----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "Wilson Lui" <wilsonlui@atitec.com>; "'David Robbins K1TTT'"
<k1ttt@arrl.net>; <towertalk@contesting.com>
Sent: Tuesday, July 06, 2004 2:07 PM
Subject: Re: [TowerTalk] Lighting
> Thanks Dave and Wilson.
>
> > Grounding does not prevent strikes. What a proper
> grounding system does do
> > is allow for any lightning strike that does happen is
> condected safely into
> > the surrounding soil and not arc through any
> equipment/structure trying to
> > find a lower resistance path to earth.
>
> That's my opinion also, based only on the physics involved.
>
> I notice a large group of people actually think lighting
> does not hit grounded structures because grounding causes
> the charges to bleed off or dissipate.
>
> I'm curious where that idea actually came from. Does anyone
> know?
>
It might go back as far as Ben Franklin, inventor of the lightning rod.
There is much "lore" (no better word for it.. anecdotal, not based on any
sound theoretical basis, etc.) about one kind of air terminal or another.
The problem is it's really, really, really hard to do objective tests.
There were some researchers at Erico (in Australia) who designed a HV power
supply that can recreate the E field time history before the strike to do
the testing, but I think they ran out of funding or were transferred or
found other jobs before they got any substantive results. The usual HV
testing deals with the "after the strike" currents (e.g. the classic 2
microsecond rise time (10%-90%) and 50 microsecond fall (to 50%) double
exponential used for lightning impulses) or for switching surges (much
slower rise and fall times).
As it happens, there's not much commerical market in validated lightning
prevention.. Nobody has anything that's 100% guaranteed, so you'd have to
have a afterstrike damage protection scheme anyway, and once you have that,
you don't care as much about whether you do or don't get hit. There's
plenty of market for one sort of air terminal or another, but, it appears
(to me) that the selection would be based more on speculation than any hard
science.
_______________________________________________
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