In a message dated 97-04-17 06:18:01 EDT, W8IK@ibm.net (Joe Subich) writes:
> > The practical hemisphere of influence for ground rods is twice
> >their length. In other words, a ten foot ground rod has a hemisphere
> >of approximately 20 feet. So does the adjoining ground rod. The rods
> >should be spaced 20 feet apart so that their respective hemispheres
> >just touch. If the rods are closer together, you aren't getting the
> >maximum effectiveness from them. Farther apart is better than closer
> >together.
>
> > If your question was "how far to space the ground rods from the
> >tower" then the answer is to get them as close as practical to the
> >tower legs. The connecting wire should have big lazy bends in it and
> >not sharp ones. The sharp bends are high impedance points which is
> >what you're tryin to avoid.
>
> Which is it, Steve? If ground rods should be four times their length
> from the adjacent rod how can they be as close as practical to the
> tower legs?
First of all, the spacing distance BETWEEN ground rods is twice their
length not four times (see above); i.e. ten foot rods should be 20 feet
apart.
The topic (I think) was where to place the first rod from the tower.
You want to get that one close to the tower so that you don't have an
unnecesarily long ground wire from the tower leg to the ground rod. Then
subsequent rods in series should be spaced as per above.
>
> For example, if I were to install three eight foot ground rods on a
> tower (one on each leg), should they all be adjacent to the base
> (about 6' apart if they're outside the concrete base) or should they
> be spaced on a 32' triangle (with heavy cables around the triangle
> and connecting a rod to each leg)?
Good question. In Roger Block's Polyphaser lightning book, the
illustrations have the first ground rods right on the edge of the concrete
base pad (granted, they do appear to be big pads like for a self-supporting
commercial tower). I'd say the first one goes as close as practical to the
base concrete pad while observing the hemisphere distance. Remember, you can
use different lengths of ground rod to 'adjust' their hemisphere. Imagine
three adjoining and touching circles with 20 foot diameters (ten foot ground
rods) with the tower base right in the middle and the ground rods being in
the middle of each circle. That would satisfy our design parameters.
Be sure to observe the 8-inch minimum radius for wire bends in the
ground wire. The additional rods are in a radial spoke configuration.
What is the purpose of the ground system? It is there to offer a low
resistance and inductance path to ground for transient lightning events.
What are the lowest resistance things you can do? One is to make the tower
leg/ground rod connection as effecient as possible which includes making it
an effective length and properly installed.
>
> I can't believe a single eight to 10 foot ground rod is sufficient
> even if the tower, rebar and ground rod are bonded together.
>
No body said it was. It's the START of a good ground system.
73, Steve K7LXC
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