|THE UFER GROUND
|The term "Ufer" grounding is named after a consultant working for the US Army during World War II. The technique Mr. Ufer came up with
was necessary because the site needing grounding had no underground water table
and little rainfall. The desert site was a series of bomb storage vaults in the
area of Flagstaff, Arizona.
The principle of the Ufer ground is simple,
it is very effective and inexpensive to install during new construction. The
Ufer ground takes advantage of concrete's properties to good advantage. Concrete
absorbs moisture quickly and looses moisture very slowly. The mineral properties
of concrete (lime and others) and their inherent pH means concrete has a supply
of ions to conduct current. The soil around concrete becomes "doped" by the
concrete, as a result, the pH of the soil rises and reduces what would normally
be 1000 ohm meter soil conditions (hard to get a good ground). The moisture
present, (concrete gives up moisture very slowly), in combination with the
"doped" soil, make a good conductor for electrical energy or lightning
Ufer techniques are used in building footers, concrete floors,
radio and television towers, tower guy wire anchors, light poles, etc. Copper
wire does not function well as a "Ufer" ground due to the pH factor of concrete
(+7pH is common). The use of steel reinforcement as a "Ufer" ground works well
and concrete does not chip or flake as has been found with copper. The use of
copper wire tied to the reinforcement rods outside the concrete shows none of
The minimum rebar necessary to avoid concrete problems
The type of concrete, its content, density, resistivity, pH
Amount of concrete surface area in contact with the soil.
Soil resistivity and ground water content.
Size and length of the
reinforcement rod, wire, or plate.
Size of the lighting strike current.
The following chart shows the conductivity of lightning current per foot of
Rebar (reinforcement rod). Only the outside Rebar can be counted. Rebar in the
center of the footer or foundation does not count in this calculation. In a
trench footer only the rebar in the sides and bottom of the footer can be
Rebar Diameter In Inches /Surge Amperes Per Foot
.375 / 3400
.500 / 4500
.625 / 5500
.750 / 6400
1.000 / 8150
Mr. Ufer did not know what he had found until he
experimented with various lengths of wire in concrete. Today's informed engineer
benefits from Mr. Ufer's discovery and will tie in the bars of steel
reinforcement in a building or other foundation to the building electrical
ground. When bonded to the electrical ground, building steel, etc., the
buildings reinforced floor and foundation become part of the building grounding
system. The result is a much improved grounding system with a very low overall
resistance to earth reference.
If Ufer grounding alone was enough, the
manufacturers of ground rods would go out of business. But a Ufer ground alone
it is not adequate. Few buildings, even those under construction today are built
to take advantage of the Ufer ground. It is common to see the use of "Ufer
grounding" in military installations, computer rooms, and other structures with
very specific grounding specifications. It is not common in most industrial
plants, office buildings and homes. More common today is grounding to national
and local electrical codes. This will involve one or more driven ground rods
connected (bonded) to the neutral wire of the electrical service entrance. The
purpose of this bond is what is known as life safety ground. It is used for many
other things but the code required life safety ground is why it is there to
Ground rods come in many forms, but the most common used in
electrical service grounding are galvanized steel ground rods. Please remember,
the best day a ground rod will normally see (resistivity) is the day it is
installed. Corrosion, glazing, etc., all are factors that lessen the
effectiveness of ground rods.
Ground rods in general are divided into one
of the following sizes; 1/2",5/8",3/4" and 1". They come in steel with
stainless, galvanized or copper cladding and can be solid stainless or mild
(unclad) steel. They can be purchased in unthreaded or threaded sections that
vary in length. The most common lengths are 8' and 10'. Some will have a pointed
end, others will be threaded and can be coupled together to form longer rods
The effectiveness of a 1" ground rod over the 1/2" ground
rod is minimal when resistance readings are taken. The larger rods are chosen
for more difficult soil conditions. Clay or rocky conditions often dictate the
use of power drivers, similar to an impact driver used by mechanics when working
on your automobile. They are typically electric or pneumatic. The power drivers
when used with the heavy 1" ground rods will drive in most soils.
copper clad rod when compared to a 1/2" copper clad rod in the same soil
conditions will yield about a 23% improvement is performance. The surface area
of the 1/2" rod is 1.57 compared to the 1" rod at 3.14 ( 3.14 x .5 = 1.57 and
3.14 x 1 = 3.14 ). So, for double the surface area, you only get about 23%
improvement in performance.
The cladding of ground rods is to protect
the steel from rusting. Most think the cladding, (copper on a steel rod) is for
the increase in conductivity of the rod. It does aid in conduction, but the main
purpose of the cladding is to keep the rod from rusting away. Not all clad
ground rods are the same and it is important the clad rod have a reasonably
thick cladding. High quality industrial quality copper clad steel ground rods
may cost a little more but they are worth the small extra cost.
ground rod is driven into rocky soil, it can scratch off the cladding and the
rod will rust. Rust is not conductive when dry, in fact it is a good insulator.
When it is wet it is still not as conductive as the copper on the rod. Soil pH
can be tested and that should determine the type of rod used. In high pH soil
conditions only high quality clad rods should be used. If the soil is extremely
acidic, stainless rods are the best choice.
One of the most popular
ground rods is the galvanized (hot dipped zinc) steel ground rod. This rod is
used with copper and aluminum conductors to form the service entrance ground in
most buildings and homes. This is a poor choice for ground restivity over time.
The joint between the ground rod and conductor are made above or below the
surface of the ground and in most cases subject to constant moisture. Under the
best of conditions the joint between two dissimilar materials will result in
corrosion and increased resistance over time.
When dissimilar materials
are joined, electrolysis occurs. If Aluminum is used with copper that is not
tinned the aluminum will pit to the copper leaving less surface area for contact
and the connection could come loose and even allow arcing. Any sharp blow or
impact could cause the connection to be broken. When installing in the soil it
is not recommended tinned wire be used. Tin, lead, zinc and aluminum are all
more anodic than copper and they will sacrifice (disappear) in the soil. When
the connection is made above the surface of the soil in the electrical
distribution panel tinned wire is acceptable.
Another treatment for
joint corrosion problems is using a joint compound to prevent moisture bridging
between the metals. The more popular compounds are copper or graphite particles
imbedded in a grease compound. Using similar material is a better solution as
even joint compounds can loose their effectiveness if not maintained but their
use is preferable to a dry joint. Joint compounds work by imbedding particles
into the metals to form a virgin junction of low resistance void of air when
they are placed under pressure. The act of tightening the clamp on the conductor
and rod provide this pressure.
The problem of dissimilar material is not
found in copper clad steel rods. ( Please see the Noble Chart at the end of this
chapter. ) Of all the reasonably priced choices, the copper clad steel rod with
a copper conductor is your best choice. If money were no object a gold
conductor, and ground rod would be ideal, but hardly economically
The effective performance of ground rods is reduced by soil
conditions, lightning currents, physical damage, corrosion, etc., and should be
checked for resistance on a regular basis. Just because your ground was good
last year it does not mean it is today. Have it checked by the fall of potential
A driven rod, when compared to a back filled rod, is much
better. The density of undisturbed soil is much higher than even compacted soil.
The connection of the soil is the key to the rod performance.
ground rods is not difficult but proper procedures must be followed and the
resulting rod(s) should be checked for performance. Testing for resistance by
the fall of potential method is the only way to be sure what looks good is good,
a low resistance ground.
© Copyright PSI 1995