[TowerTalk] Grounding/Bonding Lessons Learned

[Patrick Greenlee]

Paul, Thanks for sharing your experience.  Regarding a hard to ignite 
shot of Cadweld:  The process is quite like Thermite.  I would expect 
that you can place a small quantity of powdered magnesium on top of the 
difficult shot and use a fuse consisting of a small thin strip of 
magnesium ribbon to ignite the magnesium powder.

Patrick    NJ5G

On 3/29/2015 12:18 AM, Paul Christensen wrote:
> Thought I would share some lessons learned while recently installing a large
> ring ground system.   The goal was to form a ring ground around two towers,
> AC power entry point, and a small telecom shelter.  We followed the
> "Motorola R56 Standards and Guidelines for Communications Sites."
>
>   
>
> CADWELDS
>
>   
>
> Out of two dozen Cadweld one-shots used, only one would not ignite.  That
> was the first device tried.  When pouring in the granules into the crucible,
> it's important to: (1) NOT shake the vial; (2) slowly pour the mix; and (3)
> delicately pour the final igniting power at the very end.  Erico, the
> manufacturer, really needs to separate the two powders into separate vials.
> If the fine igniting powder accidentally becomes mixed with the granules,
> then ignition absolutely, positively will not happen with any form of
> igniting  and you will have wasted a $10 one-shot.  Don't bother heating it
> either with a propane torch.  It takes a spark with the igniting mix on top
> to start the chain reaction.
>
>   
>
> We found children's sparklers to be an excellent spark source.  Much time
> can be wasted with a flint striker, especially in breezy conditions.
>
>   
>
> SILVER SOLDER
>
>   
>
> Where we did not bond with Cadweld one-shots, we used 15% silver brazing
> bars.  In the past, I used acetylene-air for the task.  This time, I
> purchased a Lincoln Electric Oxy-Acetylene torch set and wow, what a
> difference.  Acetylene-air produces excessive carbon/soot and quickly turns
> the copper black - to the point where solder will not flow.   In the past,
> I'm not sure how I managed to use acetylene-air, but adding a small amount
> of O2 into the mix completely burns out the carbon and with a little
> practice, the right acetylene-O2 mixture will produce nothing but an intense
> blue cone (a neutral flame) at the end of the torch.  That cone heats the
> copper surface without adding any impurities from the gas.   With no flux,
> the brazing bars immediately flow to the copper surface.  When cooled, the
> result is nearly as good and strong as an exothermic bond.   Some folks have
> successfully used MAPP gas, but often does not get hot enough when bonding
> to large surfaces with high mass.
>
>   
>
> For the AC premise grounding, we used Cadwelds to join multiple ground rods.
> #2 wire solid wire extended from the rods up the power entry panel.  That
> satisfied the NEC but we augmented that grounding with a 4" copper strap
> ring around the perimeter of the telecom shelter.
>
>   
>
> I don't believe the current NEC (and most local codes) allows
> silver-soldering in place of clamps or exothermic bonding.  Just a hunch,
> but I suspect the main reason is that it's too much of an art form; no two
> silver-solder bonds are the same and it takes some skill to get a good bond.
> By contrast, if one just follows the Cadweld instructions,  the result is
> repeatable.
>
>   
>
> Paul, W9AC
>
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