ERICO Practical Guide to Electrical Grounding - Manual (Page 93)

Although the welded type are more expensive, they assure
that the couplings will not become a high resistance
member in the current path over the life of the system. One
loose coupling will render all of the lower rod sections
useless.
Also available is a connection which is a combination of a
screw coupling and a welded coupling. After the screw
coupling is installed, two CADWELD connections are
made to weld the coupling to both the top and bottom rods.
(Fig. 5-6)
When using deep driven rods to reach soils of low
resistance, tests have shown that the rods do not have to be
separated more than 10 feet for maximum efficiency. This
is probably due to the fact that only the lower 10 feet of rod
is in the lower resistance soil.
USE A GROUND ENHANCEMENT
MATERIAL
Several materials are available to lower the resistance of the
installed rod electrode. They are placed around the rod
which has been installed in an augured hole. Although they
have a resistivity higher than the metal rod, their resistivity
is lower than the surrounding soil. This, in effect, increases
the diameter of the rod. Following are some of the
materials commonly used as ground enhancement materials
along with their resistivities;
concrete : 3000 to 9000 ohm-cm (30 -90 ohm-m)
bentonite (clay) : 250 ohm-cm. (2.5 ohm-m) (Shrinks and
looses contact with both rod and earth
when it dries)
GEM™ : 12 ohm-cm (0.12 ohm-m) or less.
(Permanent, sets up like concrete and
does not shrink or leach into soil)
USE A CHEMICAL TYPE OF
GROUNDING ELECTRODE
Several makes of chemical types of ground electrodes are
available. They are essentially a copper pipe with holes in
it. The pipe is filled with a salt, such as magnesium sulfate.
The salt slowly leaches from the holes in the pipe
infiltrating the soil. The salts must be periodically replaced
for the electrode to remain effective. Also, the
Environmental Protection Agency (EPA) may object to
adding salts to the soil. Chemical type electrodes are
discussed in more detail on page 15.
USE A SALT AROUND THE ROD
Adding salt to a trench around the ground rod is an
inexpensive method to add salts to the soil. The salts must
be periodically renewed. The EPA may also object to this
method. Some salts may corrode the grounding conductors.
This approach to lowering the ground resistance is not
recommended.
CONNECTIONS
The connections to the ground rod can be as important as
the rod itself. (Connectors are discussed further in the
following section.) Often, a large conductor is connected to
one or two ground rods. In many cases, this is a mismatch
since the rod cannot carry as much current as the conductor.
Table 5-1 lists the equivalent copper conductor size for
various rod sizes based on fusing formulas.
One must also consider the current flow into the rods. If the
current heats the surrounding soil to 100
o C or higher, the
moisture evaporates and the soil resistivity increases. The
maximum one second fault current for a 5/8” x10’ ground
rod in 100 ohm-meter soil is 27 amperes to limit the
temperature to 60
o C. (Ref IEEE Std 80-1986)
In areas where the amount of available land is limited and
the soil resistivity is high, the use of multiple rods with
interconnecting conductors will lower the system
resistance. When this is not sufficient, using GEM around
either the rods or the conductors, or both, should be
considered. (Fig. 5-4)
85 Chapter 5: Selection of Components Used In Grounding
Ground Rods
Table 5-1
Rod Size Type Closest Equivalent
Copper Size
1/2” (0.447 D) Sectional (1/2” Thread) #1 AWG
Copper-Bonded
1/2” (0.475 D) Plain Copper-Bonded 1/0 AWG
1/2” (0.505 D) Sectional (9/16” Thread) 1/0 AWG
Copper-Bonded
1/2” (0.5 D) Galvanized Steel #2 AWG
5/8” (0.563 D) Copper-Bonded 3/0 AWG
5/8” (0.625 D) Galvanized Steel #1 AWG
3/4” (0.682 D) Copper-Bonded 4/0 - 250 KCMIL
3/4” (0.75 D) Galvanized Steel 2/0 AWG
1” (0.914 D) Copper-Bonded 400 KCMIL
1” (1.0 D) Galvanized Steel 250 KCMIL
Grounding Book 4/14/99 10/5/99 6:02 PM Page 85 (Black plate)