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DISSIMILAR METALS AND CORROSION CONTROL CHAPTER 4: EXTERNAL GROUNDING (EARTHING) TABLE 4-7 SUITABILITY OF METALS FOR BONDING Metal assumed to have the Larger Surface Area Electrode metal or item assumed to have the Smaller Surface Area Steel Galvanized Steel Copper Galvanized Steel Suitable Suitable Suitable Suitable Steel in Concrete Not Suitable Not Suitable Suitable Suitable Galvanized Steel in Concrete Suitable * Suitable Suitable Lead Suitable * Suitable Suitable Key: Suitable = Materials suitable for bonding. Not Suitable = Materials not suitable for bonding. * = Materials suitable for bonding, but the galvanizing on the smaller surface may suffer. This table is based on Table 8 of BS 7430:1998. 4.5.3 MISCELLANEOUS GENERAL INFORMATION • Galvanized steel is strongly electronegative to copper and steel encased in concrete, therefore, careful consideration must be give before galvanized ground rods are used at a site that contains a concreted encased electrode. (See BS 7430:1998, clause 11.2 for additional information.) See “Concrete-Encased Electrodes” on page 4-20. • Steel encased in concrete has a potential similar to that of copper; therefore, may be bonded to copper or copper-clad ground rods. (See BS 7430:1998 and IEEE STD 142-1991 for additional information.) 4.5.4 METHODS TO HELP REDUCE CORROSION Tinned Copper Listed below are some general requirements and guidelines to help prevent corrosion of the grounding (earthing) electrode system and other metallic items at the communications site: • The same metal shall be used throughout the grounding electrode system whenever possible. • Aluminum or copper-clad aluminum grounding conductors shall not be used (NFPA 70-2005, Article 250.64). • Copper shall not come into incidental contact with galvanized steel. • Copper shall not come into incidental contact with aluminum. • Precipitation run-off from copper and copper alloys can attack galvanized parts (BS 6651:1999 and IEC 61024-1-2, section 5.2); therefore, bare copper conductors or copper bus bars shall not be installed above galvanized steel, such as a tower, unless the steel is protected against the precipitation run-off (IEC 61024-1-2, section 5.2). 4-38 68P81089E50-B 9/1/05
STANDARDS AND GUIDELINES FOR COMMUNICATION SITES DISSIMILAR METALS AND CORROSION CONTROL CAUTION Extremely fine particles are shed by copper parts, which result in severe corrosive damage to galvanized parts, even where the copper and galvanized parts are not in direct contact (IEC 61024-1-2, section 5.2). • Where a galvanized tower is not protected against precipitation run-off from copper and copper alloys, the tower ground bus bar (TGB) shall be constructed of tinned copper or other suitable material. • Where tinned conductors or galvanized ground rods are used, care shall be exercised during installation so that surfaces are not damaged. If surfaces of these ground elements are damaged, the potential for deterioration from galvanic action increases (ANSI T1.313-2003, section 11.5 and ANSI T1.334-2002, section 13.6). • Exothermically welded joints on galvanized material shall be coated with a zinc-enriched paint to prevent corrosion. • Copper/aluminum joints shall be avoided wherever possible. In cases where they cannot be avoided, the connections shall be exothermically welded or made using an AL/CU listed bimetallic transition connector (IEC 61024-1-2, section 5.2). Use a listed conductive anti-oxidant compound on all mechanical connections (ANSI T1.334-2002, section 9). • Solid, bare, tinned copper conductor should be used to minimize galvanic corrosion between tower legs and other parts of the grounding electrode system (ANSI T1.313-2003, section 10.7). • Grounding connections to galvanized towers shall be exothermally welded whenever possible. When exothermic welding is not possible, the tower grounding conductor shall be constructed of tinned-copper (IEC 61024-1-2, section 5.2). • Select appropriate grounding electrode system components using Table 4-5. • When soil conditions are not favorable, such as highly acidic or alkaline, grounding electrode system component corrosion may be reduced by encasing the components in a grounding electrode encasement material. See “Grounding (Earthing) Electrode Encasement Materials” on page 4-27, and BS 7430:1998, section 19.6.1, for additional information. • When soil conditions are not favorable, such as highly acidic or alkaline, grounding electrode system component corrosion may be reduced by installing electrolytic ground rods encased in a grounding electrode encasement material. See “Grounding (Earthing) Electrode Encasement Materials” on page 4-27 and “Electrolytic Ground Rods” on page 4-16. • When soil conditions are not favorable, such as highly acidic or alkaline, the useful life of a copper ground rod can be extended by using solid copper ground rods instead of copper-clad rods, if soil conditions allow driving of the solid copper rod. • When soil conditions are not favorable, such as highly acidic or alkaline, the useful life of buried grounding conductors can be extended by using larger conductors, such as 70 mm2 csa (#2/0 AWG) instead of 35 mm 2 (#2 AWG). • Use a listed conductive anti-oxidant compound on all mechanical connections (ANSI T1.334-2002, section 9). The anti-oxidant compound shall be liberally installed between the two metals (see Figure 4-27 on page 4-40). • See “Guy Anchor Points” on page 4-54 for information regarding proper grounding techniques to help minimize galvanic corrosion of the guy anchor. 68P81089E50-B 9/1/05 4-39
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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 81 and 82: 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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