Red wrote:
> When considering grounding systems, do not overlook two major
> categories: soil characteristics and characteristics of the energies you
> are trying to ground.
>
> Important soil characteristics are conductivity and reactance. It is
> useful to think of the soil as a lossy capacitor, especially in the
> spectrum occupied by lightning. Even if conductivity is very poor,
> capacity will provide a path for dissipating lightning energy. That
> path is more effective at the higher frequencies and drops to zero at DC
>
> The energy in lightning is mostly composed of components at frequency of
> 1 MHz and below. Earth dissipates it by a combination of conduction and
> reactive current. Model a system of ground rods and radials as a number
> of capacitors paralleled by resistors and interconnected by elements
> composed of distributed capacity, inductance, and resistivity.
A more accurate way of modeling the system would be for the resistance
and capacitance to be in series.
There is a time constant involved in the dissipation of energy to earth.
The resistance of the rod couples to the soil which acts as a large
capacitor being charged through that resistor.
The more conductive the rod is to earth the lower that series resistance
is. The more effective area of earth that the rod has an influence on
the greater the capacitance. There is greater effective area by greater
length rods but not by larger diameter rods. The capacitive effect is
not directly from rod to earth.
Yes there is a small amount of direct capacitance but no where near as
great as the earth.
The area of influence of the rod is roughly a sphere about the diameter
of the rods length in the soil. The larger that sphere (longer rod) the
greater the influence it has and the more energy it will transfer in a
given amount of time.
This is why placing rods too close to one another does not work as well
as spacing them apart. Too close and those spheres of influence overlap
each other. Each sphere size can get rid of only so much energy in a
given amount of soil in a given amount of time.
Placing rods too far apart, the connecting cables inductance starts to
get too high and the next rod begins to look like a high impedance and
is less effective.
The same thing happens with very long rods in the ground. It would seem
that if an 8 foot rod is good then a 20 foot rod should be better but
that is not usually the case. The inductance of the long rod gets high
and very little energy reaches down toward the end. Better use is made
of several shorter rods.
However in very dry soil longer rods are sometimes effective if they can
get down to some moister.
73
Gary K4FMX
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