I followed the guidance and have 4 spokes of 40 feet
with two rods each plus a couple next to the tower. Plus a bare
buried run of 110 feet to the shack/shop Ufer foundation 250ft perimeter
footing ground.
What I found in my grounding exercise by actual resistance measurements was:
1. #2 wire has a lot of surface and it acts the same as a ground rod if
buried 8 inches or so.
2. Rod resistance varies a lot, soil is not homogeneous
3. A couple of rods wouldn't drive full depth, so they were cut off but
still welded into the field.
3. Wet vs dry and thawed vs frozen ground can change the field
resistance by 50% (frozen not likely a FL problem) dry=7.9 ohms;
wet=5.3 ohms, wet w shallow freeze=6.4 ohms
4. My Ufer ground is about as good as it gets, always in wet soil.
Maybe 700 sq ft of concrete in earth contact.
There is much debate about the wisdom of connecting the tower ground
system to the house ground for a distance over 50 ft or so.
My layout is not symmetric and as long as the rods a 2x length apart I
don't think it is not critical. Going longer than 50ft I believe has
little benefit for the HF component of a strike.
Some rental yards have clamp on resistance meters, I bought a $150
import on ebay and learned a lot in using it. My target was to get
close to the commercial 5 ohms, since the tower clears the tree line by
50ft. Since WWA is one of the lowest USA strike zones, I declared
victory with the above values (although this season started with several
T-storms, unusual for here).
With the meter, I could see the improvement with each rod driven so that
is one big advantage. Rods varied from 25 to 75 ohms each. Stop adding
rods when you reach your target. Disconnecting sneak paths or loops is
important, obvious when readings are single digit.
A concise and clear explanation of grounding by Times is at
https://cdmelectronics.com/wp-content/uploads/2014/05/Times-Protect-Brochure-R.pdf
Grant KZ1W
On 5/18/2020 12:28, 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.
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