The use of ground rods spaced at 2x the rod length was covered in the
Polyphaser book on lightning and perhaps elsewhere. As I recall (it has been
a few years since I read it) the idea was better dissipation of charge
during a strike. The wire between the rods doesn't dissipate the charge all
that well relatively speaking. Closer than 2x rod length spacing allows the
ground charge to swell and reduces the effectiveness of the rods in
dissipating the charge. The Polyphaser book suggested several ground radials
with ground rods along each radial with the rods spaced at 2x the rod
length.
73, Larry W6NWS
-----Original Message-----
From: Jim Lux
Sent: Sunday, September 02, 2012 8:59 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Thoughts on a Grounding System for my new tower
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.
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