Date: Wed, 15 Nov 2017 09:45:56 -0800
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Ground wire between tower and ground stakes
<The simple facts are that 1) bigger will carry more current before it
<melts, 2) bigger is harder to work with, 3) stranded makes bigger easier
<to work with, 4) stranded does corrode faster when exposed to air or
<water, 5) insulation prevents that exposure except at connection points.
<Many years ago I came into a big roll of insulated #4 stranded, and
<that's what I'm using. All of life is a compromise. Good engineering is
<making the best compromise for any given application. That's also true
<of life.
<Many things are done the way they are in broadcasting and commercial
<radio because of 1) tradition and 2) they have the labor/budgets to do
<it better. 120 radial systems for AM broadcast stations are an example
<of both. So is #2 solid for cell sites.
<73, Jim K9YC
## when was the last time you saw 2 awg stranded cu... actually melt, much less
3 of em ?
## If its cadwelded at one end... and a compression lug used at the other end,
and say
bare, stranded cu used, corrosion between each end doesnt enter into the
equation.
Provided the terminations on each end are good.
I use stranded and insulated 2 gauge cu from each tower leg to 3 x 8 ft rods
. Cadwelds
are below ground several inches. Then about 60 ft of BARE stranded 2 gauge
cu... from
grnd setup at base of tower to spg..using a C compression fitting.
This 60 ft run is buried in the grnd a few inches..per code.
That puts the base of the tower and the spg at the same potential. The 60 ft
of bare stranded
cu, buried in the ground, provides for additional surface area, below ground.
Thats like another
60 ft of rod implanted into the ground.
## I find it hard to believe that lightning would melt 4 gauge cu.... or even
6 gauge cu. Even
though the peak current is sky high, the duration is extremely short.
## typ cell site back in 2008 and 2009, b4 I retired from the telco was aprx
$500K to install.
2 gauge solid vs 2 gauge stranded, doesnt begin to enter the equation, nor the
cost of heliax
at the time. They were buying hundreds of rolls of the stuff, and got a
discount on bulk. Labor
costs and transporting to the site, site prep, etc, engineering costs, etc, is
the major expense.
Roof top microwave towers would typ use 3 to 4 inch wide bare cu strap, and
.125 thick, over the side of the flat roof,
and down to an extensive grounding system, typ 10 to 12 ft long, cadwelded
cu rods into the ground,
around the entire circumference of the building in question. Typ 10 ft
spacing between rods, all
below ground, below any asphalt and or concrete etc. A 50 x 50 ft office =
200 ft circumference =
20 rods used. Then 4 x .25 thick solid cu buss bar back to main office
ground. They wanted a 2 ohm or less ground setup.
At one remote office site near a lake, we had to use 2 x 450 ft deep holes,
each was 4 to 6 inches in diameter, filled
with carbon granules. The price tag was horrendous.
At one mountain top site, that was helicopter access only... at $350.00 per
hr for the chopper, and they bill from the time
they left the airport, till they got back, the microwave site was on solid
granite. So all the concrete had to be air lifted in,
this time with a 2 bladed chopper, at $1700.00 per hr. Then ditto with
the annual filling of the diesel fuel tank, which
held 20,000 litres, done each June. Ground system consisted of 200
radials, 8 and 10 gauge, over the bare rock. After
25 years, it gets hit with lightning, building caught fire, above ground
diesel fuel tank let go, all the fuel burned, and all that
was left was the charred remains of the 200 radials. You could see the burn
marks on the granite. Needless to say, an entire town
had no LD ph service, nor TV signals for several months.
## 120 radials used for AM broadcast, but they are longer than .25
wavelengths. Typ they are .4 wavelengths,
some are even longer. Towers are any where from 90-225 degs. 102 degs is
typ.
Try a ground mounted 10m vertical sometime, you wouldnt use 8 ft long
radials. Even 33 ft long radials on a ground mounted
40m vertical is nothing to write home about.
Jim VE7RF
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