On Mon, 2008-08-18 at 08:57 -0500, Mike Gorniak wrote:
> You make a good point. I would suggest, however, that a single ground rod is
> probably inferior to the system you describe. Also, the typical broadcast
> station uses AWG 10 for the radial system. That's a far cry from 0000 !
>
> In any event, the station that I was referring to was located in a swampy
> area. Radials alone may work in some circumstances, but I like what the tower
> manufacturers and the NEC have to say on the matter...a solid connection to
> ground rods is a good idea!
>
> 73,
>
> Mike
> NM7X
>
Even or especially in swampy ground, the earth is made of layers.
Sometimes fertile and conductive, sometimes gravel, and sometimes clay.
I own land with a clay layer down about 5'. That clay is waterproof all
by itself. And many porcelain insulators are fired from clay. I've dug
up buckets of that clay with my back hoe that had water standing on it
and when I dumped the water, the clay appeared to be dry. It sure wasn't
mud. That causes what is called a "perched" water table and is
characteristic of many high ground swamp areas, like the county marsh
adjacent to my land.
To get conductivity to the earth's core you have to penetrate those
insulating gravel and clay layers with a driven ground rod. Hydraulicked
is not good because it blows a much larger hole than the rod so the
earth isn't in intimate contact with the rod. NEC calls for 8' or longer
rods. Electric utilities drive longer sectional rods until the measured
ground resistance is low enough to satisfy them.
Remember a direct hit may cause a current of 1 to 10 kiloamps. Even a 10
ohm rod (which is idealistic and hard to achieve) leaves 10 to 100 KV
voltage drop between the rod and the earth. Earth is not a great
conductor either so that voltage may be between two points 20 or 30 feet
apart with separate ground rods. Which is why you don't want to be
walking or running on the ground when lightning hits nearby, the voltage
between your feet may be enough to kill you. Same thing is true when a
power boom truck is grounded and accidentally touches a hot wire. The
"step" potential near that ground rod can be high enough to be fatal.
Its generally considered that rods space horizontally less than a rod
length apart aren't a lot better than a single rod. Multiple rods
perform better with a wider spacing.
In sandy ground, its possible for lightning currents to melt the sand to
glass and insulate the existing rods. So rods may need checking and
replacement after direct hits. Or new rods added away from the glass
insulated rods.
73, Jerry, K0CQ
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