[TowerTalk] Info/Advice re: Exothermic Welds

[K8RI]

On 8/20/2012 1:54 PM, Art Greenberg wrote:
> I've recently moved to north-central Florida and want to get a couple of
> multi-band antennas going this fall. The plan is to install a 43-foot
> vertical and a 135-foot doublet fed with ladder line. In both cases I'll
> place a remote matching network at ground level, and run coax and control
> lines for the matching network from there into my shack in a shallow-ish
> trench (possibly with conduit) and through an entrance panel. I will run a
> ground wire in each trench with the coax, and will install ground rods at
> both ends of each trench and at intervals along the way. The entrance
> panel will contain a grounding plate on which lightning arrestors are
> located, and I'll run a ground wire (or a wide copper strap) from the
> grounding plate to that ground system, and to my electrical service ground
> rod. I'm looking for an acceptable way to bond the ground wire and copper
> strap to the ground rods. I've seen the term "cadweld" used, and I watched
> a utility crew use an exothermic weld technique to connect a ground wire
> to a ground rod at the base of a utility pole on my property. I have no
> direct experience with the process.


Before you start, keep SAFETY paramount. Do not look directly at a 
burning charge. It is very bright. Do not use matches to ignite the 
charge as this stuff is very hot. It will burn through concrete. Follow 
the directions that come with the "One Shots" to the letter.

"One Shot CadWelds(TM) are relatively easy to use, but bear in mind you 
are working with Thermite and I know of nothing that will put it out 
once it is burning.  This stuff has a place in history as starting some 
very large fires.

This is what I did with my grounding system: 
http://www.rogerhalstead.com/ham_files/ground.htm

As the top of the ground rod should be below the surface along with the 
rest of the ground system, I found no need to put putty around them.  I 
just packed the sand/dirt around the "One Shot" and had no problems with 
the charge powder leaking out.  It is a fine grain powder, not a liquid.

The "one shot" comes with a very fine grain powder igniter charge tamped 
into the plastic cap of the container in which the charge is shipped. 
The igniter charger is relatively easy to ignite.  The Thermite itself 
is not so easy to ignite.

> Looking at on-line catalogs I see reusable molds for various
> configurations, clamping "handles" and welding powder all sold a-la-carte.
> I have not seen a mold that would work to connect wide copper strap to a
> ground rod.

The molds are expensive, the powder is relatively inexpensive.  Shipping 
is likely to be quite pricey
Even a dozen "one shots" are far less expensive than any molds I've see 
and I used to purchase "One Shots" by the carton at the local electrical 
supply.


>
> The utility crew didn't use a reusable mold, it was what looked like a
> ceramic cylinder that they placed over the connection and left in place
> when the weld was complete.
>
> I imagine that the single-shot mold might be faster to use but more
> expensive than a reusable mold.

The one shots are a lot cheaper unless you are doing a lot of connections.

I have over 600 feet of bare #2 copper CadWelded(TM) to 32 or 33 8' 
ground rods in the ground system. 
http://www.rogerhalstead.com/ham_files/cablebox.htm  Watch for line wrap.

  If each trench run is 100 feet and I use
> 10-foot ground rods spaced at 20 feet, that's a total of 12 ground rods to
> bond to including the existing electrical service ground. So its probably
> going to make sense to buy reusable molds and a clamping handle. Are there
> tradeoffs other than cost that I should consider? Does it really make
> sense to buy two different molds, one for "T" connections and another for
> a dead-end connection, or can I use the "T" mold for everything with just
> a stub of wire on the second port?

You just plug the second port with clay or dirt.

  Is there such a thing as a mold
> (one-shot or reusable) for bonding wide copper strap to a ground rod?

I believe so, but I've always used "one shots" and normally purchase 
what I plan to use over the next year or so.   Remember if buying the 
stuff in bulk that it'll burn a hole right through a concrete floor.

I do reloading and I'm far more comfortable with kegs of smokless powder 
in the basement than I am a few pounds of Thermite.

>

73, good luck, and stay safe!

Roger (K8RI)

> I'm looking for advice and opinions as well as information. Is my plan
> reasonable? Suggested reading and sources for the molds, etc. would be
> appreciated.

Go to the Polyphaser site.

>
> TIA.
>