"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

"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

"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, W3CQH wrote:
"So then, what is the reason they drive a separate ground rod in when
they install a commercial tower whose legs are in concrete?"

Working with electricity you soon learn you need a rubber mat or rubber
soled shoes to protect against electric shock from the mains when you
are on bare concrete. It is often damp directly on the earth. When
poured as a slab, concrete is placed on plastic as a vapor barrier to
keep wood flooring from warping.

A tower base after years is not completely set but still contains
moisture and is still hardening.

Tower bases are bypassed by ground cables outside the concrete block so
that the moisture inside does not turn to steam during a lightning
strike and blow the concrete asunder.

Best regards, Richard Harrison, KB5WZI

John Doe wrote:


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



Tradition?
Maybe it's easier to just drive the rod and hook it up than to explain
the subtleties of grounding to the inspector?
The contractor owns stock in a ground rod manufacturing company?

In article ,
"John Doe" wrote:



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



There is a GIANT difference between RF Grounding and Lightning
Protection. Don't even think of confussing the two....

You wrote:

There is a GIANT difference between RF Grounding and Lightning
Protection. Don't even think of confussing the two....

And there's a third reason and set of requirements for grounding which
is different from both of those -- AC safety ground. Its requirements
are dictated by the NEC and local electrical codes, and it shouldn't be
confused with either of the other two.

Roy Lewallen, W7EL

On Wed, 11 Jul 2007 11:35:11 -0700, Roy Lewallen wrote:


And there's a third reason and set of requirements for grounding which
is different from both of those -- AC safety ground. Its requirements
are dictated by the NEC and local electrical codes, and it shouldn't be
confused with either of the other two.

Is there such a thing as a ground that is good for both AC safety ground
and lightning protection ... or, both of those plus RF ground?

Rick (W-A-one-R-K-T) wrote:
On Wed, 11 Jul 2007 11:35:11 -0700, Roy Lewallen wrote:


And there's a third reason and set of requirements for grounding which
is different from both of those -- AC safety ground. Its requirements
are dictated by the NEC and local electrical codes, and it shouldn't be
confused with either of the other two.

Is there such a thing as a ground that is good for both AC safety ground
and lightning protection ... or, both of those plus RF ground?

Sure. One way to do it is to make a proper AC safety ground, a good
lightning ground, and an effective RF ground, then bond them all
together (provided it's permitted by code).

Roy Lewallen, W7EL

"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