One generally wants more than one lightning rod ("air terminal" in lightning
protection-speak), particularly if they don't stick up very far above that
which is being protected. The currently fashionable approach is to consider
the "protected zone" that which would be under a sphere with radius=height
of terminal, rolled around the terminal (think of the top of a circus tent).
Before, it was the "cone of protection" idea with 45 or 30 degree angles.
Either way, you get the general idea.
There is also some discussion that having a sharp pointed electrode will
"spray" corona when the field gets high, reducing the local field strength.
I'm not so sure about that one, because the current just isn't all that
high, nor is the actual field before a strike (<20 kV/m), compared to the
breakdown field for air (about 3 MV/m). The sharp point probably just
provides a "preferred" place for the stroke to connect.
On a facility I worked on out at White Sands Test Facility in Las Cruces NM,
we had 6 pointed rods, about 2 feet long , sticking up above what was
essentially a metal shed some 20x30 ft with 10 ft high roof. Each rod was
connected to a copper downlead to a UFER type ground in the slab. The rods
were also joined around the eaves by a continuous cable. Now then...
lightning protection installations seem to be idiosyncratic, so I don't know
that this scheme was any good. It never took a hit as far as I know, even
though there were some fairly good sizes thunderstorms in the area. For all
I know, this was just the favorite scheme used by the contractor that built
----- Original Message -----
From: "Pete Smith" <firstname.lastname@example.org>
To: "Michael Tope" <W4EF@dellroy.com>; "Jerry Keller" <email@example.com>;
"(Reflector) TowerTalk" <firstname.lastname@example.org>
Sent: Saturday, May 31, 2003 7:34 PM
Subject: Re: [TowerTalk] Length of Mast
> At 06:45 PM 5/31/03 -0700, Michael Tope wrote:
> >Actually it brings up a good question for the lightning technology
> >on Towertalk. How come you only see lightning rods on barns and not
> >houses? Is that just because most barns are old, and people have stopped
> >using lightning rods, or is the their some other reason?
> My house -- a farmhouse -- came with an array of 6 lightning rods,
> with woven aluminum down-conductors. Lightning rods are fairly common on
> houses out here. I am very unclear on whether they offer any protection
> all, because the ground conductors seem to violate the low-inductance
> requirement that we hear from Polyphaser. It's not clear to me why
> lightning would "prefer" the rods to other ground conductors that might be
> available in the neighborhood, or on nearby power poles.
> 73, Pete N4ZR
> The World HF Contest Station Database was updated 9 May 03.
> Are you current? www.pvrc.org/wcsd/wcsdsearch.htm
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