On 5/8/13 6:18 AM, Patrick Greenlee wrote:
Jim, $8-$10 per connection certainly gives me motivation to seek
alternatives. I have done some reading and consulting since previous
posting. I have the equipment and the skill and now after some
consultations and reading I have an understanding of the methods
needed. I realize not everyone is so equipped and for them Cadweld
"shots" at $10 are likely a wise choice. I would never say never
regarding my use of Cadweld. Who knows, maybe one day I will need to
make a connection out in the boonies where there is no electrical power
and terrain prevents getting close with my welding trailer and my back
pack oxyacetylene rig is unavailable for silver solder alternatives.
If you have an inspector to please, then hard (silver) solder or brazing
might be a tough sell. From an engineering standpoint, I don't see much
difference between a one-shot and a properly done welding job with
copper filler between copper things. The challenge is whether you can
guarantee "properly done". is the welder certified to do copper/copper
welds? Do they periodically do test welds and take coupons for testing
and inspection, etc. This is a life safety kind of application like
structural or pipeline welds, and process control is important. I'm not
sure an oxy-gas weld would meet the bar: it might need to be GMAW (MIG,
with copper wire?) or GTAW (TIG) so you have the gas shield to prevent
inclusions and stuff. That said, I've seen some impressive oxy-gas
welds on exotic materials over the years, so I suspect it can be done.
Brazing, by its nature, uses a filler metal with a lower MP than the
things being joined (otherwise you'd call it welding<grin>). The lower
MP, and the potential for higher electrical resistance, raises the
(unlikely) specter of the metal melting out due to temperature rise, as
well as a host of long term aging effects from differential coefficient
of thermal expansion which might lead to cracking, etc. (certainly a BIG
deal with soft solder)
Doing hard solder or brazing might fit in the general bucket like bolted
clamps. If you're in an environment where it can be regularly checked
for integrity (e.g. a broadcast station or commercial comm site, with a
regular maintenance and inspection plan), then the issue of long term
cracking or degradation is less. You'd find it in the regular
inspections, fix it, and move on.
In an environment contemplated by the electrical code, it's more a "set
and forget" with no expectation of inspections or maintenance. That's
why the code likes thermite. It's all the same metal, it's fairly
"workmanship" insensitive, etc.
For proper mounting of such items as AlphaDelta TransiTrap surge
protectors to ground rods I prefer welding a stainless steel tab at or
near the top of the rod. A drilled hole in this tab makes a fine mount
for the protective device. Similarly a ground wire or braid terminated
with a soldered terminal can be through bolted to a similar tab and then
weatherproofed with your favorite goo. I personally prefer this to the
screw clamp sold for ground rod connection at the electrical supply
house. Much of my hands on experience comes from the marine applications
environment where salt air and some salt water splashing is the normal
environment. Where I live there are no permits required and no
inspections. I prefer to try to understand the science behind "code"
and meet or exceed its performance rather than slavishly follow it.
In this case, the science that pushes for Cad-weld is the dissimilar
metal problem: how do you make sure that the bond between the metals (SS
and copper cladding, or more likely SS and the underlying steel, since
the copper will probably burn off) will hold.
And frankly, One-shots have the same problem of dissimilar metals, when
the rod is copper clad: the claddings not that thick so at some point
you have a copper/steel interface. The only thing about the exothermic
weld is that there's a LOT of test data and historical experience. The
companies that make them have run hundreds of samples through test
campaigns/cross sectioning and microscopic examination/etc. So while
we can speculate about potential CTE mismatch and microscopic cracking,
in the case of a Cadweld or oneshot, there's actual test data to look at.
Some one-off "stick a tab on the rod with a TIG or torch" doesn't have
that sort of backup testing and historical experience. Might work, might
not. We could speculate for days, weeks, months and not get a lot closer
to a definitive answer.
It's your own system, so you get to make the call about risk
acceptance, esp since there's no third party inspector to convince.
I would agree that a screw clamp is lame.
A welded connection of identical metals is best.
Somewhere in between is anything else.
My situation is not unique but it does place me a few sigma out on the
bell curve. Most hams don't have the freedom I exercise. What with
permits, inspections, home owner associations, height restrictions,
small lot sizes, RFI/TVI etc. I feel sorry for the average Joe trying to
pursue a nice hobby while surrounded by Philistines.
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