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Re: [TowerTalk] Lightning protection - length of grounding rods

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Lightning protection - length of grounding rods
From: David Robbins <k1ttt@verizon.net>
Reply-to: k1ttt@arrl.net
Date: Thu, 29 Jan 2015 11:41:34 -0600 (CST)
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
" Personally for #3 - I take the strategy that on a ham's budget, we are never
going to do a good job at a major strike. I am just going for not exploding
the tower base. So I put 3 ground rods in the hole itself with direct
connects at the bottom of the buried base section. So that at least there
is no concrete path of choice to ground on a direct hit. Beyond that I
assume the ground itself will dissipate the energy just like any other
forest strike."

as far as this one... it is extremely rare that lightning explodes a tower 
base.  Just think of how
many times high voltage power lines get hit every year and how few reports of 
tower foundations being
damaged there are.  And then go find all the pictures hams aren't posting about 
their poured by hand,
ungrounded, no rebar foundations having been blown up by lightning... no, not 
everyone listens
to the prime directive and does what the manufacturer says... heck, i'll admit 
it, I have a tower here that
has a foundation like that... well, except it does have one rod next to it that 
is attached to the house 
perimeter ground.  

now, about rods at the bottoms of the legs...  first of all, 3 rods in the same 
foundation hole aren't going
to be far enough apart to be considered separately so they aren't much better 
than a single rod.  then,
keep in mind two things.  the diameter of the tower legs is bigger than the 
rods and they are probably 
connected to the rebar either by design or accident, and the conductivity of 
the concrete is higher 
than the conductivity of the soil.  so, the current comes down the tower into 
the foundation, sees the 
nice big fat ufer ground and since most of the opposite charge is near the 
surface because it was 
attracted by the leader coming down from above it heads out through the 
concrete before it even 
gets to the rods at the bottom that have a higher impedance.  there have been 
some published 
experimental results, and algorithms, (and unfortunately more that are still 
proprietary or copyrighted
and still paywalled) showing that the current from a vertical rod or foundation 
mostly goes out radially 
and not off the tip.  also,the current does not flow uniformly, it usually 
forms streamers though the soil 
(google 'fulgurite' for extreme examples) so you can expect that there will be 
lots of streamers through 
your concrete, or just across the top of it, as the charges rush towards each 
other.  my preference is
to go from the tower legs to a rod near the foundation just as a way to spread 
out the current a little
bit more than the ufer ground will by itself, though the effect is small 
electrically, its effect on inspectors
and visitors is large.... especially when they see the huge conductor I used, 
though most don't seem to
notice that the conductor is fatter than the cross brace it is clamped to or 
the rod itself.



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