[TowerTalk] Sparky Pays a Visit

[Jim Lux]

At 01:41 PM 8/3/2005, Steve London wrote:
>So, just for amusement, let's see what folks would do in my situation...
>
>I live on a very dry hilltop in SW New Mexico.  There are large limestone
>boulders at the surface, or within a few inches of the surface. You don't 
>dig in
>this stuff with a shovel or backhoe - it takes a jackhammer or excavator 
>with a
>jackhammer attachment. There is no moisture in the ground, except for a 
>few days
>after a monsoon rain event.
>
>For what small good it might do, I have ground rods at each end of my 
>beverages.
>   The deepest I could get them was about 2 feet. The resistance between the
>ground rods (which are about 500 feet apart) is around 5M ohms.  We're 
>talking
>DRY here !
>
>Frankly, what the local power company does for grounding seems 
>worthless.  They
>drill a hole in the ground for their pole, run their copper ground wire to 
>the
>bottom of the pole, loop the copper wire in a spiral around the bottom of the
>pole, and plop the pole in the ground.  With no moisture in the ground, I 
>can't
>believe that does anything.  When my house was built, they didn't even try to
>drive in a ground rod, they just used the rebar and concrete in the 
>foundation
>as a Ufer ground.  I understand that is okay for safety grounding, but
>completely worthless for lightning protection.


Interstingly, the "concrete encased grounding electrode" aka "the Ufer 
ground" was invented by Mr. Ufer for just your situation: lightning 
protection of ammo vaults in a desert environment.

As you've noted, a ground rod is worthless in dry soil.  Even apparently 
dry concrete is actually a relatively good conductor, and has a huge 
contact area with the soil.  They can add things to the concrete mix to 
enhance the conductivity as well (used in, for instance, electric railroad 
installations). For transients, the capacitance and inductance are 
important too.  You want big capacitance and low inductance, and what 
better way to do that than to use a large area electrode laying on the 
ground (i.e. the concrete slab).

http://www.mikeholt.com/mojonewsarchive/GB-HTML/HTML/UferGroundPsi~20030930.htm 
gives a short writeup.

http://home.earthlink.net/~w6rmk/lightning.htm collects lots of other 
references.