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GROUNDING (EARTHING) ELECTRODE SYSTEM COMPONENT AND INSTALLATION REQUIREMENTS CHAPTER 4: EXTERNAL GROUNDING (EARTHING) 4.4.1.2.4 EFFECT OF PARALLEL GROUND RODS Figure 4-13 below shows the effects of adding additional ground rods (15.9 mm (0.625 in.) diameter by 3 m (10 ft.) long) together in parallel. As seen in the figure, the addition of one ground rod to the first ground rod (for a total of two rods) significantly reduces the resistance to earth of the ground rod system. Each subsequent ground rod added in parallel has less of an effect on the resistance to earth of the ground rod system. Approximate Resistance to Earth in Ohms 35 30 25 20 15 10 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Number of 3 m (10 ft) Ground Rods Installed 6.1 m (20 ft) apart in 10,000 ohm-cm soil FIGURE 4-13 RESISTANCE TO EARTH DUE TO PARALLEL GROUND RODS 4.4.1.3 ELECTROLYTIC GROUND RODS At sites where, due to poor soil conductivity (high resistivity) and/or limited space, an acceptable grounding (earthing) electrode system resistance cannot be achieved using standard ground rods, commercially available electrolytic ground rods should be considered. See MIL-HDBK-419A Volume I, section 2.9.5, and UL 467-2004, section 9.2.7 for additional information. Electrolytic ground rods (Figure 4-14) are available in straight or L-shaped versions and in various lengths from 3 m (10 ft.) to 6.1 m (20 ft.), or longer as a special order. Electrolytic ground rods are generally constructed of 54 mm (2.125 in.) diameter hollow copper pipe. This copper pipe is filled with a mixture of non-hazardous natural earth salts. Holes at various locations on the pipe allow moisture to be hygroscopically extracted from the air into the salt within the pipe, therefore forming conductive electrolytes. These electrolytes then leach out of the pipe into the soil, improving soil conductivity. Electrolytic ground rods are inserted into a pre-drilled hole, or in the case of L-shaped rods, placed into a trench at least 762 mm (30 in.) deep, and encased in a grounding electrode encasement material. See “Grounding (Earthing) Electrode Encasement Materials” on page 4-27. Electrolytic ground rods should be considered for use in grounding electrode systems covered by concrete or pavement, such as parking lots. By allowing moisture to enter, the design of the electrolytic ground rod improves the resistance of the grounding electrode system. 4-16 68P81089E50-B 9/1/05
STANDARDS AND GUIDELINES FOR COMMUNICATION SITES GROUNDING (EARTHING) ELECTRODE SYSTEM COMPONENT AND INSTALLATION REQUIREMENTS NOTE: Unless prohibited by local environmental authorities, condensation from the site's HVAC system may be routed to the ground rod area to keep the soil moist, improving conductivity. Electrolytic ground rods may provide significant improvement over standard ground rods of the same length and may last several years longer than standard ground rods. The resistance to earth of electrolytic ground rods is generally more stable in environments with variations in temperature and moisture. Requirements for the use of electrolytic ground rods are listed below: • Electrolytic rods shall be UL listed (or equivalent). • Electrolytic rods shall be installed per the manufacturers' recommendation. • Electrolytic rods shall be free of paint or other nonconductive coatings (NFPA 70-2005, Article 250.53 and NFPA 780-2004, section 4.13.2.2). • Electrolytes within the rod shall be environmentally safe and approved by the environmental authority having jurisdiction. • L-shaped electrolytic rods shall be installed perpendicular to the building or shelter. • L-shaped electrolytic rods (horizontal portion) shall be installed at least 762 mm (30 in.) below the earth's surface. • Grounding electrode encasement materials (also known as backfill) shall be environmentally safe and approved by the environmental authority having jurisdiction. See “Grounding (Earthing) Electrode Encasement Materials” on page 4-27. • Electrolytic rods should be maintenance free. 68P81089E50-B 9/1/05 4-17
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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 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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