We've got experts onboard here, but it's still worth
observing that static drain devices...be they hedgehogs
on tower tops, or short whips trailing from several points
on aircraft wings are STILL limited in the amount of charge
they can dissipate.
At some point, charge builds up faster than they can drain,
and there is a greater arc. It was the stbd wing of a 747
en-route to London that took the hit, as we climbed out from
EWR 10 years ago. Once our hearing recovered from the thunder
clap, Cap'n came on the intercom and said, Folks,
that surely was exciting, but it was only a static discharge,
we weren't hit by lightning, not to worry...
Right. I was on the overwing exit row and looking at the
wingtip when the ESD hit, and the acoustic wave rattled the airframe.
Flying through T-storms will do that. But we WERE still flying,
and all systems were apparently still working, so what the hey.
Similarly, fast moving storms will cause strikes to towers,
or sailboat masts, I don't care how many discharge points you've
got.
It's interesting that the secondary strike, or streamer, damage
from hits is a function of the quality of the discharge path.
A sailboat in salt water, with a properly grounded mast, may be
hit, but likely won't sustain hull damage. Properly grounded
is defined by Lloyds as a 1 sq. ft. discharge plate connected by
#4 cable.
Take that same boat in fresh water, and you'll need 100 sq. ft.
to provide the same drain. The result? Sailboats in fresh water
tend to have scads of holes punched or burned through the hull,
when they're hit. AND, they tend to be hit more frequently than
those in salt water.
I virtually never heard of a problem in coastal waters, whereas
in Lake Champlain, VT, there were at least 2 or 3 a year, over a
10 year period.
On a more serious note, I don't recall pidgeon shit on the Tour Eiffel.
Either the observation decks are above their service ceiling, or those
pointy things work for something!
n2ea
jimjarvis@ieee.org
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See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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