On 12/30/2012 6:06 PM, Hans Hammarquist wrote:
If there is anything predictable about lightning it's lightning's
unpredictability.
Anecdote or not. If you push enough current though any conductor it will "explode".
(This is actually use in one type of detonators.) I don't know how much current required to crack
concrete and if a direct lightning strike can be enough, but I don't think we have to worry so much
about that as such a "super" strike will cause more harm in other places around the tower
hit.
It depends on the area interface of the conductor and concrete, IE:
The more conductor and the more volume of concrete the more current it
takes to do damage. moisture, concrete makeup, and soil conductivity all
play a part. The volume of concrete and size of the rebar cage are the
most predictable. So a large UFER ground is more predictable than just
a lot of ground rods and wire.
I have my present house designed to take a "super" strike. I am using 3/4" copper pipes
in all four corners of the house. According to the engineer, helping with this design, my house is ready
for one of these "super" strikes, observed on occasions. I believe it is a lightning with a
peak current of 300 kA, maybe it was even more. Anyhow, after three confirmed, direct hits in the house,
I feel relatively sure I did something right.
There is no guarantee against damage a "super strike". You are only
playing the odds. even with strikes that fall within the ordinary realm
you are still playing the odds.
Each properly implemented precaution increases the odds in your favor,
but they never reduce the chances of damage to zero.
My system consists of 32 or 33 8' ground rods, cad-welded(TM) to over
600 feet of bare #2 copper in what now days could be considered an
enhanced "Ring ground"
The "UFER ground" is considered to be one of the most reliable.
However the "Single point ground" where every thing enters the house is
the most likely to prevent damage in the house.
A large rerod cage inside an even larger concrete block reduces the
current density by increasing both the surface area and contact area to
a point where the current is "unlikely" to do any damage.
I believe someone explained this in detail earlier. Broadcast stations
and the military quite commonly use this method of grounding. Detailed
explanations are readily available on the net!
73,
Roger (K8RI)
Happy New Year everyone,
Hans - N2JFS
-----Original Message-----
From: Jim Lux <jimlux@earthlink.net>
To: towertalk <towertalk@contesting.com>
Sent: Sun, Dec 30, 2012 5:22 pm
Subject: Re: [TowerTalk] Earthing a tower
On 12/30/12 10:33 AM, Michael Mraz wrote:
My dear old SK friend Al Caplan, who used to manage the Hy-Gain amateur
division when it was part of Telex, was emphatic about proper grounding
when I bought my HG-52 crankup. He said that lightning could split the
concrete around the re-bar if the tower took a direct strike. It was a
long time ago, but my recollection was that this nasty event actually
occurred to some unfortunate ham who had to abandon the old foundation
and install a new one in a different location.
this anecdote and ones similar come up all the time. The short answer
is, "not if the system was properly constructed in the first place".
If the rebar penetration of the concrete surface is incorrectly done,
then it's a problem. For instance, if the top of the concrete holds
water at the steel/concrete join, it will eventually wind up inside a
tiny crack, which then causes corrosion of the iron. That in
combination with water in the crack will cause spalling or cracking when
lightning heats it up.
And when discussing "well, what it the installation isn't done right,
then disaster strikes"... I'd contend that the odds of getting a Ufer
done right are a whole lot higher than some of the bolt on clamp and
grounding rod schemes I've seen over the years.
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