Jim Brown wrote:
> 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.
>
I was speaking in generalities which always gets me in trouble! Mr Ufer
developed his
ground system during WW2 to protect ammunition storage bunkers in the
desert southwest.
The theory being that, during the infrequent rains, the concrete would
absorb moisture and
retain it longer than the surrounding earth. I assume that "ordinary"
concrete was used.
I think it's obvious that, the lower the R (concrete and surrounding
soil), the less dependent
on C the system becomes.
> 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.
>
My understanding of the Ufer ground is that it was a conductor embedded
in the concrete
foundation of the bunker that went completely around the perimeter of
the bunker. I don't
know if the bunkers had a concrete floor with additional conductors
embedded. The idea
was to provide maximum C and minimum R in that particular circumstance.
> 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.
>
Agreed.
73, Roger
--
Remember the USS Liberty (AGTR-5)
http://ussliberty.org/
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