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

STANDARDS AND GUIDELINES FOR COMMUNICATION SITES GROUNDING (EARTHING) ELECTRODE SYSTEM COMPONENT AND INSTALLATION
REQUIREMENTS
• Within the first 152 mm (6 in.) from the ground rod, 52% of the total resistance to earth is achieved.
• The area immediately around a ground rod is the most important for reducing its resistance to earth.
See “Grounding (Earthing) Electrode Encasement Materials” on page 4-27 for information on
reducing resistance.
• In high resistivity soil areas, decreasing the soil resistance in this area is useful in improving
the effectiveness of the grounding electrode system.
• In porous soil areas, decreasing the contact resistance with the ground rod in this area is useful
in improving the effectiveness of the grounding electrode system.
Unless specified elsewhere in this chapter, ground rods should be installed apart from one another by
the sum of their respective lengths, so their spheres of influence do not overlap (See Figure 4-6). This is
especially important when only a small number of ground rods are installed, such as around tower
ground rings.
NOTE: In a given area, more ground rods installed closer together (such as one length apart from one another)
will achieve a lower resistance to earth than fewer rods installed further apart (such as twice the length
apart from one another). For example, five 3 m (10 ft.) ground rods installed 6.1 m (20 ft.) apart in an
24.4 m (80 ft.) straight line will achieve a resistance to earth of 7.8 ohms (assuming 10,000 ohm-cm
soil). Nine 3 m (10 ft.) ground rods installed 3 m (10 ft.) apart in the same 24.4 m (80 ft.) straight line
will achieve a resistance to earth of 5.7 ohms.
4.4.1.2 GROUND RODS
Typical ground rods are shown in Figure 4-7. Requirements for ground rods are listed below. See IEEE
STD 142-1991, section 4.3.1 and UL 467-2004 for additional information.
FIGURE 4-7 TYPICAL GROUND RODS
4.4.1.2.1 GROUND ROD SPECIFICATIONS
• Ground rods shall be UL listed (or equivalent).
• Ground rods shall be constructed of copper-clad steel, solid copper, hot-dipped galvanized steel, or
stainless steel (ANSI-J-STD-607-A-2002, section C.4.3, and ANSI T1.334-2002, section 5.3.2).
See NFPA 70-2005, Article 250.52 and UL 467-2004, section 9.2.1 for additional information.
68P81089E50-B 9/1/05 4-11