Grant Saviers wrote: >>I would like to be enlightened why #00 wire is
needed to attach to two ground rods which are likely several ohms
resistance to the "remote earth"? Especially when #6 was ok in the last rev
of the standard and when from an I^2*R and fusing current it would do the
job.<<
I'm only quoting the TIA-222-G code; I'm not aware of the facts or reasons
that motivated the committee to upgrade the grounding standards from
222-F. However, I will say that because the committee is composed of
experts and engineers from leading tower designers, tower erectors,
broadcasters and cell owners, I generally accept their judgment. Just as
the committee toughened standards for ice loading in response to tower
failures, I presume they toughened the grounding standards in response to
lightning-induced problems.
Beyond that, my personal speculation is that going with a big #00
essentially makes the radial wire act as a horizontal ground rod, thus
increasing the total volume of earth available to lightning to dissipate to
and of course lowering resistance of the system as well.
Also, I misquoted TIA-222-G: #00 is not specifically required, it says
connections between tower and rods shall be "not smaller in surface area
than 2/0 solid." I interpret that to mean that the spec could be met with
multiple strands of #4 or #6 in parallel.
>>Further, it is a bit of a mystery why Ufer grounding is not included
...This forum has opined extensively about why Ufer's beat ground rods. <<
Again, I'm not aware of the committee's reasoning. Just my personal
opinion: I don't see how the math checks out for Ufer grounds for towers. A
multiple radial and ground rod system might offer on the order of 10,000
cubic feet of earth for lightning to dissipate to. With an Ufer on a tower
with a 6x6x6 footing, that's only 216 cubic feet of volume available to
dissipate one trillion joules of energy. If I'm building a tower I'd rather
direct the lightning energy away from the tower versus concentrating it in
the footing, IMHO.
73 Jim K6OK
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