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MINIMUM SITE GROUNDING (EARTHING) REQUIREMENTS CHAPTER 4: EXTERNAL GROUNDING (EARTHING) 4.7.4.3 SUPPLEMENTAL GROUNDING (EARTHING) Sites with high (poor) soil resistivity shall require enhancements to the grounding (earthing) electrode system if the resistance requirement of the site cannot be met. Sites in high lightning prone geographical areas, and sites normally occupied (such as dispatch centers), should also include enhancements to the grounding electrode system, regardless if the resistance requirements are met. Some techniques for enhancing the grounding electrode system are described below: • Installation of radial grounding conductors. • Installation of concrete encased electrodes in new construction. • Installation of longer ground rods. NOTE: Doubling the length of a ground rod will reduce its resistance value by approximately 40% (assuming homogeneous soil). • Installation of electrolytic ground rods. • Use of grounding electrode encasement materials. • Specific design by Motorola Engineering or other engineering firm. 4.7.5 TYPE “A” SITE GROUNDING (EARTHING) In sites defined as Type “A”, a single ground rod may be sufficient if it can achieve 25 ohms or less throughout the year. If a single ground rod cannot achieve 25 ohms or less throughout the year, then the grounding (earthing) electrode shall be augmented by an additional grounding electrode (NFPA 70- 2005, Article 250.56 and MIL-HDBK-419A, section 2.2.2.1), or alternate methods shall be used. Such alternate methods may include the following: • Installation of concrete encased electrodes as part of new construction. • Installation of a longer rod that can achieve 25 ohms or less throughout the year. In general, doubling the length of a ground rod will reduce its resistance value by approximately 40%. • Installation of a parallel ground rod. The ground rods shall maintain a minimum separation of 1.8 m (6 ft.) from one another (NFPA 70-2005, Article 250.56). For maximum parallel efficiency, the ground rods should be separated by the sum of their respective lengths. See “Grounding (Earthing) Electrode Resistance Characteristics and Sphere of Influence” on page 4-9 and Figure 4- 31 for additional information. 4-48 68P81089E50-B 9/1/05
STANDARDS AND GUIDELINES FOR COMMUNICATION SITES MINIMUM SITE GROUNDING (EARTHING) REQUIREMENTS 2.4 m (8 FT) FIGURE 4-31 PARALLEL GROUND ROD INSTALLATION EXOTHERMIC WELDS OR IRREVERSIBLE HIGH COMPRESSION FITTINGS NOTE: It is recommended to use two grounding electrodes as the minimum installation, even if 25 ohms is achieved with a single grounding electrode. • Installation of a small ground ring (Figure 4-64 on page 4-89). • Installation of an electrolytic ground rod. At an existing building, the AC power system grounding electrode system will typically serve adequately as the communications Type “A” grounding electrode system. A supplemental Type “A” grounding electrode system may need to be installed if the site has an RF transmission line (or other communications cables) entry point at a different location than the AC power utilities entrance. If a supplemental Type “A” grounding electrode system is installed, it shall be bonded to any other grounding electrode system at the site. See “Common Grounding (Earthing)” on page 4-5. For an example of bonding a supplemental grounding electrode system to an existing AC power service grounding electrode system, see Figure 4-54 on page 4-77 and Figure 4-64 on page 4-89. NOTE: A single 2.4 m × 15.9 mm (8 ft. × 0.625 in.) ground rod requires a soil resistivity of approximately 6,250 ohm-cm or less throughout the year in order to achieve a resistance value of 25 ohms; a 3 m × 15.9 mm (10 ft. × 0.625 in.) ground rod would require 7,500 ohm-cm; a 4.9 m × 15.9 mm (16 ft. × 0.625 in.) ground rod would require 11,200 ohm-cm; and a 6.1 m × 15.9 mm (20 ft. × 0.625 in.) ground rod would require 13,600 ohm-cm. See Chapter , “Soil Resistivity Measurements,” for additional information. 68P81089E50-B 9/1/05 4-49 2.4 m (8 FT)
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© 2005 MOTOROLA, INC. ALL RIGHTS R
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TABLE OF CONTENTS Chapter 1. Introd
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STANDARDS AND GUIDELINES FOR COMMUN
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CHAPTER 1 INTRODUCTION 1 This manua
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CHAPTER 2 SITE DESIGN AND DEVELOPME
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CHAPTER 3 COMMUNICATION SITE BUILDI
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Page 91 and 92: STANDARDS AND GUIDELINES FOR COMMUN
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CHAPTER 5 INTERNAL GROUNDING (EARTH
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CHAPTER 6 POWER SOURCES 6 6.1 LOCKO
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CHAPTER 7 SURGE PROTECTIVE DEVICES
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CHAPTER 8 MINIMIZING SITE INTERFERE
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CHAPTER 9 EQUIPMENT INSTALLATION 9
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APPENDIX A ELECTROMAGNETIC ENERGY I
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APPENDIX B SOIL RESISTIVITY MEASURE
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APPENDIX C PROTECTING AGAINST ELECT
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APPENDIX D GROUNDING (EARTHING) ELE
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APPENDIX E GENERAL CONVERSIONS AND
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APPENDIX F R56 COMPLIANCE CHECKLIST
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I NDEX Index Index AC power cabling
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