Dave wrote:
"Rick (W-A-one-R-K-T)" wrote in message
news
When electrically grounding one's station, is there any particular
advantage of a single 8-foot ground rod over two 4-foot ground rods, or an
8-foot length of heavy gauge wire buried 6-12 inches under the ground?




yes. first there is the electrical safety code that you must comply with
which i believe generally calls for 8' rods. 4' rods may not be below the
frost line in the winter so may not provide any useful grounding for part of
the year. shallow buried wire has the same problem.

on rf issues, lots of shallow buried radials can help reduce ground loss
under certain antennas... but these are not substitutes for good electrical
safety grounds.


The NEC allows a wide variety of electrical safety grounds (and,
interestingly, a ground rod is not usually allowed as the only grounding
electrode). The old standby of "cold water pipe" is specifically not
allowed any more (too many places with plastic pipe from street to house).

The preferred ground in most jurisdictions is a concrete encased
grounding electrode (aka a Ufer Ground, after the inventor's name Herb
Ufer).. 20 ft of appropriate conductor encased in concrete.

As far as rods go, you can dig a trench and lay it sideways and meet the
code requirement. 2 rods 4 feet long might meet code (if all of the rod
is buried and they are far enough apart).


However, in addition to any regulatory requirements, there's a
difference between a "good ground" for
a) electrical safety
b) RF
c) lightning

A grounding system that's good for one isn't necessarily good for the
others.


There's a good writeup on grounds, with particular attention to
antennas, cable TV, telephone, etc. at Mike Holt's website (He's a
electrical code guru that does seminars, etc.)

http://www.mikeholt.com/ is the site, look for the "low voltage
handbook", which is a free download and has all the relevant code
sections explained, with diagrams, etc.

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?

"Jim Lux" wrote in message
...
Dave wrote:
"Rick (W-A-one-R-K-T)" wrote in message
news
When electrically grounding one's station, is there any particular
advantage of a single 8-foot ground rod over two 4-foot ground rods, or
an
8-foot length of heavy gauge wire buried 6-12 inches under the ground?




yes. first there is the electrical safety code that you must comply with
which i believe generally calls for 8' rods. 4' rods may not be below
the frost line in the winter so may not provide any useful grounding for
part of the year. shallow buried wire has the same problem.

on rf issues, lots of shallow buried radials can help reduce ground loss
under certain antennas... but these are not substitutes for good
electrical safety grounds.
The NEC allows a wide variety of electrical safety grounds (and,
interestingly, a ground rod is not usually allowed as the only grounding
electrode). The old standby of "cold water pipe" is specifically not
allowed any more (too many places with plastic pipe from street to house).

The preferred ground in most jurisdictions is a concrete encased grounding
electrode (aka a Ufer Ground, after the inventor's name Herb Ufer).. 20 ft
of appropriate conductor encased in concrete.

As far as rods go, you can dig a trench and lay it sideways and meet the
code requirement. 2 rods 4 feet long might meet code (if all of the rod
is buried and they are far enough apart).


However, in addition to any regulatory requirements, there's a difference
between a "good ground" for
a) electrical safety
b) RF
c) lightning

A grounding system that's good for one isn't necessarily good for the
others.


There's a good writeup on grounds, with particular attention to antennas,
cable TV, telephone, etc. at Mike Holt's website (He's a electrical code
guru that does seminars, etc.)

http://www.mikeholt.com/ is the site, look for the "low voltage handbook",
which is a free download and has all the relevant code sections explained,
with diagrams, etc.

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.

--
Jim Pennino

Remove .spam.sux to reply.

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.

"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

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

Then there is the matter of whether to extend the rebar into the soil
or totally encapsulate in a concrete base for a tower...

And the matter of applying a protective coating of paint to the rebar
to minimize corrosion...

I have a couple of clothes line poles that are in pretty good
condition above and below the ground line but rusted completely
through at the ground line!

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"

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?

Concrete is a conductor, and generally a better conductor than the soil
around it (it's hygroscopic), so rather than the sort of iffy contact
between the rod and soil, you have a much larger contact area between 20
feet of wire and concrete, and an even larger contact surface area
between the concrete and the soil. Run some numbers, and it turns out
that capacitive coupling from concrete to soil is probably lower
impedance than resistance.


There ARE high resistivity concretes (used for things like supporting
rails on electric trains), but that's unusual.

There's lots and lots of field tests, lab work, and theoretical analysis
to back up the consistent good performance of Ufer grounds.