standards and guidelines for communication sites - Radio And ...
standards and guidelines for communication sites - Radio And ... standards and guidelines for communication sites - Radio And ...
GROUNDING (EARTHING) ELECTRODE SYSTEM COMPONENT AND INSTALLATION REQUIREMENTS CHAPTER 4: EXTERNAL GROUNDING (EARTHING) 2X Rod Length FIGURE 4-6 MINIMUM GROUNDING ELECTRODE SPACING FOR MAXIMUM EFFECTIVITY Table 4-2 provides the relationship between percentage of total ground rod resistance and the radial distance from the ground rod (IEEE STD 142-1991, Table 9). . Optimum Spacing 2X Rod Length TABLE 4-2 TOTAL GROUND ROD RESISTANCE VS. DISTANCE FROM GROUND ROD Distance from Electrode Surface (r)** ft Meters Approximate Percentage of Total Resistance 0.1 0.03 25% 0.2 0.06 38% 0.3 0.09 46% 0.5 0.15 52% 1.0 0.3 68% 5.0 1.5 86% 10.0* 3.0* 94% 15.0 4.6 97% 20.0 6.1 99% 25.0 7.6 100% * 94% of the resistance to remote earth occurs within a radius equal to the length of the ground rod. This radius is commonly used as the ground rod's sphere of influence. ** Ground rod resistance at a radius (r) from a 3 m × 16 mm (10 ft. × 0.625 in.) ground rod (From IEEE STD 142-1991, Table 9) The following observations can be made from the above table (IEEE STD 142-1991, chapter 4): • Within the first 2.5 cm (1 in.) from the ground rod, 25% of the total resistance to earth is achieved. 4-10 68P81089E50-B 9/1/05
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
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GROUNDING (EARTHING) ELECTRODE SYSTEM COMPONENT AND INSTALLATION REQUIREMENTS CHAPTER 4: EXTERNAL GROUNDING (EARTHING)<br />
2X Rod Length<br />
FIGURE 4-6 MINIMUM GROUNDING ELECTRODE SPACING FOR MAXIMUM EFFECTIVITY<br />
Table 4-2 provides the relationship between percentage of total ground rod resistance <strong>and</strong> the radial<br />
distance from the ground rod (IEEE STD 142-1991, Table 9).<br />
.<br />
Optimum Spacing<br />
2X Rod Length<br />
TABLE 4-2 TOTAL GROUND ROD RESISTANCE VS. DISTANCE FROM GROUND ROD<br />
Distance from Electrode Surface (r)**<br />
ft Meters<br />
Approximate Percentage of<br />
Total Resistance<br />
0.1 0.03 25%<br />
0.2 0.06 38%<br />
0.3 0.09 46%<br />
0.5 0.15 52%<br />
1.0 0.3 68%<br />
5.0 1.5 86%<br />
10.0* 3.0* 94%<br />
15.0 4.6 97%<br />
20.0 6.1 99%<br />
25.0 7.6 100%<br />
* 94% of the resistance to remote earth occurs within a radius equal to the length of the ground rod. This<br />
radius is commonly used as the ground rod's sphere of influence.<br />
** Ground rod resistance at a radius (r) from a 3 m × 16 mm (10 ft. × 0.625 in.) ground rod<br />
(From IEEE STD 142-1991, Table 9)<br />
The following observations can be made from the above table (IEEE STD 142-1991, chapter 4):<br />
• Within the first 2.5 cm (1 in.) from the ground rod, 25% of the total resistance to earth is achieved.<br />
4-10 68P81089E50-B 9/1/05
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