Some thoughts about lightning [RE-wrenches]
R. Sparks Scott
sharkey at mrsharkey.com
Tue Feb 27 08:06:40 PST 2001
As as mentioned in passing in a previous post, lightning rods are best
used to prevent lighting from striking at all, and providing a path for
current once a discharge has occurred.
"Lightning rods", and more sophisticated static discharge devices, are
meant to prevent the buildup of static charge and eventual ionization of
the surrounding atmosphere, leading to a strike. The physics of this are
fairly simple. The charges preceding a lightning strike build up on objects
on the ground, seeking the shortest path to the opposite atmospheric pole
in the sky. Actually, lightning doesn't strike down, it strikes up, when
the charge on the ground becomes too much for the air to insulate. The
first step in this process is corona discharge, or the buildup of excessive
electrons on an object, leading to ionization.
Static discharge devices prevent corona by dissipating the buildup of
electrons and providing a low resistance path to ground. Modern devices are
constructed of many sharp spines, or needles, set in a conductive rod or
halo- shaped bracket, literally a "crown of thorns". Simple lightning rods
are effective for the same reason, even with only one point to provide
discharge. Obviously, more lightning rods means more points to dissipate
the buildup of electrons.
So, to protect a PV array, your best bet would be to install a row of
pointy aluminum rods along the top edge of the mounting, with a solid
ground conductor running straight as can be to a deep ground rod. If
lighting is common in the area, a line of pointy rods all along the top
ridge of the building the PV's are mounted on would be more helpful than a
single lightning rod.
There are many other tricks to keeping lightning and lightnign induced
spikes out of equipemtn, but that will be another post, my hard drive
crashed yesterday, and this laptop keyboard is less comfortable to rant
upon...
-S
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