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

[jimlux]

On 11/3/17 9:58 AM, Jeff wrote:
> In one of the Polyphaser documents they talk about the rise time and 
> length of a strike duration.  Forget the details but the conclusion was 
> that rods further out than about 50' would not be effective because the 
> strike would be over before the outlying rods came into play.
> 
> If that's true then it seems there is an implied maximum density of the 
> field based on the 50' maximum radius for rods and the 2x rod length 
> spacing.


Its a bit more complex than that..
you've got what's really a weird sort of transmission line, so while the 
pulse from the lightning stroke at one end of your 50 foot line is done, 
the wave is still propagating out. I guess 50 ft, if the propagation 
velocity is 0.1c, is about 1/2 microsecond, which is getting towards the 
rise time, but not the fall time, of the lightning impulse.

Probably a bigger factor is that since it's spreading the current in a 
2d sheet (assuming you've got multiple wires/rods radiating out from the 
middle) is that the current density is pretty low when you get out there

But, yeah, if the rods have 8 ft in the ground, and they're 16 ft apart, 
each rod has an "active area" of about 200 square feet.  a 50 foot 
radius circle is 7800 square feet - so, 40 rods..

if you could magically make the current distribute evenly that's 1kA/rod 
if the stroke is 40kA.

The IEEE grounding spec goes into all this.. LONG before you get to 
driving dozens of rods, you're probably better off just burying a AWG 2 
wire.

There's good math to tell you how to optimally lay out multiple electrodes.

Is it worth lots of rods?  Probably not.


Remember that these commercial specs are not driven by rigorous analysis 
for the optimum number - they're so that a program manager can put 
together an RFP, and contractors can bid, and they'll get something that 
"works" and can be easily verified that it was built properly. 
Management and acquisition labor is expensive - You don't want to write 
an RFP that says "protect this from lightning"  because you'll get 50 
proposals which then have to be read, evaluated, ranked, and selected.

Much easier to say "Install 37 rods at a spacing of 16 feet on three 
separate lines, etc."

Then, all you have to do is look at the cost data in the proposal and 
see if it is reasonable.




If you go look on FedBizOps you can probably find RFPs asking for more 
detailed analysis for bigger jobs, where you don't want to "cookbook" it 
(for things like ammunition dumps or an entire airport, etc.).



Or, laying concrete with a wire.