>From: "Jim Smith" <jimsmith@ns.net>
To: <towertalk@contesting.com>
>Date: Sat, 21 Nov 1998 19:45:45 -0800
>
>I've looked through several of my catalogs including Blackburn,
>Burndy, Ideal, Ilsco, Teledyne, and Thomas & Betts. They
>manufacture a variety of clamps using a variety of alloys, and
>platings for use in different installations with the specific
>intent to avoid a problem with dissimilar metals. Bronze clamps
>are made for the purpose of bonding directly to copper, and
>galvanized steel.
No argument here. What happens to 'em is entirely dependent on
the service environment.
Of the catalogs cited, I'm aware of only one clamped connection
(Burndy) that is specifically rated for direct burial (and
certified by the NRC for that purpose). That clamped connection
requires a hydraulic press for installation and is for copper to
copper only - not for dissimilar materials. I'm not saying
others don't exist. just that I'm not aware of them.
>Some of them are specifically rated for direct burial when
>connected to steel, and for use in corrosive environments.
I am very interested in this area. Can you point me to a specific
clamp rated for this purpose? I am aware of several that are
rated for application within a concrete pour. But I don't know
of any that claim to be survivable long term in soil.
>I've seen lots of these clamps in old installations on
>galvanized pipe, and bronze hubs on galvanized conduit, but
>never any corrosion.
What caused the soil surrounding these clamps to be removed so
that you could observe them? Most of the connections we bury
stay buried until there is an observable problem or they find ore
under the radio site.
I have a bronze clamp for connection of the power system ground
at the service entry point to my house to a galvanized cold water
pipe. This clamp is above grade in a very dry climate. The
house was built in 1983. I go out annually and inspect this
clamp. In the 4 years that I have been doing this, I have been
forced to reposition the clamp twice. I have been required to
tighten the screw that clamps the conductor every time I
inspected the clamp. There is obvious degradation of the zinc
galvanic coating where the clamp touches the pipe. I suppose I
might be forced to replace this pipe in 20 years or so. But I
would be replacing it now if it was a structural member of my
tower.
I think that if this clamp was indoors and not exposed to the
(very) occasional rain soaking and the daily 30 to 40 degree
temperature variation, it would probably not be showing any
degradation yet.
But I have already had to replace two galvanized steel pipes at
this house which were in the soil and had corroded enough to leak
water. No clamp of any kind was necessary for them to fail.
The galvanic potential between the underground copper pipes
below the house slab and the buried galvanized steel pipes that
were also in the ground (leading up to the house) caused the zinc
coating on the galvanized pipes to sacrifice itself. And then the
steel, once exposed, to corrode to the point of failure in about 10
years. And this is in earth that is VERY dry most of the time.
We average about 11 inches of precipitation per year. The water
table is 200 feet below grade.
I think the problem here is that there is more copper pipe in the
earth than there is galvanized steel. So the galvanized steel
looses at a high rate.
>Certainly, U.L. is concerned with electrical safety. If the
>connection corrodes, the ground path is lost, creating a life
>threatening hazard. The connection has to be good for the life
>of the installation.
That is why the codes specify a regular inspection routine for
clamped connections which are critical to safety. If they are in
a benign environment, then inspection is all that is usually
necessary. In harsher environments, some maintenance is required
from time to time.
>You're not likely to improve the products by modifying them, and
>voiding the U.L. listing. There's no need to reinvent something
>that's already been engineered, tested, and proven millions of
>times over.
I certainly aggree with the above statement. I don't recall
having advocated modifying any particular connector. If a
connection that includes stainless intermediate material is
required, I recommend using one that is designed for the purpose
and has the materials required with it. Polyphaser and others
make bonding clamps specifically for this application. I would
use one of those.
73, Eric N7CL
-----------------------------
>Jim Smith
>KQ6UV
>
>>I'm sure that the UL is referring to the electrical safety of the
>>bronze to galvanized connection and not necessarily certifying
>>that there will not be corrosion of the galvanized participant.
>>The connection will remain safe for UL purposes as long as there
>>is significantly more material making up the galvanized conductor
>>than the bronze clamp.
>>
>>It is no skin off the UL's nose if the galvanized coating on the
>>pipe is degraded in small areas over the years. They (or the
>>NEC) specify that the connection must be regularly inspected. It
>>can be slightly relocated and retightened occasionally to restore
>>the connection's effectiveness. The galvanized pipe is generally
>>not a thin wall load bearing structural member.
>>
>>
>>Using stainless steel shim as an intermediate material in a
>>clamped connection between either bronze or copper and zinc will
>>reduce the galvanic potential difference across the physically
>>connected materials to approximately half of what it would be if
>>the copper or bronze were directly connected to the zinc. So
>>the corrosion process will be approximately half as rapid as it
>>would be without the stainless.
>>
>>Whether this is an important consideration depends entirely on
>>the environment the tower is exposed to. In an arid climate far
>>from industrial or metropolitan sources of atmospheric
>>contamination, it is probably not an important consideration.
>>
>>In a climate that frequently wets the connection, or exoses it to
>>acid rain or is near salt water, it might be a very important
>>consideration. In such an environment, unless the connection can
>>be completely protected from its immediate environment by a
>>durable moisture barier, a stainless intermediate material should
>>probably be used. Corrosion can't happen if the electrolyte
>>can't touch both sides of the connection. And it will happen
>>only half as fast with the stainless intermediate material.
>>
>>Of course this or any other kind of clamped connection between
>>dissimilar materials should never be used below grade where it is
>>continuously in contact with an electrolyte solution.
>>
>>73, Eric N7CL
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