On 12/19/13 7:39 AM, Larry Banks wrote:
I thought the rule of thumb was to space the rods at twice their length: 8'
rods every 16 feet, 4' rods every 8 feet, etc.
73 -- Larry -- W1DYJ
----- Original Message ----- From: <john@kk9a.com>
To: <towertalk@contesting.com>
Sent: Thursday, December 19, 2013 9:36 AM
Subject: Re: [TowerTalk] Filter location and grounding rods
I only use shorter ground rods when I cannot push in a longer one. Using
more shorter ground rods will cost more as there are more connections to
purchase and the soil is likely dryer near the surface so they may be
less
effective.
John KK9A
Yes.. spacing closer than twice the length reduces the effectiveness,
because the fields overlap.
But John's comment is also very appropriate. Taken to an extreme, 8
rods 1 foot long spaced every 2 feet would probably not be as good as 1
rod driven 8 feet deep.
And of course, if you have the wire to space out those rods over 14
feet, why not just bury the wire.
I know rods are a traditional approach, and have been used for
centuries, and you can be all he-man John Brown swinging that sledge to
drive them, but really, they're not a very good approach for "connecting
to the earth".
Dig a trench a couple feet deep (with a power trencher) and put a big
wire in or, even better, lay a (smaller) wire in, and fill the trench
with concrete.
No hassling with exothermic welding, no driving bendy rods through rocks
and clay, etc. The "hydraulic mining" technique sort of negates the
whole "rod driven into undisturbed soil" thing, too.
Herb Ufer did us all a favor more than 50 years ago.
Let's look at costs:
Dig a 20 foot trench, 6" wide and fill it with 1 foot of concrete with a
10 AWG wire. That's 10 cubic feet of concrete, about 1/3 yard. About
$50-60 from the U-Kart concrete place, less if you buy sacks and mix it
yourself. It's not structural, so you can be pretty casual with the
mix.. you could probably do a soil/concrete mix using the dirt you dug
out of the trench as the aggregate, although I'd worry about it falling
apart over time (OTOH, we do that around here for heavy use trails, and
it works pretty well)
Home depot is selling AWG10 bare wire for 0.39/ft, so there's another
$10 for wire.
The trencher is the most expensive part.. $100 for the 3 hour minimum
locally.
For 20 feet, I might just dig it by hand, but if I were doing a ring
ground, I'd get the trencher. You could easily trench all the way
around a house in 3 hours (even allowing for navigating around things).
A ring ground, with AWG 2 is going to be a bit pricier.. Home Depot AWG4
is $1.40 a foot 50 ft to go around a 10x10 ft shack is going to be $70,
AWG 2 is probably $100.
But still, for a couple hundred dollars, you'll have a ground that is
MUCH better than driving a bunch of rods, will be much better over the
long term, and will probably take less time to install.
One rod is "quick 'n' easy" for the utility installer to drive in decent
soil, so that's why it was done. However one rod doesn't work so well,
and as soon as you're talking multiple rods, interconnects, etc., it's
time to take a step back and do some systems engineering.
Home Depot charges $11.28 for a 8ft rod, and $2 for the clamp, just for
comparison. And I have no idea what those Zareba 6 ft galvanized rods
are for.. You can use a 8 foot galvanized rod (if sufficiently big) and
meet code. But galvanized rods have a 10-13 year expected life, while
copper clad steel is expected to last 40 years...
The Navy did a study, and come up with something along the lines of
galvanized rods might be ok for a temporary structure (trailers, etc.)
that has an expected life of less than 5-10 years, so the short life of
a galvanized rod isn't an issue. I suspect that this is the kind of
thing these short galvanized rods are intended for (and in that
application, they're probably not paying $15 each for them, like at Home
Depot)
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