[TowerTalk] Mot R36 related to NUMBER of installed ground rods

[jimlux]

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.