On 11/3/17 11:22 AM, Clay Autery wrote:
Mmmmm.... the speed of light is REALLY REALLY REALLY fast... The speed
of current through big copper is still REALLY fast... 50 feet? Nah....
Rise time and duration for WHICH part of the strike event? It's not a
single pulse...
the speed is slower than you think.. one way is to think of it as a wire
immersed in a dielectric, for which the speed will be sqrt(epsilon).. if
you use the ham favorite of 5/13 soil, sqrt(13) is 3.6 so the
propagation speed will be 27% of speed of light, or about 3.6 ns/ft
Another way is to consider it as a transmission line with a inductance
of about 1 microhenry/m, and a capacitance to ground of 10 pF/m (1cm
wire 10 cm above the soil) (not a whole lot different than RG-213, which
is, I think 13pF/meter)
And the 2x rod spacing is a guide... rod spacing depends on the soil
conditions... And nothing says you can't space them closer... IN FACT,
there are conditions under where it is likely advisable to do so...
I don't think the rod spacing depends on soil properties *if* the soil
is uniform resistivity (on a scale comparable to the rod size.. whether
it's sand, 3" rocks, or 12" rocks doesn't matter). It's a geometry thing
and independent of the soil resistivity.
Table 13 in IEEE 141 gives the formulas, and rho (soil resistivity) is a
single term.
Now, if you had a big insulating wall, then driving a rod on each side
might be a good idea.
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