Bill,
I'm sorry it took me this long to get some time to devote to your
question. I didn't see any postings in response so I'll comment
now anyhow.
To: <towertalk@contesting.com>
>Date: Sat, 05 Sep 1998 19:17:30 -0500
>From: Bill Aycock <baycock@hiwaay.net>
>
>I am putting up a new (to me) tower, and am in the mixed
>'plan-ahead' - 'doit' mode. What this means is that I have
>decided on the basing, dug a hole, and cast some concrete. I
>decided not to ground within the base, for various reasons, but
>will have extensive grounding around the tower.
>
>I am now contemplating the corrosion situation with regard to
>the interaction of Copper- Bronze- Iron (things DO get
>scratched) and hot-dipped galvanized. I have read lots here,
>but, the confusion comes when I see what (supposed-to-be)
>professionals do.
>
>The local power people will clamp copper grounding wire directly
>to galvanized iron rod. This is supposed to promote local
>corrosion, according to what I read here. To make this worse-
>the actual clamp is Bronze, with a copper bolt to tighten the
>clamp to the wire and rod. The contact surfaces here are :
>copper to Bronze-copper to galvanized- Bronze to galvanized-.
>An additional arrangement I have seen is a copper coil on the
>base of a pole, running up to the top, where it is hooked into
>the galvanized fittings that hold the transformer up.
The power folks are not too out of step here as long as the
connections are not below grade. A lot depends on the nature of
the environment that the connection is in. In areas of heavy
industrial or natural corrosive atmospheric pollutants, even
above grade connections (like I'm assuming these are) need to
have some form of atmospheric barrier (tar, paint, grease,
silicone rubber, etc.) applied after the connection is made. But
in relatively benign atmospheres, the connections you describe
are quite reasonably left exposed.
Bronze is very close to copper in the Galvanic Series. Depending
on exactly what chemical composition for bronze it is, it is just
above or below copper in the series. But the distance from
copper is not great enough to cause severe corrosion problems
with the copper. In the connection you describe, the zinc in the
galvanic coating would be the anode in any galvanic corrosion
process involving the copper or the bronze and the coating. The
zinc coating would be the thing that corroded away. But for this
to happen requires the presence of an electrolyte solution in
contact with both metals. If the connection will be dry most of
the time, it will take quite a long time for the corrosion to do
enough damage to affect the system.
The galvanic coating on the rod is to protect the steel from
corrosion. The idea is that the electrochemical surface
uniformity for the galvanic coating is much better than it is for
steel so the galvanic coating will corrode away much more slowly
when in an electrolyte solution than the steel would. Of course,
this presumes that the galvanic coating is not violated by rock
scratches when the rod is driven into place. Where I live, this
is a ludicrous assumption.
Since a major fraction of the rod is in continuous contact with
an electrolyte solution, I'd be more worried about the rod
failing than the connection. Essentially, it is a race to see
which fails first.
>The telephone people will apparently attach to anything that
>looks like it might go to earth somewhere. In one case- I found
>the "ground" was a piece of four-conductor house phone wire (#20
>or 22 ?) with all four wires together, leading under the house,
>to the house panel ground. Maybe that was why a strike over a
>mile away came in the phone line and blew my computer?
Yeah, the phone company and the TV cable guys just don't seem to
be up to speed about why they are using a ground and what to to
make a good connection to a ground. The awareness of even the
ones that do have some idea of what they are doing terminates at
about 3 KHz in the RF spectrum.
In times passed, the phone company was pretty good at this but
lately, I see some really bizarre stuff that they have done. It
is really hard to get good technical help these days. I
personally know a few phone company folks who _DO_ now how this
is done. But what actually happens at anyone's particular site
is just a random function of who gets assigned to the job.
Usually, we insist on overseeing this part of any installations
for our company.
>What I have read, and intend to use, is stainless shim to
>protect copper from Galvanized, where I clamp grounding
>conductor to Rohn 25. I know that exothermic welding is 'best',
>but want to be able to change, later, as I add radials, without
>having to cut .
Exothermic welding (or competent torch welding or brazing) is
"best" and should be used exclusively as the connection method
for similar materials which will be installed below grade and
therefore will not be inspectable after installation. You would
_NOT_ want to use any of these methods to connect to a tower
leg. Although, it would probably be OK for a connection to the
central area of a tower base plate.
The connection you describe is the proper way to get a good
mechanical bond to a tower leg. The stainless is about halfway
between copper and galvanic coating in the Galvanic Series so the
corrosion potential is cut in half. It is reduced far enough
that in a benign atmosphere, no further protection should really
be required (beyond regular inspections). By the way, the
stainless is protecting the galvanic coating. Not the copper.
We frequently make these connections in a less than benign
atmosphere (dissolves uncoated aluminum antenna elements in about
18 months). The procedure we use with pretty good success is as
follows:
1. "Shine" the tower leg where the connection will be made with
stainless or bronze scouring pad (like mom used to have in
the kitchen before the plastic ones). Rub lightly, don't
scratch through the plating.
2. Immediately coat the shiny area with silicone high vacuum
grease.
3. Coat the connecting surfaces of the copper conductor and
clamping device with silicone high vacuum grease (Shine up
the copper a bit first if it needs it.).
4. Wrap the tower leg with the stainless shim.
5. Clamp the connection together.
6. Pass 10 amps or so through the connection and measure the
voltage difference between the clamp and the tower leg with a
DVM. It should read less than 0.1 volt.
7. Wipe any excess grease from the outside of the connection
with a dry paper towell. Be careful not to spread this stuff
around on the area very far away from the connection.
8. Heat the area around the connection (+/- 6 in) with a heat
gun until it is hot to the touch (to dry any moisture).
9. Spray the entire area of the connection and the tower leg for
6 inches either side of it with a medium thick coating of
"Plasti Dip" (R) spray-on heavy duty flexible rubber coating.
This is the familiar tool handle dip coating but it is
available in spray cans. Inspect carefully to be sure there
are no voids in the coating.
These connections normally outlast the service life of the tower.
But they should be inspected annually anyhow (in the nasty
atmosphere present at some of our sites). It is pretty easy to
cut the rubber coating off, and peel it away from the connection
area (it doesn't adhere well where there had been silicone
grease) for inspection. Once the connection is inspected, the
coating is easily resprayed.
The coating is made by:
PDI, Inc.
P.O. Box 130
Circle Pines, MN 55014
(612)-785-2156
according to the can I have here.
We get ours at a local ACE Hardware store or from Grainger.
I hope you find this useful.
73, Eric N7CL
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