On 7/18/13 7:53 AM, Donald Chester wrote:
Lightning pulses have an extremely short rise time, so they should be
treated as RF, not the same as DC or 60 Hz a.c.
Yes, but... Unlike for an antenna, for instance, you don't care as much
about loss. If some of the energy goes into heating up the wire (as
long as it doesn't melt), you don't much care. Very different than your
top band vertical, where dissipating energy in resistance is bad.
The best lightning
ground is not several rods driven into the ground, but a radial
system similar to what you would use with a vertical antenna. A dozen
or so 20-ft. buried radials using #8 wire or larger would be far
superior to one or more 8' ground rods.
Actually, the goal in lightning grounding is to safely dissipate the
energy and limit the "step potential" (the voltage gradient in the
soil). A big hunk of concrete with some wire inside it does this really
well (and is cheap). I would agree that radials are better than rods,
but that's just because the surface area in contact with the soil is
probably better. AWG 8 would be be huge overkill, I think.
Ground rods make a good a.c.
safety ground, but the best safety ground does not necessarily make
the best lightning ground, and an excellent lightning ground might
not pass a NEC inspection for a.c. power service.
This is, in my opinion, partly the case. A code compliant electrical
safety ground may not be adequate as a lightning ground. I would think
that a lightning grounding system compliant with NFPA 780 probably meets
any of the requirements in the NEC (NFPA 70), as long as it is
appropriately bonded to the electrical system.
My 160m vertical, 127' of Rohn 25, is mounted on that base insulator
I got from the BC station. I have 120 quarter-wave radials (133'
each) made of #12 bare soft-drawn copper wire, buried a couple of
inches below the surface. I know this was overkill, but I had the
wire and at the time (30 years ago) I thought this was absolutely
necessary for minimum ground losses, although since then I have read
enough on the subject to realise I could have used half that many
radials and the additional loss would have negligible.
Yes, radials are a good way to reduce ground losses for RF. And, a
radial ground system will be a fine lightning ground. But if all you
are interested in is lightning protection, an RF radial system is overkill.
Incidentally, I have periodically inspected the radials, and after 30
years buried in the soil, they show very little deterioration of the
copper wire.
Copper is pretty tough in most soils.
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