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MINIMUM SITE GROUNDING (EARTHING) REQUIREMENTS CHAPTER 4: EXTERNAL GROUNDING (EARTHING) 4.7.4 GROUNDING (EARTHING) ELECTRODE SYSTEM RESISTANCE REQUIREMENTS In order to disperse lightning energy into the earth without causing dangerous over-voltage, the shape and dimensions of the grounding (earthing) electrode system are more important than a specific resistance value of the grounding electrode system. However, a low resistance grounding electrode system is generally recommended (IEC 61024-1-2). Attempts should be made to reduce the grounding electrode system resistance to the lowest practical value (MIL-HDBK-419A, section 2.2.3). NOTE: Although grounding electrode system resistance is important and should be met whenever possible, it alone does not determine the suitability of the grounding electrode system to properly dissipate and control lightning energy. The resistance of the grounding electrode system is only a general measure of merit. Proper design and installation of the grounding electrode system, installation of ground rings, ground rods, radial grounding conductors, and the bonding of systems and equipment, is as important as the resistance to earth. Effective grounding electrode system shape and dimensions are achieved through the proper installation of the required and recommended grounding electrode system components listed throughout this chapter. The required and recommended grounding electrode system components are, but not limited to, the following: • Building ground ring. See “External Building and Tower Ground Ring” on page 4-22. • Tower ground ring. See “External Building and Tower Ground Ring” on page 4-22. • Ground rods properly installed and spaced around the building and tower ground rings. See “Ground Rods” on page 4-11. • Radial grounding conductors for high lightning prone geographical areas, sites that are normally occupied (such as 911 dispatch centers), sites with high soil resistivity, or when bedrock prohibits the driving of ground rods. See “Radial (Counterpoise) Grounding Conductors” on page 4-24. • Proper bonding of all grounding electrode system components. See “Bonding to the External Grounding (Earthing) Electrode System” on page 4-40. • Proper bonding of all ancillary equipment. See “Bonding to the External Grounding (Earthing) Electrode System” on page 4-40. The grounding electrode system resistance requirement is determined based on the classification of the site. Communications sites are classified by this standard into two categories: • Type A: See “Type “A” Sites - Light Duty” on page 4-46. • Type B: See “Type “B” Sites - Standard Duty” on page 4-47. 4.7.4.1 TYPE “A” SITES - LIGHT DUTY Type “A” sites shall have a grounding (earthing) electrode system resistance design goal of 25 ohms or less for a single grounding electrode (NFPA 70-2005, Article 250.56; MIL-HDBK-419A, section 2.2.2.1). If the design goal of 25 ohms cannot be achieved throughout the year with a single grounding electrode, then the grounding electrode shall be augmented by at least one additional grounding electrode (NFPA 70-2005, Article 250.56). See “Soil Resistivity Variability and Factors Affecting Soil Resistivity” on page B-1 for information regarding the seasonal variations in grounding electrode resistance. 4-46 68P81089E50-B 9/1/05
STANDARDS AND GUIDELINES FOR COMMUNICATION SITES MINIMUM SITE GROUNDING (EARTHING) REQUIREMENTS NOTE: It is recommended to use two grounding electrodes as the minimum installation, even if 25 ohms is achieved with a single grounding electrode. Type “A” sites typically have the following characteristics: • No tower associated with the site • Non-critical installation • Not part of a larger system infrastructure • Single control station • RF alarm/reporting site • Small telemetry cabinet utilizing an existing utility pole to support the antenna • Single voting receiver site • May be located in a commercial office or residence 4.7.4.2 TYPE “B” SITES - STANDARD DUTY Type “B” sites shall have a grounding (earthing) electrode system resistance design goal as low as practical and not over 10 ohms (AS 3516.2-1998, BS 6651:1999, FAA STD 019d-2002, MIL-HDBK- 419A, and MIL-STD-188-124A); Motorola recommends a design goal of 5 ohms or less (also see the International Association of Electrical Inspectors publication, “Soares Book on Grounding and Bonding” - 9th Edition, Appendix B and the United States National Weather Service Manual 30-4106- 2004, “Lightning Protection, Grounding, Bonding, Shielding, and Surge Protection Requirements”). If the design goal of 10 ohms (or recommended 5 ohms) cannot be achieved with the minimum accepted grounding electrode system, reasonable efforts shall be made to achieve the design goal using supplemental grounding techniques. The supplemental grounding techniques are described in “Supplemental Grounding (Earthing)” on page 4-48. Type “B” sites typically have the following characteristics: • A tower is associated with the site • 911 dispatch center • Communications dispatch center • Base station and/or repeater site • Telecommunication repeater equipment is installed, such as cellular, PCS, or wide-area repeater site • Large installation or multiple systems, such as telephone or electronic switches, LANs/WANs, and Mobile Switching Offices (MSO) are installed • Critical public safety or military installation 68P81089E50-B 9/1/05 4-47
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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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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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