[TowerTalk] Tower lightning ground system layout

[K9MA]

Why do you want to go 65 feet out? Seems to me the minimum distance from 
the tower to keep the ground rods 16 feet apart would be better. For 4 
rods 90 degrees apart, that's just over 11 feet. If you want longer 
buried radials, they can extend beyond the ground rods, with lighter 
wire, if necessary.

While I don't have any hard data, I've always felt that in most soil the 
radials themselves might be a more effective lightning ground than 
ground rods, assuming they are heavy and numerous enough. (I do know 
that in average soil the resistance of a single ground rod is some Ohms, 
not great if you get a 100 K Amp strike.) My inclination in your 
situation would be to install as many shorter AWG 2 radials as possible, 
placing ground rods wherever you can at the 16 foot spacing.

I have 1200 feet of AWG 12 and 13 ground rods in my 60 by 120 foot lot. 
AFAIK, it hasn't been tested by a direct strike, yet. Its integrity is 
regularly tested by my YL digging in the garden, though.

Perhaps one of the experts will weigh in on this. Much as I'd love to 
have a Faraday cage under and around the house, the really optimal, 
recommended grounding configuration seems rarely to be practical.

73,
Scott K9MA


On 5/18/2020 14:28, Art Greenberg wrote:
> I am working on the layout for my tower lightning ground system.
>
> I have on hand about 275 feet of #2 bare solid copper. I also have 17 8-foot ground rods.
>
> I've read that lightning protection "radials" reach the point of rapidly diminishing returns at lengths beyond about 70 feet.
>
> Originally I planned to have three runs of 50 feet about 120 degrees apart and make a fourth run go to my entrance panel and mains ground. But it seems my distance estimating skills are lacking. I just measured that distance and its more than 100 feet, and well beyond being an effective length.
>
> If I instead go with four runs of about 65 feet spaced at about 90 degrees, the fourth run will be limited in length by a driveway. I can't rotate the whole pattern very much to improve that due to another obstacle. My apparent options:
>
> 1 - I can abandon the idea of equal angular spacing to make that run a bit longer. I think I can get the full 65 feet but I'll be going into a wooded area with the possibility of having to deal with shallow tree roots and I definitely won't be able to make a perfectly straight line of it.
>
> 2 - I can turn it into two or three shorter runs in a fan configuration (also abandoning equal angular spacing), but the angular spacing between the fan runs will result in the set ground rods on each run that are 16 feet from the base of the tower being much less than 16 feet apart. I imagine the optimal spacing rule of 2 times rod length still applies.
>
> 3 - I can put a bend in a single run to turn it parallel to the driveway to get the full 65 feet. I would have to abandon equal angular spacing to avoid an acute (less than 90 degrees) bend. What would be the best way to lay out this bend (e.g., multiple gentler bends vs. a single bend, smooth curve or something else, what about ground rod placement, etc.)?
>
> Any of these options means acquiring more ground rods. I think I have a sufficient number of Uni-Shots already.
>
> I'm thinking option 1 is best, but I'm uncertain. Is there a clear advantage to one approach vs. the other?
>
> While I'm asking ... Should I be thinking about using more shorter runs, say 5 runs of 55-ish feet spaced 70 degrees apart, or 6 runs of 45-ish feet spaced 60 degrees apart?
>
> Yeah, I'm probably over thinking this. But I live in Florida and summer thunderstorm season is about to begin.
>
> Thanks.
>

-- 
Scott  K9MA

k9ma at sdellington.us