To: abostick@gethen.UUCP (Alan Bostick)
Organization: There's Unix there in Oakland
Lines: 35

In article <6179@aw.sei.cmu.edu> rsd@ae.sei.cmu.edu.UUCP (Rich D'Ippolito) writes:
>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.
>
>* Electric field theory will explain the reason for the sharp point.

Because of the wire connecting the lightning rod to the the surface of
the earth, the tip of the lightning rod is at the same potential as the
ground.  The static charge builds up not on the ground, but in the AIR,
in the clouds in question, due to convective circulation between the
base of the cloud and its top.

Because of the sharp tip of the lightning rod, the electric field
strength there is much greater than on the (more or less) flat ground.
It becomes much easier for the insulating air to break down in the
vicinity of the rod.  Hence, the preferred path for the lightning is for
it to strike the rod, and travel down the wire to the ground.

Remember that those thousands of amperes are flowing for a small
fraction of a second.  If the wire is stout enough it will be able to
sustain the flow for the needed time without melting.

					Alan Bostick
					ucbvax!unisoft!gethen!abostick