----- Original Message -----
From: <jimjarvis@comcast.net>
To: <towertalk@contesting.com>
Sent: Thursday, January 27, 2005 7:19 AM
Subject: [TowerTalk] concrete and lightning
> I was responsible for a 500' self supported installation,
> AM/FM/commercial tenants, and significant effort was placed on routing
strike current away from the foundation
> concrete. That stick gets hit 8-10 times a year, with no
> damage, and it's been up since 1979.
>
> The polyphaser site indicates that concrete will absorb moisture, and
release it slowly.
That seems to be the consensus in the IEEE Standards, as well... Why they
advocate concrete encased grounding elctrodes.
>Depending on whether there is a good conductive path through the
concrete... j bolts or welded rebar cage....you might get current to ground
that way...or, if the cage system doesn't touch earth below th e foundation,
strike current would have to flow through the concrete alone...producing
added heat.
In theory(!) the concrete is lower resistivity than the surrounding soil, so
the heat dissipated in the concrete is less than that in an equivalent
amount of soil next to a bare rod. I suppose the difference is the question
of heating to the point of destruction of the concrete.
Or, as they mention in IEEE Std80, the possibility that the encased
electrode might have become corroded (due to improper installation, or a
continuous current flow), providing a place for water to sit which can then
boil and turn to steam. They point out that a proper encased electrode has
such a small space (if any) between electrode and concrete, that all you're
tallking about is the water in the pores of the concrete, which is quite
small.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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