On 12/30/2012 2:14 PM, ve4xt@mymts.net wrote:
Another repetition of the exploding base myth.
If concrete is so explosive, and such a poor conductor, why is it often used to
assist with rod-earth connections?
As well, the very existence of the Ufer ground pretty much debunks the
exploding base mythology.
The NWS even has video showing what could be construed as the exploding
base and it is no myth, BUT we need to be able to separate cause from
hearsay.
I have seen pictures of concrete blown apart from lightning strikes and
some of those were from the NWS so I believe them. However in those
cases we need to ask WHY did the concrete either blow apart, or crack so
badly it had to be replaced.
Concrete by itself is not explosive. The resistivity in ohms per cubic
centimeter is relatively high and after cure the moisture content is
relatively low. This means that in a "properly designed and constructed
tower base" (please note the emphasis on that phrase!) that current
density will be quite low and should cause no structural damage even
with a so-called super strike, BUT in a poorly designed and/or
constructed base it is easy to realize conditions where the current
density can be very high in some localized spots/locations and in those
instances the damage can be spectacular, particularly if you are close
when the strike occurs!
The UFER ground depends on a low current density to work well and to
maintain its structural integrity. It does this by having a large
volume, a large contact area with the surrounding soil, and a large,
intimate contact with the tower through the rerod in the base.
Typically the point where the metal enters the concrete becomes cracked
or the metal becomes rusted. In either case moisture can enter the
concrete and flow along the metal. This leaves a small gap that is a
wet, high resistance area that the high current must jump across. This
produces some very hot, high pressure steam almost instantly and that
produces a tremendous pressure with a force comparable to Dynamite.
So with a properly designed ground system, be it UFER or ring It's
likely that more damage will be caused by voltage induced into the house
electrical wiring than will come in on the coax from the tower and this
is assuming a good single point ground where the cables enter the house.
So, yes a super strike can blow a poorly designed and/or implemented
ground, or base apart, but it's unlikely it will do so with an adequate one.
73
Roger (K8RI)
73,
Kelly
Sent from my iPhone
On Dec 30, 2012, at 12:33, "Michael Mraz" <mike.n6mz@gmail.com> 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.
73 Mike N6MZ
At 17:17 30-12-12, Patrick Greenlee wrote:
I have a friend who is a ham and is EE with 30+ years of antenna design experience. He
tells me emphatically, "concrete is not a good conductor." Given this, I'd be
trying to not depend on concrete as part of a grounding system. Rebar installed
correctly in steel reinforced concrete is probably at least 2 inches under the surface of
the concrete on any side, top, or bottom. Weld plates installed when the concrete was
wet (to give you something to anchor to) or bolts or other components set into the
concrete are typically NOT in contact with the rebar cage inside the concrete (which is
not a good conductor anyway.) So again, it is better to not rely on the concrete to
conduct.
73 Patrick AF5CK
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