On Wed, 18 Apr 2007 10:15:12 -0400, Roger D Johnson wrote:
>My understanding is that it appears as a leaky
>capacitor to the lightning strike. There must be sufficient
>capacitance available to absorb the peak of the strike as the
>energy leaks off through the semi-conductive concrete. A few
>rebars bonded together in a small tower pier do not a Ufer ground
>make!
I think you may misunderstand the relative conductivities of
concrete and soil. Both vary widely, depending on their chemistry.
Some concrete mixes are good enough insulators to be used as
insulators for electric railways, or to have pavement heaters
imbedded in them, while others have very good electrical
conductivity. It is entirely possible that the conductivity of a
concrete mix may be greater than that of the soil it is embedded
in.
Your comment regarding the connection to earth being comparable to
a leaky capacitance is, however, quite good -- IF you apply it to
virtually all earth connections. Soil is rather resistive. We are
able to measure relatively low values only because there is so
much soil in parallel to provide a path. It is also useful to
think of a radial system, insulated or not, as providing a
parallel path to earth. That path, like the path via a ground rod
or Ufer, will have R, L, and C components. Since the energy
contained in lightning has a broad peak in the MHz range, the
radial system may conduct more of the strike to earth than ground
electrodes.
I haven't read every word of the magazine piece, but a quick skim
of it looks pretty good. I do, however, strongly disagree with the
use of chemically enhanced grounds unless the enhancement will be
regularly maintained. Here today, gone tomorrow, as the rain
washes it away. :) I also agree with comments regarding the
measurement of impedance to earth at audio, and its relevance to
lightning -- that is, not much. If you think of lightning as
anything less than a multi-megawatt MHz-range transmitter, you're
likely to regret it.
73,
Jim Brown K9YC
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