[TowerTalk] static bleed & lightning discharge

[David Robbins K1TTT]

The concept behind the conventional 'franklin' lightning rod is to provide a
preferred point for the lightning to hit and a good path to ground from
there.  The pointed rods provide that sharp point for the streamer to get
started and the heavy cable provides a good drain around the outside of the
building to prevent the charge from trying to work its way through the less
conductive wood and small interior wiring.

The radio towers that are insulated are that way for rf considerations.  You
will usually see a large spark gap that provides a preferred discharge route
for lighting currents around the insulated base.  The insulated base and
insulators on guy wires are much too small to stop the lightning, or even to
stop the charge build up before a stroke.
 

David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
 

> -----Original Message-----
> From: towertalk-bounces at contesting.com [mailto:towertalk-
> bounces at contesting.com] On Behalf Of j4976 at juno.com
> Sent: Friday, March 18, 2005 22:36
> To: jimjarvis at ieee.org
> Cc: towertalk at contesting.com
> Subject: Re: [TowerTalk] static bleed & lightning discharge
> 
> 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 at 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 at 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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