ERICO Practical Guide to Electrical Grounding - Manual (Page 22)

14 Practical Guide to Electrical Grounding
Section 250-83 contains requirements for other (frequently
referred to as “made”) electrodes. These electrodes can be
used to supplement the grounding electrode system or are
to be used when none of the grounding electrodes covered
previously are available. Local metal piping systems, such
as water wells, can be used but metal underground gas
piping systems shall not be used as the grounding electrode.
The most common made electrodes consist of rod, pipe or
plates. Ground rods can be constructed of iron or steel, of at
least 5/8 inch in diameter. Nonferrous ground rods, such as
copperbonded steel or stainless steel can also be used,
provided they are not less than 1/2 inch in diameter and are
listed. Design life of the facility being protected should be
considered when choosing ground rod material. Galvanized
steel ground rods are often used for grounding structures
such as a telephone booth with an anticipated service of 10
years or less. A UL Listed copperbonded steel ground rod
with a copper thickness of 10 mils (0.25 mm) will last 30
years or more in most soils. A 13 mil (0.33 mm) copper
thickness copperbonded steel rod will last 40 years or more
in most soils. Unusual soil conditions demand additional
considerations. Contractors should be aware of the many
factors that influence the impedance of grounding systems
that utilize ground rods. The dimension of the ground rod
does have some affect on its resistance. Typically, the larger
the diameter of the ground rod, the lower its resistance, but
to a very minor extent. A far more important factor in
determining the resistance of the ground rod is the depth to
which it is driven. Usually, the deeper the ground rod is
driven, the lower its resistance. Another very important and
frequently unknown factor is the resistivity of the soil
where the ground rod is driven. As stated above, the higher
the moisture content of the soil, the lower its resistivity.
ERICO GEM
TM , Ground Enhancement Material, is the
answer in situations where reducing earthing resistance and
maintaining low resistance permanently is required. GEM
reduces the resistance of the electrode to the earth and
performs in all soil conditions. GEM can be used around
ground rods in an augured hole or installed in a trench as
permitted by Section 250-83 (c) (3), of the NEC. See Figure
1-35 (Page 13). As with all of the grounding electrodes, the
connection is critical to maintaining the integrity of the
grounding system. While listed clamps or fittings are
permitted, exothermic welding provides the best solution to
the contractor needs. ERICO offers a complete line of cable
to ground rod connections, including the CADWELD
ONE-SHOT
® connection, which can be used for both plain
or threaded copperbonded and galvanized steel or stainless
steel rods. See (Figures 1-36 and 1-37).
CADWELD Ground Rod Connections
Fig. 1-37
GR GT
NX NT
GR
GB
GT
GY
ND
NC
CADWELD One-Shot ® Connections
Fig. 1-36
Grounding Book 4/14/99 10/5/99 6:01 PM Page 14 (Black plate)