ERICO Practical Guide to Electrical Grounding - Manual (Page 40)

32 Practical Guide to Electrical Grounding
electrodes as required by the NEC (250-81 and 250-86) and
the several lightning codes such as NFPA 780. No
exceptions! Failure to bond all grounding electrodes
together can result in dangerous voltage differences
between exposed metal connected to ground points,
especially during lightning strikes or ground faults. Such
voltage differences injure people and destroy equipment.
Transient earth clamps are available that act as an open
switch during normal operation but turn on during an
overvoltage event to bond the systems together. These are
approved for use in some countries for bonding between the
separate ground electrode systems.
The purpose of establishing a low resistance connection to
earth is to conduct lightning current away from people,
equipment and structures. A low resistance grounding
system is desirable in a lightning protection system but not
essential. In an area where the soil resistivity is high, an
extensive network of conductors still may not provide a low
grounding resistance. But, the potential distribution about
the building is substantially the same as though it were
setting on conductive soil with a low resistance grounding
scheme. The resulting lightning protection is also substan-
tially the same. The minimum electrode requirements vary
with the soil type.
Moist clay. The electrode shall extend vertically at least 10
feet into the earth. The rod size shall be at least
1 /
2
inch by
8 feet (5/8 x 8 for buildings over 75 feet high). (Fig. 2-11)
Shallow top soil. If bedrock is near the surface, the
conductors are laid in trenches extending away from the
building. The trenches shall be 1 to 2 feet (0.31 to 0.62 m)
deep and 12 feet (3.7 m) long in clay soil (Fig. 2-12) and 2
feet (0.62 m) deep and 24 feet (7.4 m) long in sandy or
gravelly soil. (Fig. 2-13) In rare cases where this is imprac-
ticable, the lightning cable shall be buried in 2 feet (.62 m)
deep trenches. Where this is impossible, the cable may be
laid directly on top of the bedrock at least 2 feet (0.62 m)
from the foundation or exterior footing. This cable must
be terminated on a buried copper plate at least 0.032 in
(0.813 mm) thick and 1 square foot (0.093 square m) area.
(Fig. 2-14)
Sandy or gravelly soil. In sandy or gravelly soil, the
lightning conductor shall extend away from the building in
a trench at least 12 inch deep. The ground rod shall be 20
feet long or greater or there shall be 2 or more rods
separated at least 10 feet driven vertically to a minimum 10
feet below grade. (Fig. 2-15)
If the soil is less than 12 inch thick, a counterpoise (or
network of conductors) in a trench or rock crevices shall
surround the structure. The counterpoise conductor must be
copper, sized to meet Class I main cable size. If the
structure is over 75 feet in height, the cable must be sized to
meet Class II main size copper. These cable sizes are listed
in the various lightning codes. In extreme cases, copper
plates may also be required.
10' Min.
10' Min.
10' Min.
10' Min.
Sandy Or Gravelly Soil Alternate Layouts
Fig. 2-15
Grounding Book 4/14/99 10/5/99 6:01 PM Page 32 (Black plate)