Hi Jim,
Thanks for the reply. I am now dangerously close to learning
something. This is a good thing and it is the reason I subscribe
to the reflector.
The important thing that I've learned from this thread so far is
that the devil is indeed in the details.
I looked through the Burndy book. I am trying to get copies of
some of the others you mentioned. I also reviewed the material I
have here on clamped connections and galvanic corrosion.
My conclusion now is that we are both correct but talking about
different circumstances. This discussion did cause me to get a
good understanding of why clamped connections between dissimilar
metals can be rated for subgrade use under some circumstances.
It also clarified in my mind about why we (where I work) can't
normally use them in our applications. And why they may not be a
great idea in a number of different ham related applications.
It turns out that it has mostly to do with the relative surface
area of the cathodic material (copper, bronze, etc.) compared to
the surface area of the anodic material (steel, zinc, etc) which
is exopsed to the electrolyte solution.
As long as the surface area exposed to the electrolyte of the
anode material is MUCH MUCH larger than the surface area exposed
to the electrolyte of the cathode material, the "corrosion
current density" for the anode will be far lower than it is for
the cathode.
The current density for the cathode is not terribly important
because the cathode is not the sacrificial member of the system.
But the sacrificial process speed is proportional to the current
density. So lowering it for the anode material is of paramount
importance. Corrosion is still going on, but it is at an
extremely low rate.
The relative surface area consideration is one of the more
important details but by no means the only one that bears on the
issue.
The clamps seem to be fairly carefully designed and require
correct assembly to achieve the following results.
1. Inclusion of some kind of insoluble grease (sometimes with
conductive powder filler) within the connecting area to fill
voids and prevent ingress of electrolyte into the junction
2. Connecting surfaces made of deformable material to permit
maximum contact area and improve the ability of the grease
to make an effective seal.
The grease / filler is really key because it insures that any
corosion that takes place does not happen within the area of the
connection.
Not found in the connector books but referred to in the
construction guidelines for structures including steel structural
members are frequent references to galvanic protection systems.
These systems can be either self powered using sacrificial anodes
or active bias systems requiring a continuous source of current.
Particularly in the MIL handbook on the subject, these systems
are not presented as optional. It is easy to imagine that there
would be very little corosion trouble with subgrade galvanized
steel or even bare steel if the installation was protected by
such a system.
I now think I understand why my observations have been at such
variance with common practice in electrical construction work.
I suspect that in most large scale steel construction, the vast
majority of the buried conductor surface area is steel. And, it
is probable that correctly engineered large scale construction
with a lot of subgrade steel incorporates some kind of active
"reverse bias" protection system.
In virtually all of the instances that I have been involved with,
the steel ground rod was virtually the only steel element in the
below grade ground system. There was always a much much larger
area of copper exposed to the soil than there was steel. And
since no reverse current protection system was employed, the
galvanized stuff dissolved just as might be expected.
The water pipes at my house suffered for the same reason. There
is probably a 10 or 20 to 1 ratio in surface area copper to steel
in that case. What I'll do the next time I have to dig that pipe
up is put a short section of PVC pipe between the copper and the
steel to break the return current connection.
The implications of all this for the above grade tower leg clamp
that started the discussion in the first place are quite
interesting.
It is clear that it would be possible to correctly apply a
dissimilar metal clamp to a tower leg. But it requires careful
attention to a few details to be absolutely sure that the
connection does not promote deterioration of the tower's galvanic
coating.
1. The connection must contain an occlusive insoluble filler to
preclude ingress of electrolyte into the junction.
2. Steps must be taken so that electrolyte is not held in
contact with both sides of the junction simultaneously. This
usually will require some type of moisture proof coating of
both sides of the junction for a distance that precludes
bridging by a continuous pool of electrolyte. Unfortunately,
"almost" moisture proof is probably worse than no coating in
this case. So whatever material is used, it must be
continuous and it should form a bond to the protected
surfaces.
The interesting thing about this application is that (depending
on the climate and other environmental factors) most of the time,
there is no electrolyte exposure unless some is trapped in the
junction or under the moisture barrier. But if there is
electrolyte contact, the zinc is in trouble because very little
of the surface area will actually be in contact with the
electrolyte. It is probably reasonable to assume that it will be
a roughly equal area of both anode and cathode that is exposed to
electrolyte.
The likelyhood that field assembly personnel would correctly
apply such a connection 100% of the time is near zero for the
people we seem to end up with (especially at remote third world
sites). I'm sure that this realization is what motivates
PolyPhaser and others to use stainless intermedaite material in
their tower bonding clamp kits. Then, if the occlusive grease
turns out to wash away after a long time or is not correctly
applied, and the moisture barrier is not correctly applied or
fails over time, the corrosion of the joint that ensues will
proceed at the slowest possible rate. Hopefully slow enough that
the inspection interval will reveal the situation before the
integrity of the zinc coating on the tower leg is compromised.
>From: "Jim Smith" <jimsmith@ns.net>
To: <towertalk@contesting.com>
>Date: Mon, 23 Nov 1998 20:21:15 -0800
>
Snip...
>I'm speaking of a variety of clamps for different purposes. If
>you give me a very specific application, I can see what would
>work best. For example, the Thomas & Betts cat.# 3849 is a clamp
>U.L. approved for attaching copper or aluminum wire to copper
>tubing, galvanized pipe, or ground rods in corrosive
>environments, and is also U.L. approved for direct burial.
Thanks, I'll check that one out.
Snip...
>The example I'll use though is a military contractor that
>handles many types of explosives. They take their grounding
>seriously. I've done many jobs for them, installing many miles
>of underground conduit, and many miles of copper cable in their
>ground grid. Each building would get bonded to two rings around
>it, which in turn were bonded to the next building. Ground
>cables were taken to any kind of metal object, or structure with
>metal. Every trench would have a ground cable run through it,
>attached every so often to a ground rod before the conduit was
>placed in. You can't dig through this maze with a backhoe
>without locating many of the splices in the grid.
Sounds like they were following the MIL 419 to the letter... See
below...
Snip...
>Working for the military contractor mentioned above, I've dug up
>galvanized conduits that were almost completely eaten away after
>long term exposure to corrosive chemicals, but I don't often see
>corrosion in our area under normal conditions. There must be
>some unusual factor involved in your area if the pipes can't
>last 15 years.
>
Could there be active cathodic protection systems involved at
these facilities? Y'all bury any graphite or solid zinc
electrodes?
Snip...
>>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.
>>
>>73, Eric N7CL
>
>I've never seen this inspection requirement before, or any type
>of maintenance requirement in the NEC. Can you point me to it?
>Perhaps it's a local requirement due to unusual conditions in
>your area.
You are no doubt correct that the requirements for regular
inspections of clamped connections don't appear in the NEC (can't
find the copy I used to have so I can't check this directly right
now). But I do see the requirement in two places. One is local
building codes. The other is in the military handbook for
grounding and shielding sensitive facilities. Specifically
MIL-HDBK-419A Vol I and II. Both sources (building codes and MIL
handbook) are fairly adamant on the point.
The mil handbook frequently refers to the NEC so I probably
mistakenly assumed that is where the requirement came from.
In the MIL handbook, in the chapter on installation practices for
an earth electrode system, it says:
"d. All bonds in concealed locations must be brazed or welded.
Any bonds between dissimilar metals such as between a copper
wire and cast iron or steel pipe, must be thoroughly sealed
against moisture to minimize corrosion. Bolted clamp
connections are to be made only in manholes or in grounding
wells and are to be readily accessable for verification of
integrity."
In the chapter on facility maintenance, they suggest annually as
a minimum inspection interval for all clamped bonding
connections.
>
>Many people due not realize the NEC has no authority in, and of
>itself. The authority belongs to your local building department
>which will adopt portions, or all of the NEC with their own
>amendments to it to suit the particular needs of the local
>community. For example, both the city, and county here have not
>allowed ground rods for grounding electrodes in the past due to
>the unreliable results in our soil, but they are very common in
>surrounding areas.
>
>Jim Smith KQ6UV
73, Eric N7CL
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