Recently I put up a big new tower and a ground system for it and learned
a few things.
The actual ground conductivity is a bit of a mystery and highly variable
over short distances, so I bought a clamp on ground resistance measuring
meter on ebay, a Chinese knockoff for $300. It was quite informative
about what was working and what not. After a short learning curve, it
worked well and would be a good investment to share within clubs, etc.
(DY1000A Digital Clamp-on Grounding Earth Resistance Meter Earth
Tester) I checked into renting a meter and the one month rental was
more than $300.
The new code requirement is for #2 awg to the rods from the
panel/tower. However after expressing skepticism about why this is
necessary, I learned that this size conductor is essentially a ground
rod if down about 10" in the earth as it is essentially a horizontal
rod, just about as effective (surface area) as pounding a 10' rod into
the earth.
I started off believing I could drive two 10' x 5/8" rods with couplers
to 20' depth with a 1.5" capacity Bosch hammer drill with a ground rod
driver attachment but that was a no go. I drilled the first 36" of the
rod position with a rotary bit as we have a hardpan layer at about 12"
to 24" depth. Five new rods were 10' and one of 4' when going deeper
was a no go. The recommended spacing is 2x the ground rod depth and in
a star configuration for towers. The code specified resistance to
achieve for a tower is less than 10 ohms. I measured each lead and the
resistance reduced by an average of 35% from late summer bone dry
conditions to after about 4" of rain. 8.0 ohms to 5.3 ohms net parallel
resistance for 5x 10' plus 4x 8' rods. I believe that much of that
resistance reduction after 4" of rain was from the #2 horizontal runs,
about 90' of wire. So I declared victory, and will measure again
shortly now that we are well into the rain season here in Redmond, WA.
and the ground is becoming saturated at depth.
Cadweld "One Time" copper thermite welding is the way to go for
connecting the ground lead to the rod - good for the life of the rod w/o
any need for inspection and zero ohms. They are about $12 a shot from
your local electrical distributor. There are many variations of number
and size of conductors. They also appeal to any pyro instincts you
might have. For problem sites e.g. rock ledges, there are conductive
concrete mixtures that can be poured around horizontal rods and wire
leads to dramatically increase the conductive surface area and produce a
useful ground resistance, but that is not needed at my site.
My reference "far earth ground" for measurements is the Ufer foundation
of a steel building, about 250' of perimeter concrete. It is impossible
to measure that resistance since the steel frame connects everything to
everything, including the service transformer with HV underground
feeder. When that transformer was installed the PSE "ground" was the
concrete vault Ufer, no separate ground rods. I suspect the net of these
Ufer grounds are less than 1 ohm to the "far earth ground."
While my QTH is in one of the lowest lightning frequency areas of the
USA, the new tower goes 50' plus over the tree line and is on a ridge,
so I became a bit paranoid about a strike. The total investment for the
ground system was about $850 including the meter. That seemed like
reasonable insurance considering the rotators, hardline, controllers,
and shack equipment dependent on a good ground system.
Grant KZ1W
On 10/29/2015 7:43 AM, Paul Christensen wrote:
That's one reason why I like to augment a NEC-required mechanical connection
with silver-solder. The mechanical connection will degrade with time but it
will take significantly longer for a silver-soldered connection to degrade,
absent some really acidic soil condition.
Local code here now requires two (2) grounding electrodes at the service entrance spaced
to cover the "sphere of influence." That means two eight-foot rods require 16
ft. spacing. As I recall, the 2008 issue of NEC allows one electrode if one can
demonstrate 25-ohms earthing resistance. Not sure if that's still true with the newest
NEC changes. But what measures 25 ohms today may measure a lot higher as time goes by.
Paul, W9AC
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Dan Edward
Dba East edwards
Sent: Thursday, October 29, 2015 10:28 AM
To: Matt Murphy <matt@nq6n.com>; Tom W8JI <w8ji@w8ji.com>
Cc: Jim Murray <adkmurray@yahoo.com>; low bad reflector <Topband@contesting.com>
Subject: Re: Topband: RFI - and lots of it
I, for one, wonder how good my service entrance ground is...mine is probably 20
years old, and while the top looks ok, there's no way to know what's going on
down 4, 6 and 8 feet...my utlities are underground, if that means anything...
as a starting point, should a guy drive a new one? or 2 or 3, spaced some
distance apart?
( gosh, top band sounds lousy this fall, for me, anyway...sigs are well down
from normal strength..) 73, w5xz, dan
snip...
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