I snipped the following text by Gary Coffman (clear back in 1994) from the
Polyphaser Web site as reference for the (dumb) question I have regarding
his references to a single point ground that everything must be connected
to. In this description, the center of the tower grounding system is that
'single point.'
My question is:
Does this mean that if my tower is 250 feet from the house and garage area
that I should run CATV, Telephone, etc. grounds to the single point ground
250 feet away?
Can anyone point out what I'm missing here?
73,
Frank - W0ECS
To: <towertalk@contesting.com>
Date: Thu, 17 Mar 1994 06:29:11 GMT
Reply-To : gary@ke4zv.atl.ga.us (Gary Coffman)
Subject : Re: Grounding and lightning protection
(snip)
Note that in *addition* to the ground window, every antenna or support whose
construction will allow it should have a separate ground conductor run to
the station ground field. This will relieve the downleads, and suppressors,
of part of the current load they'll have to carry during a strike.
A single 8 foot ground rod is *not* an effective ground field. Ideally we'd
copper plate the Earth to form an effective ground field, but that's
impractical. So we make do with driven ground rods. In average soil, a
single 8 foot ground rod will have a resistance to Earth of about 230 ohms.
That will place a connection to that rod at 920 kV during a 4000 ampere
strike. Not good. As currents start to flow into the ground, the soil
becomes temporarily *saturated* with charge. This limits the amount of
current that can be quickly dumped into any individual Earth connection. So
we need a bunch of Earth connections. How many is a bunch? Well good
practice says that the total resistance to Earth should be less than 25
ohms, so that means at least 10 rods are required. How far apart should the
rods be to avoid overlapping saturation zones? The rule of thumb is that
ground rods should be no closer together than the *sum* of their lengths.
That means that any two rods in the ground field need to be at least 16 feet
apart.
The rods should be laid out in a star pattern with the rods connected to
each other by no less than 1.5 inch bare copper strap buried not less than
18 inches below grade level. Note that these straps can be considered
horizontal ground rods themselves and can reduce the number of driven rods
needed in the system by about a third. So assume 7 rods, one central and six
radial at a 16 foot separation. Make all connections to the central rod.
That's your *single point ground*. Tie power company, phone company, and
CATV grounds to this point as well as attaching your station ground and
separate antenna grounds to this point. Never never never daisy chain
grounds. All grounds must be tied to this single point, and only to this
single point. (Note, if you have a tower, it can serve as the central rod.
With its base planted in concrete, it forms a Ufer ground superior to a
single driven rod. Note too that if you have metallic underground plumbing,
that should also be tied to your single point ground by a strap connection.)
(snip)
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