[TowerTalk] Tower lightning ground system layout

[jimlux]

On 5/18/20 1:26 PM, K9MA wrote:



> 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.

Maybe the real question here is:
Is this a lightning ground (the goal of which is to keep the voltage 
rise "reasonable" and to tie everything together so it goes up and down 
together (i.e. have all the cables to the tower routed together, so 
their inductance and capacitance to "ground" is the same)

or are you using the ground network as a low RF resistance if you use 
your tower as a radiator?

For AM broadcast (the source of the famous 120 radials approach) those 
two goals are similar - that is, a ground network that is "perfect" from 
a FCC standpoint for radiated field strength at 1 MHz from a 1/4 
wavelength vertical happens to also be a fairly decent lightning ground.

Some of the "lightning grounding" as well as "electrical code bonding" 
recommendations are also based on physical ruggedness, more than 
inductance or current carrying capacity - i.e. you don't want your NEC 
required bonding to be accidentally severed by gardening activities.

> 
> 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.


And often, references to commercial site grounding guidelines (R56, and 
similar for air traffic control towers) are based upon the specific 
economics and availability requirements for those facilities. Expensive 
labor and very high availability drive some of the recommendations.





> 
> 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.
>>
>