standards and guidelines for communication sites - Radio And ...

STANDARDS AND GUIDELINES FOR COMMUNICATION SITES SPECIAL GROUNDING (EARTHING) SITUATIONS
• Using multiple large copper plates (0.88 to 1.8 m2 (10 to 20 ft2 .)) buried to an optimal depth of 1.5
m to 2.4 m (5 to 8 ft.). The plates are placed vertically on edge and bonded to the grounding
electrode system using exothermically welded 35 mm 2 csa (#2 AWG) solid copper wire. Placing
the plates on vertical edge allows the plates to be buried with a minimum of excavation and may
make it possible to obtain more surface area contact with the soil when backfilling. The use of a
number of well-placed ground plates in parallel is preferred to placing longer rows of ground plates
(IEEE STD 142-1991, section 4.2.4). Encasing the ground plates in a grounding electrode
encasement material can further increase the effectiveness of the grounding electrode system.
4.11.6 SHALLOW TOPSOIL ENVIRONMENTS
Some sites are located in areas where bedrock is near the surface or where the top soil is less than
305 mm (1 ft.) deep. These areas require installation of specialized grounding (earthing) electrode
systems and may require the support of an engineering firm.
Requirements and recommendations for grounding electrode systems in areas with shallow topsoil are
provided below. Reasonable attempts should be made to use as many options as possible and as needed
to meet the ground resistance requirements of the site. See NFPA 780-2004, section 4.13.8.1 for
additional information.
• Consult with Motorola Engineering or other engineering firm.
• Installation of concrete-encased electrodes as part of new construction. See “Concrete-Encased
Electrodes” on page 4-20.
• Installation of ground rings around the building and tower, with the ground rings buried as deep as
the soil will allow. The ground rings should be encased in a grounding electrode encasement
material. A conductive concrete may be the best grounding electrode encasement material for use
in shallow topsoil environments, since the conductive concrete would not require a covering of
topsoil for protection. See “External Building and Tower Ground Ring” on page 4-22.
• Installation of ground plates along the length of the ground rings instead of vertical ground rods.
The ground plates should be encased in a grounding electrode encasement material. See
“Grounding (Earthing) Electrode Encasement Materials” on page 4-27 and “Ground Plate
Electrodes” on page 4-19.
• Installation of a grounding electrode grid system, using ground plates instead of vertical ground
rods. The grounding conductors and ground plates shall be buried as deep as the soil will allow.
The grounding conductors should be encased in a grounding electrode encasement material. A
conductive concrete may be the best grounding electrode encasement material for use in shallow
topsoil environments, since the conductive concrete would not require a covering of topsoil for
protection. See “Sites With Limited Space for the Grounding (Earthing) Electrode System” on
page 4-90 and Figure 4-66 on page 4-91 for additional information on grounding electrode grid
systems.
• Installation of electrolytic ground rod systems. See “Electrolytic Ground Rods” on page 4-16.
68P81089E50-B 9/1/05 4-97