I'm firmly in the camp that says exploding foundations are a myth. I've
spent hours and hours searching the internet and haven't been able to
find a single documented case of a grounded tower foundation that
exploded. I haven't been able to find anyone else who has ever
discovered such a reference either. That includes externally grounded,
Ufer grounded, or ground wires running out the bottom of the base. In
addition, Polyphaser says it is a myth. Check their website.
That being said, I also agree that it is always prudent to provide as
many low an impedance paths to ground as possible. In my opinion, that
includes ground wires connected to the tower legs that fan out radially
from the tower, and it includes making a Ufer ground by connecting the
tower to the rebar cage, AND in many cases it includes having ground
wires connected to the tower base inside the concrete that exit the
bottom of the foundation to buried ground rods.
That last one seems to generate the most debate, but here's why it makes
sense to me. The tower base is already embedded into the concrete
foundation, in many cases quite deeply. The base of my
soon-to-be-installed AN Wireless HD-70, for example, runs to within just
a few inches of the bottom of the five foot deep foundation. No way
anyone can convince me that the massive tower base itself is not already
the lowest impedance path to that point. I can string a dozen #2 wires
from the tower above the base to ground rods surrounding the tower, but
they won't provide a viable lightning shunt to those three large tower
legs (with cross braces) that are already five feet into the
foundation. If I were worried that the Ufer ground was insufficient and
if I wanted to prevent the bottom of the foundation from blowing out in
a lightning strike, I'd have more confidence bringing some heavy copper
wire from the bottom of the tower base out to ground rods buried under
the footing than I would trying to shunt the current around it. I'd not
want to let the ground rods penetrate the concrete, of course, but I'd
have no worry about the copper wire doing so ... it isn't going to
corrode either inside the concrete or in the soil, at least not enough
to matter to me or my heirs. For the sake of argument here, I'm
ignoring the non-trivial issue of making a reliable connection to the
tower base inside the concrete.
I've asked before on this reflector and didn't get any result, but I'll
ask again. Can anyone identify a specific reference (book, scientific
paper, internet link, etc) that documents a case where a tower
foundation exploded from a lightning strike because of a conductor that
exited it below ground? The only related instance I ever found was
where the structure hadn't been grounded at all and the strike had no
place to go except out the side of the foundation.
Here's a second question. Why would a low impedance copper wire
generate more heat, and therefore create more damage to a concrete
foundation, than a higher impedance path through the concrete itself?
Before you answer, consider that that the two paths are in parallel.
The copper wire isn't replacing the broader impedance path of the bulk
concrete around it ... the copper wire is only replacing the volume of
concrete it displaces.
Third question ... why would a solid copper wire that can exist forever
within concrete without corroding, and exist virtually forever in normal
soil without corroding, corrode when leaving the concrete to enter the soil?
Thoughtful replies, whether supportive or contradictory, would be
sincerely appreciated. Since I will soon be installing my tower, this
is more than just an exercise for me.
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