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) ROOF-MOUNTED ANTENNA MASTS AND METAL SUPPORT STRUCTURES CHAPTER 4: EXTERNAL GROUNDING (EARTHING) IMPORTANT: In ice prone areas, if isolating the cable bridge/ice bridge from the tower creates an unprotected area on the RF transmission lines, some type of ice shield shall be installed above the isolation point in order to protect the RF transmission lines. The ice shield shall be bonded to the tower only. 4.7.11 METALLIC BUILDING SIDING GROUNDING (EARTHING) Although metallic building siding is not required to be grounded by this standard, grounding the metallic siding can provide additional safety at the site (NFPA 70-2005, Article 250.116-FPN). It is recommended to bond the metallic building siding in at least one location on the building, preferably near the electrical service entrance. 4.8 GROUNDING (EARTHING) ROOF-MOUNTED ANTENNA MASTS AND METAL SUPPORT STRUCTURES All roof-mounted antenna masts and metal support structures shall be grounded (earthed) (NFPA 70- 2005, Article 810.15). NOTE: Rooftop mounted towers are not covered in this section. See “Grounding (Earthing) Rooftop Mounted Tower Structures” on page 4-79 for information about rooftop tower grounding requirements. In new construction, provisions shall be engineered into the building design for effective roof-mounted antenna mast and support structure grounding. Typically this will include a grounding point, or multiple grounding points, with at least two direct connections to the building's grounding electrode system. The direct connection to the building's grounding electrode system may be made using effectively grounded structural building steel (preferred) and/or down-conductors. Other engineered antenna mast and support structure grounding systems may include metallic antenna support structures that are directly and effectively bonded to effectively grounded structural building steel. When a lightning protection system is installed on the building, roof-mounted antenna masts and support structures shall be bonded to the lightning protection system (IEC 61024-1-2 and NFPA 780- 2004, section 4.17). The conductor shall be of the same size as the Main Roof Perimeter Lightning Protection Ring (FAA STD-019d-2002, section 3.7.9.4). Conductor bonding shall be made using exothermic welding, listed irreversible high-compression fittings, or other fittings listed for use in lightning protection systems. See “Bonding to the External Grounding (Earthing) Electrode System” on page 4-40. No additional grounding shall be required of roof-mounted antenna masts and support structures when bonded to a lightning protection system. 4-74 68P81089E50-B 9/1/05
STANDARDS AND GUIDELINES FOR COMMUNICATION SITES GROUNDING (EARTHING) ROOF-MOUNTED ANTENNA MASTS AND METAL SUPPORT STRUCTURES When a lightning protection system is not available, roof-mounted antenna masts and metal support structure shall be grounded by directly bonding to the building's grounding electrode system, or to a supplemental grounding electrode system (FAA STD-019d-2002, section 3.7.9.4). Bonding to the grounding electrode system shall use effectively grounded structural building steel when available. When effectively grounded structural building steel is not available, bonding to the grounding electrode system shall use at least two down conductors whenever practical (ANSI T1.334-2002, section 8.2). When effectively grounded structural building steel is not available and the use of two down conductors is not practical, the use of a single down conductor should be approved by an engineer. Available water piping systems may also be used as a grounding down-conductor, but should only be used under the advice of an engineer. Bonding to the structural building steel shall be made using exothermic welding, listed irreversible high-compression fittings, or other fittings listed for use in lightning protection systems. See NFPA 780- 2004, section 4.16 for additional information regarding the use of structural building steel as a main grounding conductor. Figure 4-53 shows examples of acceptable building steel bonding connections. FIGURE 4-53 ACCEPTABLE STRUCTURAL BUILDING STEEL BONDING CONNECTIONS Down-conductors shall be routed to the grounding electrode system from opposite sides of the roof whenever practical (ANSI T1.334-2002, section 8.2). Down-conductors shall be physically separated from one another as much as practical (ANSI T1.334-2002, section 8.2). Additional requirements for down-conductors are as follows: • Grounding down-conductors shall be sized as follows: • The conductor shall be a 35 mm 2 csa (#2 AWG) or coarser, bare, copper or equivalent, for buildings not exceeding 22.9 m (75 ft.) in height (NFPA 780-2004, Table 4.1.1.1(A)). • The conductor shall be a 70 mm 2 csa (#2/0 AWG) or coarser, bare, copper or equivalent, for buildings equal to or exceeding 22.9 m (75 ft.) in height (NFPA 780-2004, Table 4.1.1.1(B)). • Grounding down-conductors shall be protected from physical damage (NFPA 70-2005, Article 810.21(d) and NFPA 780-2004, section 4.9.11). See “Protecting and Securing Grounding (Earthing) Conductors” on page 4-30. • Grounding down-conductors shall be permitted to be run either outside or inside the building or structure (NFPA 70-2005, Article 810.21(g)). 68P81089E50-B 9/1/05 4-75
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STANDARDS AND GUIDELINES FOR COMMUNICATION SITES GROUNDING (EARTHING) ROOF-MOUNTED ANTENNA MASTS AND METAL SUPPORT<br />
STRUCTURES<br />
When a lightning protection system is not available, roof-mounted antenna masts <strong>and</strong> metal support<br />
structure shall be grounded by directly bonding to the building's grounding electrode system, or to a<br />
supplemental grounding electrode system (FAA STD-019d-2002, section 3.7.9.4). Bonding to the<br />
grounding electrode system shall use effectively grounded structural building steel when available.<br />
When effectively grounded structural building steel is not available, bonding to the grounding electrode<br />
system shall use at least two down conductors whenever practical (ANSI T1.334-2002, section 8.2).<br />
When effectively grounded structural building steel is not available <strong>and</strong> the use of two down conductors<br />
is not practical, the use of a single down conductor should be approved by an engineer. Available water<br />
piping systems may also be used as a grounding down-conductor, but should only be used under the<br />
advice of an engineer.<br />
Bonding to the structural building steel shall be made using exothermic welding, listed irreversible<br />
high-compression fittings, or other fittings listed <strong>for</strong> use in lightning protection systems. See NFPA 780-<br />
2004, section 4.16 <strong>for</strong> additional in<strong>for</strong>mation regarding the use of structural building steel as a main<br />
grounding conductor. Figure 4-53 shows examples of acceptable building steel bonding connections.<br />
FIGURE 4-53 ACCEPTABLE STRUCTURAL BUILDING STEEL BONDING CONNECTIONS<br />
Down-conductors shall be routed to the grounding electrode system from opposite sides of the roof<br />
whenever practical (ANSI T1.334-2002, section 8.2). Down-conductors shall be physically separated<br />
from one another as much as practical (ANSI T1.334-2002, section 8.2). Additional requirements <strong>for</strong><br />
down-conductors are as follows:<br />
• Grounding down-conductors shall be sized as follows:<br />
• The conductor shall be a 35 mm 2 csa (#2 AWG) or coarser, bare, copper or equivalent, <strong>for</strong><br />
buildings not exceeding 22.9 m (75 ft.) in height (NFPA 780-2004, Table 4.1.1.1(A)).<br />
• The conductor shall be a 70 mm 2 csa (#2/0 AWG) or coarser, bare, copper or equivalent, <strong>for</strong><br />
buildings equal to or exceeding 22.9 m (75 ft.) in height (NFPA 780-2004, Table 4.1.1.1(B)).<br />
• Grounding down-conductors shall be protected from physical damage (NFPA 70-2005, Article<br />
810.21(d) <strong>and</strong> NFPA 780-2004, section 4.9.11). See “Protecting <strong>and</strong> Securing Grounding<br />
(Earthing) Conductors” on page 4-30.<br />
• Grounding down-conductors shall be permitted to be run either outside or inside the building or<br />
structure (NFPA 70-2005, Article 810.21(g)).<br />
68P81089E50-B 9/1/05 4-75
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