ERICO Practical Guide to Electrical Grounding - Manual (Page 92)

However, when copper is interconnected electrically to
steel in the presence of an electrolyte, the steel will corrode
to protect the copper. Since the ratio of steel to copper in
the grounding system is usually large, the amount of steel
corrosion is usually so small it can be neglected. However,
in cases where the steel to copper ratio is small, the
corrosion aspect must be considered, for example as in a
pole having both a ground rod and a guy anchor. These
may be electrically connected. If the guy anchor is steel and
the electrode is a copperbonded rod, an insulator in the guy
wire should be used to break the electrical interconnection.
Otherwise, galvanic corrosion on the guy anchor may
occur. Ground rods are discussed further on page 14.
If the soil resistivity is very high, a backfill material is used
around the ground rod to lower the system resistance. Care
must be considered in choosing the material used. It should
be of a material compatible with the ground rod, conductor
and connection material.
See the discussion on ERICO GEM™ below. (Fig. 5-4)
Often, one ground rod will not provide the ground
resistance required for the particular installation. The NEC
requires the ground resistance with one rod, pipe or plate
electrode to be 25 ohms or less. If it is over 25 ohms, a
second electrode is required, connected to the first electrode
and separated by 6 feet or more. The resistance of the two
electrodes does not have to meet the 25 ohm maximum
resistance requirement.
More often, a maximum resistance is called out in the job
specifications. This may be 5 ohms or sometimes as low as
1 ohm. Depending on the earth resistivity at the site, a low
resistance may be difficult to acquire. There are several
ways to lower system ground resistance:
Use multiple rods. Unless the surface layer of soil (top 8
to 10 feet) is of a relatively low resistance, the use of
multiple rods may not be effective. Multiple rods should be
separated 8 to 10 feet for maximum efficiency and
economy requiring a larger area which may not be
available.
Use deep driven rods. Many high resistance sites have a
high resistivity soil in the upper levels (for example a rocky
surface) but a lower resistivity at lower levels. Deep driven
rods will reach this low resistivity layer. Sometimes it is
necessary to drive 100 to 150 feet to reach this low
resistivity layer. Since a continuous rod cannot be installed,
the method of splicing the rod sections must be carefully
examined. The methods available are threaded couplers,
compression (threadless) type and welded type. ERICO
has a full line of ground rods and ground rod accessories.
(Fig 5-5)
Practical Guide to Electrical Grounding 84
Fig. 5-3
Rods are also available with factory attached
copper wire pigtails.
ERICO GEM
Ground Rod
Fig. 5-5
Threaded
Coupler
CADWELD Connection Compression
(Threadless)
Coupler
Plain Rod Threaded Rod
Fig. 5-4
GEM is easily installed. Auger a 3 to 6 inch hole to
a depth equal to 6 inches less than the rod length.
Drop the rod down the hole with the lower end
centered and driven in 6 inches. Fill the hole using
either dry GEM or premixed (slurry) GEM material.
Grounding Book 4/14/99 10/5/99 6:02 PM Page 84 (Black plate)