Doug,
Concrete is a fairly good conductor-- much more conductive than most soils.
Electric utilities, radio stations, and amateur-radio operators often use "Ufer
grounds", which consist of steel rebar encased in poured concrete as the
lightning-protection grounds for large power transformers and antenna towers.
For example, if I sink an 8-foot-long 1/2" diameter copper-clad-steel ground rod
into my backyard I would get about 25-ohms ground resistance. If I drill a 6"
diameter hole 8-feet-deep and put a piece if 1/2" rebar into it, then fill the
hole with concrete, I will get about 2-ohms ground resistance (essentially
proportional to 1/surface area). Some sources of information on Ufer grounds
are:
* The Grounds for Lightning and EMP Protection, by Block (PolyPhaser).
* http://www.scott-inc.com/html/ufer.htm
* http://www.psihq.com/iread/ufergrnd.htm
* http://www.contesting.com/_towertalk/9709/0636.html
Quite a few articles and books on Electrostatic Discharge (ESD) control mention
that plain concrete is ESD-safe. When we were laying out the IBM Lexington
card line about 1984 we briefly considered leaving the floors as unsurfaced
concrete. We couldn't stand the contamination from concrete dust, though, so we
wound up spending $1,000,000 on conductive vinyl tile over the concrete.
John Barnes KS4GL
Advisory Engineer
Lexmark International
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