In cases like this "ground potential" is a relative term due to ground
currents. The resistance of the ground itself can cause large potential
gradients over short distances. This is exactly why it's possible to get
killed by lighting without being directly hit: the potential across the
distance between your feet can be more than sufficient to generate a
voltage that the soles of your shoes cannot withstand. Peak currents in a
cloud-to-ground strike can be exceed 50 kA on a regular basis and it
doesn't take much R to get a big E with that I. The currents involved are
truly mind-boggling.
The advice your brother got is essentially correct: a *very* conductive
grid needs to be installed all around the protected site such that large
currents will not induce large voltage gradients. Copper sulfate won't do.
Experimental weather radars I've seen are protected against lightning with
a 00 ga copper wire mesh within the ground (3-5' deep) with many ground
rods, all bonded together using thermite. A very coarse heavy-wire grid
above the radar dish elevation acts as pseudo Faraday cage. Everything
needs to be bonded together and then bonded to this heavy ground to be
protected. Lightning protection is an expensive proposition and has to be
weighed against the potential losses, but a professional analysis and
installation is probably the best way to go.
Kim Elmore, N5OP
At 11:29 AM 7/13/2004, you wrote:
This is somewhat off-topic, however the issue of lightning protection is
important, and knowing how knowledgeable our
members are, I thought someone might have a solution to the following
problem..
My brother has a very expensive telescope in an observatory located on
Petit Jean Mtn. in Arkansas. His telescope has a
computerized tracking and "GoTo" system built into the base of the mount
which is mounted to a steel pier. The steel
pier is bolted to a 14 ton concrete substructure that is about 5 feet in
the ground under the floor. The concrete
section has lots of rebar and iron meshing inside of it.
Lightning has taken out his computer control in the base three times in
less than a year. The last two times, nothing
was plugged into the unit so the base was simply at ground potential. It
appears that because of the bedrock
characteristics of this mountain, he is getting hit from the ground
itself. I can't think of any other way.
Someone emailed him the message below this morning as a possible solution,
however I would be more inclined to install a
tower or something a bit further to serve as a lightning rod. Maybe even
treat the ground around the tower with Copper
Sulfate to something similar to help give it a good ground. The
suggestion below about running a copper perimeter 10'
around his observatory seems like it might add to the problem. 10 feet
does not sound like enough distance to be
absorbing a direct hit of lightning. Since the base of the scope and the
pier are both metal, there is really no way to
isolate the telescope computer away from this. It is much too heavy to
use nylon bolts and an insulating pad between
the pier and concrete.
Any ideas or suggestions?
Thanks & 73,
Brian / w5ami
----- Original Message -----
From: "P. Clay Sherrod" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
> No way at all....in fact I am convinced now that the lightening is
coming up through the
> ground.
> I talked to an electronics guy yesterday (he owns SoundCraft) that
suggested I build a #3
> copper perimeter ground around the base of the observatory, about 10
feet away from the
> walls with grounding rods on two opposing corners.....he says the
problem up here is that
> you cannot get a proper ground on this mountain because of the rock.
>
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Kim Elmore, Ph.D.
University of Oklahoma
Cooperative Institute for Mesoscale Meteorological Studies
"All of weather is divided into three parts: Yes, No, and Maybe. The
greatest of these is Maybe" The original Latin appears to be garbled.
