[TowerTalk] Soil resistivity-- lightning protection ground

[Jim Jarvis]

Beat Meier wrote:   (heavily snipped for compression)

Hi all
I'm still a grounding dumy but I've just bought a GEO test and have done
some soil resistivity test.  I'm still in putting my new tower of 200ft ...

<snip...summarized: w/ multiple groundrods, varying resistance, all
seemingly
too high.>

I have calculated that you must put 36 ground roads in parallel to
achive a resitance of 13.8 which is still pretty bad.  The tests have
showed what someone already told me that the soil here is not good.
<snip> it seems that going in deep does not make any difference (apart from
the cables you must use to connect the ground roads) in contrast to use
parallel ground rods.

How many copper have good ground rods. The ground roads I can buy here
have only 200um of copper (5/8''-2m or 5/8''-3m). I have seen that they peel
quickly if driven into earth and there is any obstacle ...

Thanks for any ideas or comments
<snip>

Beat, philosophically, if you're putting in the tower anyway, knowing your
ground resistance
is of little value.  I'd put in the best dissipative ground I could manage,
buy insurance,
and not spend money on instrumentation.

If it were my installation, I would concentrate on radial systems for
lightning dissipation.  Get some 2" copper strap, and make a perimeter
circle
around the outside of the tower foundation.  Connect straps from each tower
leg
to that perimeter strap, and continuing out in the directions AWAY from the
house,
for 10m or so. Sink one ground rod at the end.  Make sure the strap coming
off the tower legs has
a radius not less than .3m.  Sharp bends look inductive to high-rise-time
strike
current.

Then, I would silver-solder additional radial straps or #10 wire from the
perimeter
strap out 10-20m.  Again, AWAY from the house.  The objective is to provide
a low
impedance path away from the residence.  As many as you're able to do, and
comfortable
with.  I would think that 8-10 straps, plus another 10 or more copper wires
extending
to 20m would be about as good as you're going to get.

Make sure all cables from the tower come down to ground, and run along it,
before
entering the house.  House entry should be another grounded panel, with
appropriate
protective devices and rf chokes on feed and control lines.

You already know this will not be cheap.  But neither is the 200' tower, nor
will be
repair of damage to the house, when you're struck.

73/
N2EA
jimjarvis at ieee.org