"John Doe" wrote in message
. ..
"Mike Kaliski" wrote in message
...
"Nick" wrote in message
...
"Jim Lux" wrote in message
...
wrote:
John Doe wrote:
Isn't concrete an insulator??
"The preferred ground in most jurisdictions is a concrete encased
grounding electrode (aka a Ufer Ground, after the inventor's name
Herb
Ufer).. "
What am I missing here?
That while dry concrete is a pretty good insulator it is hard to
find such just about anywhere other than Southern Arizona.
And even there, the concrete is probably damper and a better
conductor
than the surrounding soil. Ufer's original work was developing
grounding
techniques for ammo bunkers in desert areas, since the ground rods
didn't
work.
I, too, am amazed - I though concrete, whilst it would be damp on the
outside underground bit, would be
substantially dry after setting, and a good insulator, being
essentially
sand.. I guess its porous or microporous..
Nick
Concrete never truly sets. The chemical reactions continue for
centuries.
Concrete structures put up 2000 years ago by the Romans are still
perfectly
useable today and under the surface remain chemically active. Many Roman
structures such as the Colosseum and aquaducts would not have been
possible
without concrete. Some of the techniques developed then are still in use
in
building today such as making the higher levels of a structure using
lighter, smaller aggregate.
Embedding steel or copper rods in concrete will pretty much guarantee
that
the metalwork will remain in contact with moisture and conductive salts
for
as long as the structure holds together. The constant exposure to
moisture
and corrosive salts is the main reason for failure of modern
ferro-concrete
structures. Concrete will suck up whatever moisture is around, either
from
the air or the soil and is always damp inside, hence the steel rusting
out
unless protected by heavy galvanisation and sacrificial electrodes.
Even in the worst environments, a couple of 8 foot copper rods embeded
in
concrete should give a DC or low frequency AC resistance/impedance of
less
than 200 ohms. Of course you could always use a dipole and balun. :-)
Mike G0ULI
So then, what is the reason that they drive a separate ground rod in when
they install a commercial tower whose legs are in concrete?
Howard W3CQH
Howard
It is necessary to provide a consistent earthing connection. The tower may
develop galvanic resistance at joints due to corrosion, or sections of the
tower may be insulated to prevent parasitic resonances at certain
frequencies, or any one of a hundred other 'faults'. With a single earthing
point, it is easier to monitor any deterioration in the system and achieve
specific design criteria with a known resistance which is unlikely to change
a great deal.
Mike G0ULI