[TowerTalk] Grounding of tower

[Jim Lux]

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.
>