To: <towertalk@contesting.com>
>Date: Sat, 05 Sep 1998 07:01:09 -0700
>From: "Robert L. Shohet" <kq2m@eci.com>
>
>Hi guys,
>
>I am psyching myself up for the big project (for me it is big
>anyway) and would like the benefit of your valuable experience:
>
Snip... (Others can answer most of these questions better than I
can)
>4) With respect to grounding, will the following scenario work?:
>At three of the "edges" of the hole (and BEFORE the concrete is
>poured) bang in a ground rod and connect the ground rods together
>with #2 wire, then pour the concrete and connect each ground rod
>with #2 wire to each leg of the tower?
I don't know what your soil chemistry is like. But around here
it has proven extremely inadvisable to permit steel material to
touch both concrete and soil. Most driven ground rods are steel
with a thin copper cladding. Once they are driven into the
ground, the cladding has usually been scratched through in
several places. The steel will disolve and vanish in a few years
(at least around here -Tucson AZ-).
Also, you really only need one earth terminal at the tower. The
volume of earth saturated by strike current will not be much
different between one and three rods if they are that close
together.
>
>5) If #4 is not advisable, does it become "better" if I put in a
>wooden form with the groundrods between the wooden form and the
>edge of the hole (ground rods OUTSIDE the concrete) and do the
>same thing?
I don't think it is a good idea to isolate most of the base from
the earth with wood. Others are probably more knowledgable than
I am on this point. I bring this up as a mechanical
consideration not for electrical considerations.
>
>6) Would it be advisable to connect some of the rebar to the
>ground rod before I pour the concrete or should it not be
>touching? If the ground rod and the rebar should touch, can they
>simply touch or must they be mechanically bonded together in some
>way?
If the tower itself is adequately grounded outside the concrete,
it is probably not necessary to attach the base rebar to the
earthing system electrically. On the other hand, it is not
necessarily a bad idea to do so. But if you do, there are a few
details that become important.
1. Do NOT permit any rebar steel to touch earth. It must be
100% covered by concrete. Any conductor leaving concrete and
entering earth should be copper and should have a large cross
section. "00" bare copper grounding conductor is the best
for this purpose.
2. Any connections between copper conductor and rebar must be
elecrtically (not just mechanically) bonded. Usually this
means usnig an exothermic welding process to alloy the metals
together. The materials to accomplish this are available at
your nearest electrical supply distributor.
3. If any of the strike current must travel through the rebar
cage to get to the ground conductor, then all the rebar
junctions should also be weld bonded not just "twist tied"
with a small wire clip.
4. Any subgrade ground connections should also be thermoweld
bonded. In particular, the connection between the earth
terminal and the "00" conductor coming out of the concrete
must be weld bonded. But this really applies to ANY ground
system connections that are in contact with earth and cannot
be readily inspected at least semiannually.
Number 4 above applies regardless of whether the rebar cage is
tied into the system or not.
Snip... (same reason)
>12) How important is it to ground the equalizer plates at the end
>of the guy wire (the guy wire will be broken up by a few
>insulators along the way so guying the end of the guy wire will
>probably not help a lightning strike at the tower)?
We (commercial entity I work for) don't normally ground the
equalizer plate explicitly. We ground the guys from a point just
on the tower side of the turnbuckles to the ground rod that
protects the anchor base. If the guys are broken with
insulators, it is possible (but not guaranteed) that stroke
current from a tower hit will not flow down the guys. But it is
also possible to take a hit on the last section of guy wire.
If at all possible, bury a substantial conductor from the tower
base earth terminal (or ground ring) to each of the guy anchor
ground rods.
73, Eric N7CL
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