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

SPECIAL GROUNDING (EARTHING) SITUATIONS CHAPTER 4: EXTERNAL GROUNDING (EARTHING)
See “Interpreting Test Results” on page B-10 to determine if the desired resistance to earth can be
achieved using different rod lengths and/or separation. If the resistance to earth cannot be achieved
using standard rods, electrolytic rods should be considered. Burying the grounding conductor in at least
152 mm (6 in.) of grounding electrode encasement material should also be considered as a method of
improving the resistance to ground. (See “Grounding (Earthing) Electrode Encasement Materials” on
page 4-27.)
NOTE: In shallow topsoil conditions, the above grounding electrode grid system can utilize ground plates
instead of ground rods.
4.11.3 TOWERS WITH LIMITED SPACE FOR A GROUND RING
Towers installed close to a building may not have adequate space for a complete tower ground ring or
for ground rods spaced properly to achieve the resistance requirements of the site. Depending on the
available space, the tower can be grounded (earthed) using multiple parallel rods and/or ground radials.
(See “Radial (Counterpoise) Grounding Conductors” on page 4-24 and Figure 4-65 on page 4-90.)
See “Interpreting Test Results” on page B-10 to determine the number of rods and rod spacing required
to achieve the resistance requirements of the site.
4.11.4 STONE MOUNTAIN TOPS
Some sites are located on mountaintops because of their RF propagation characteristics. In the instances
where there is no, or very little, top soil at the site, special designs will be needed. Some options for an
effectively grounded (earthed) site are listed 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:
• 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 radial grounding conductors from the tower and building throughout the property.
Install the radial grounding conductors to a depth allowed by the soil, preferably 457 to 762 mm
(18 to 30 in.) Encasing the radial grounding conductors in a grounding electrode encasement
material can further increase the effectiveness of the grounding electrode system. See “Radial
(Counterpoise) Grounding Conductors” on page 4-24.
• 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 and
“Grounding (Earthing) Electrode Encasement Materials” on page 4-27.
• Installation of horizontal ground rods or horizontal electrolytic ground rods along the length of the
ground rings instead of vertical ground rods. The ground rods shall be installed perpendicular to the
building and tower. Encasing the ground rings and horizontal ground rods in a grounding electrode
encasement material can further increase the effectiveness of the grounding electrode system.
4-92 68P81089E50-B 9/1/05