[TowerTalk] Thoughts on a Grounding System for my new tower

[Jim Lux]

On 9/2/12 5:20 PM, Jim W7RY wrote: Keep the
> rods spaced at lest 1.5 to 2.0X length of the rod. Installing rods on
> the ground conductor as it runs between the house and tower base is also
> a good idea.



I'm wondering what the theory behind that would be.  The ground wire 
itself provides a fairly good connection to "earth" for the lightning 
impulse, especially after the soil has settled in a bit.

Wouldn't it be better to spend the money on more copper wire and lay 
some more radials rather than driving rods?  (and it's less work, too).


If you had some sort of odd situation like dry sand over a wet layer 
that's 5 feet down, maybe the rods might help.

I'm sure there's a reason for the "multiple rods along the wire" 
recommendation, but I'd be interested to know where it actually comes 
from. I know why the spacing recommendation exists.. that's all about 
net ground resistance.  And driving multiple rods in the first place is 
a way to get low resistance in a limited area (e.g. drive 3 rods in a 8 
foot equilateral triangle)

The IEEE grounding book (IEEE 142) recommends that if you're doing 
multiple rods, put them in a square or circle spaced a rod length apart, 
because rods inside the periphery don't reduce the resistance very much. 
  For 3 rods, the resistance is Rrod/3 * 1.29  (they have a table of 
non-ideal performance with multiple rods.)

They comment that multiple rod systems are used to reduce the current 
density per rod (no "smoking rods"), or to reduce step potentials 
(they're looking at things like electrical substations with a long 
duration high power fault on a HV line, for instance). There's also all 
sorts of things not generally applicable to amateur tower installations 
like the popularity of large asphalt covered areas or areas covered with 
granite gravel (both high resistivity).


The IEEE book comments that "Steel rods in concrete in (irregular) 
excavations in rock or very rocky soil have been found greatly superior 
to other types of made electrodes. This electrode type provides 
additional grounding for the majority of the steel towers of HV 
transmission lines".

So maybe using rebar in concrete is a more cost effective grounding 
electrode than copper/steel ground rods?

The copper cladding on the ground rod isn't so much for conductivity as 
providing a way to clamp/bond a copper wire to it without worrying about 
corrosion. Steel is still way more conductive than soil, after all.


Or even better, pour some concrete to encase the ground wire in. 
Concrete will protect the wire from corrosion and mechanical damage, and 
will provide a better conductive path to the surrounding soil.  It 
wouldn't need to be anything neat, or formed up.  Just slump in a layer 
in the trench.  if it's, say, 1x1 foot, then the concrete will run about 
$3-4/running foot (at $90-100/cubic yard).

The concrete doesn't even have to be structural.. if you're mixing it 
yourself, you could probably just mix sand/gravel/dirt to make a sort of 
soil cement.  A real feeble mix like a 2 sack mix might work.  The 
cement here is just to hold the material together and be hygroscopic to 
hold the moisture for good conductivity.  Mind you, I haven't tried 
this..they do a lot of dirt/DG/cement mix around here for trails and 
"dirt sidewalks" that wear better than plain soil, and look natural.