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Re: [TowerTalk] static bleed & lightning discharge

To: jimjarvis@ieee.org
Subject: Re: [TowerTalk] static bleed & lightning discharge
From: j4976@juno.com
Date: Fri, 18 Mar 2005 16:36:20 -0600
List-post: <mailto:towertalk@contesting.com>
I thought that the point of a lightning rod or similar device was that a
house being a poor conductor, if hit, gets hit by a huge charge.  By
putting up a lightning rod which is a good conductor with a good path to
ground, the lightning can discharge earlier as a smaller charge, and
hopefully do less damage.

As for towers, I see lots of commercial radio and electrical utility
towers that are insulated and have lightning rods (or other) mounted on
them.  Isn't that how we should work?  I am confused by the call by some
to ground the tower instead.  Maybe both systems work ok, but I don't
understand wanting to intentionally use the tower as the lightning rod.

Maybe one of you experts could explain this to me before its too late,
and I put up a lightning research station instead of an antenna.


On Fri, 18 Mar 2005 17:20:48 -0500 "Jim Jarvis" <jimjarvis@comcast.net>
writes:
> 
> 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 
> 
> _______________________________________________
> 
> 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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> 
> 
_______________________________________________

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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