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) • Grounding down-conductors shall not be required to be insulated (NFPA 70-2005, Article 810.21(b)). Insulated conductors are recommended when the grounding conductor may come into incidental contact with other metallic objects. Incidental contact with other metallic object may be a point for RF interference. See Chapter 8, “Minimizing Site Interference.” • Grounding down-conductor shall be run in as straight a line as is practical (NFPA 70-2005, Article 810.21(e)). See “Bending And Routing Grounding (Earthing) Conductors” on page 4-29. • Grounding down-conductors shall have a minimum bend radius of 203 mm (8 in.), and the included angle shall not exceed 90 degrees as shown in Figure 4-22 on page 4-29 (ANSI T1.313- 2003, section 11.3; MIL-STD-188-124B; and NFPA 780-2004, section 4.9.5). See “Bending And Routing Grounding (Earthing) Conductors” on page 4-29. • Grounding down-conductors shall be securely fastened at intervals not exceeding 914 mm (3 ft.). (See NFPA 70-2005, Article 250.64(b), 810.21(c) and NFPA 780-2004, section 4.10 for additional information.) See “Protecting and Securing Grounding (Earthing) Conductors” on page 4-30. • Grounding down-conductors run outside from the roof top to ground shall be protected for a minimum distance of 1.8 m (6 ft.) above grade level when located in areas susceptible to damage; such areas may include, but are not limited to, runways, driveways, school playgrounds, cattle yards, public walks (NFPA 780-2004, section 4.9.11 and section 4.9.11.2). See “Protecting and Securing Grounding (Earthing) Conductors” on page 4-30. If the building grounding electrode system resistance cannot be verified or cannot provide a lowresistance to earth (see “Grounding (Earthing) Electrode System Testing/Verification” on page D-1), a supplemental grounding electrode system should be installed to ensure the resistance requirement of the site is met. The supplemental grounding electrode system shall be bonded to the existing grounding electrode system (NFPA 70-2005). See “Common Grounding (Earthing)” on page 4-5. See Figure 4-54 for an example of a supplemental grounding electrode system. NOTE: Consult the building engineer or manager to determine information about any existing building grounding electrode systems. The building engineer should also be informed before attempting to weld or drill on the building rooftop. NOTE: Depending on the available locations for grounding down-conductors and the entry point of the RF transmission lines into the building, it may be necessary to install a supplemental grounding electrode system. 4-76 68P81089E50-B 9/1/05
STANDARDS AND GUIDELINES FOR COMMUNICATION SITES GROUNDING (EARTHING) ROOF-MOUNTED ANTENNA MASTS AND METAL SUPPORT STRUCTURES TRANSMISSION LINE GROUND KIT 35 mm 2 csa (#2 AWG) CONDUCTORS ANTENNA GROUND RODS ANTENNA SUPPORT STRUCTURE FIGURE 4-54 ROOF MOUNTED ANTENNA MAST GROUNDING WITH SUPPLEMENTAL GROUNDING ELECTRODE SYSTEM IMPORTANT: When installed, supplemental grounding electrode systems shall be effectively bonded to the building grounding electrode system. See “Common Grounding (Earthing)” on page 4-5. 4.8.1 SIDE MOUNTED ANTENNA GROUNDING (EARTHING) ELECTRICAL SERVICE Typically, a side mounted antenna may be grounded (earthed) with a single grounding conductor. The use of two grounding down-conductors may not always be practical. However, two down-conductors should be used whenever practical; this is especially important at buildings that are normally occupied, such as dispatch centers. When two grounding down-conductors are not installed, it is recommended to either use a copper strap or a larger sized grounding down-conductor. 68P81089E50-B 9/1/05 4-77
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GROUNDING (EARTHING) ROOF-MOUNTED ANTENNA MASTS AND METAL SUPPORT STRUCTURES CHAPTER 4: EXTERNAL GROUNDING (EARTHING)<br />
• Grounding down-conductors shall not be required to be insulated (NFPA 70-2005, Article<br />
810.21(b)). Insulated conductors are recommended when the grounding conductor may come into<br />
incidental contact with other metallic objects. Incidental contact with other metallic object may be a<br />
point <strong>for</strong> RF interference. See Chapter 8, “Minimizing Site Interference.”<br />
• Grounding down-conductor shall be run in as straight a line as is practical (NFPA 70-2005, Article<br />
810.21(e)). See “Bending <strong>And</strong> Routing Grounding (Earthing) Conductors” on page 4-29.<br />
• Grounding down-conductors shall have a minimum bend radius of 203 mm (8 in.), <strong>and</strong> the<br />
included angle shall not exceed 90 degrees as shown in Figure 4-22 on page 4-29 (ANSI T1.313-<br />
2003, section 11.3; MIL-STD-188-124B; <strong>and</strong> NFPA 780-2004, section 4.9.5). See “Bending <strong>And</strong><br />
Routing Grounding (Earthing) Conductors” on page 4-29.<br />
• Grounding down-conductors shall be securely fastened at intervals not exceeding 914 mm (3 ft.).<br />
(See NFPA 70-2005, Article 250.64(b), 810.21(c) <strong>and</strong> NFPA 780-2004, section 4.10 <strong>for</strong> additional<br />
in<strong>for</strong>mation.) See “Protecting <strong>and</strong> Securing Grounding (Earthing) Conductors” on page 4-30.<br />
• Grounding down-conductors run outside from the roof top to ground shall be protected <strong>for</strong> a<br />
minimum distance of 1.8 m (6 ft.) above grade level when located in areas susceptible to damage;<br />
such areas may include, but are not limited to, runways, driveways, school playgrounds, cattle<br />
yards, public walks (NFPA 780-2004, section 4.9.11 <strong>and</strong> section 4.9.11.2). See “Protecting <strong>and</strong><br />
Securing Grounding (Earthing) Conductors” on page 4-30.<br />
If the building grounding electrode system resistance cannot be verified or cannot provide a lowresistance<br />
to earth (see “Grounding (Earthing) Electrode System Testing/Verification” on page D-1), a<br />
supplemental grounding electrode system should be installed to ensure the resistance requirement of the<br />
site is met. The supplemental grounding electrode system shall be bonded to the existing grounding<br />
electrode system (NFPA 70-2005). See “Common Grounding (Earthing)” on page 4-5. See Figure 4-54<br />
<strong>for</strong> an example of a supplemental grounding electrode system.<br />
NOTE: Consult the building engineer or manager to determine in<strong>for</strong>mation about any existing building<br />
grounding electrode systems. The building engineer should also be in<strong>for</strong>med be<strong>for</strong>e attempting to weld<br />
or drill on the building rooftop.<br />
NOTE: Depending on the available locations <strong>for</strong> grounding down-conductors <strong>and</strong> the entry point of the RF<br />
transmission lines into the building, it may be necessary to install a supplemental grounding electrode<br />
system.<br />
4-76 68P81089E50-B 9/1/05
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