[TowerTalk] Thoughts on a Grounding System for my new tower
K8RI-on-TowerTalk at tm.net
Sun Sep 2 22:35:40 EDT 2012
On 9/2/2012 8:59 PM, Jim Lux wrote:
> 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.
Until you have a dry spell. Currently the water table here is around 5
to 6 feet while the surface foot or so is powder dry. In the spring the
water table is about 6" down in the back yard. Note the clay "chips"
> 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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