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.
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