David,
Urban legends? I don't think so.
WCTC-AM, New Brunswick, NJ. Took multiple strikes, and had
structural cracks
in the foundation afterward. This was in the 1965's, when I worked
there as an engineer.
WBRW-AM Somerville, NJ. 4 tower 190' inline array. One tower
took a strike, during
construction and proof of the array. A crack appeared in the
foundation, from the insulator
base to earth, opened to a width of 1", showing burn marks. 1972.
Both of these were series fed sticks. i.e. insulated, with single-
point tower bases.
In the interest of length, I removed a paragraph from my post, which
said something like:
"The area of debate probably occurs when the tower mounting itself
routes lightning into
the foundation. i.e. J-bolts or tower legs extend into the
concrete. In that case, an argument
can be made for the brute force method of bonding everything together."
Guess I shudda left it in, eh?
You're right about cadwelds.
As for my argument that you want to route lightning around the
foundation as much as possible,
I'm sticking to it. In addition to two broadcast arrays, I've seen
enough exploded sidewalks from lightning,
I'd rather not dispel the urban legend myth with first-hand experience.
It is my contention that the shape and form of the re-bar does NOT
constitute a low-impedance path to
the complex lightning waveform. It is highly inductive. Tie it
together? fine. But I'd bypass it with
4" copper strap, (or 1.5", if that's what you're using), making a
nice, smooth 18" radius bend, and exiting
the concrete away from the rebar.
Wish I had digital pix of our ground treatment of the 500' WCTC-FM
self supporting tower. I was consultant
on that project in 1978. It gets hit in virtually EVERY storm.
N2EA
On May 29, 2008, at 4:45 AM, David Gilbert wrote:
>
> Jim Jarvis wrote:
>> The idea is to provide a low impedance path AROUND the concrete
>> foundation
>> for lightning to get to ground, not THROUGH it.
> Why? If it is indeed a low impedance path (and it would be
> useless if it wasn't), why is it less desirable to run it through
> the concrete than around it?
>>
>> Personally, I would not attach the rebar to the tower at all. To
>> do so only makes
>> that a more viable path for lightning.
> Why? The tower legs (or the tower base or the tower mounting
> bolts) are already embedded far into the concrete and they arguably
> form the lowest possible impedance path INTO the concrete.
> Besides, Polyphaser strongly recommends making a Ufer ground out of
> the rebar cage specifically for the purpose of lightning protection
> (see http://tinyurl.com/4grr2o). In fact, my local zoning
> department INSISTED that the tower be bonded to the rebar cage.
>
>> In addition, the corrosion which will take place
>> with the copper attachments will assure that in 10 years, those
>> embedded connections
>> can't be trusted.
>>
>>
> That's why Cadweld connections should be used to bond to the rebar,
> and it isn't any more difficult to do that than to bond the ground
> wires to ground stakes. I bought the mold and materials to do so
> on eBay for less than $30.
>
> I keep seeing this same discussion pop up time after time, and yet
> I've never seen a single coherent, fact-based argument why a ground
> wire should not be run through a concrete tower base. Lots of
> people claim to have heard about a tower base that exploded for
> that reason, but even Polyphaser says they haven't been able to
> confirm any instance of it (they call it a myth). I installed a
> tower and antenna system at my own QTH over the last few months ---
> I bonded the tower base to the rebar cage and ran six ground wires
> from the tower out through the side of the concrete base to be
> connected to ground rods in a radial pattern. Pictures of the
> entire installation can be found at http://tinyurl.com/5jj8gy. We
> get some major lightning storms here in southern Arizona during our
> summer "monsoon" season and if my concrete explodes I'll be sure to
> let everyone know and add pictures of it to the web site, but in
> the meantime it would be nice to see this discussion revolve around
> facts instead of urban legends.
>
> 73,
> Dave AB7E
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|