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STANDARDS AND GUIDELINES FOR COMMUNICATION SITES GROUNDING (EARTHING) ROOFTOP MOUNTED TOWER STRUCTURES
4.9 GROUNDING (EARTHING) ROOFTOP MOUNTED TOWER
STRUCTURES
Rooftop mounted towers may increase the lightning risk index for the buildings they are installed upon.
Due to their increased height and lightning risk probability, all exposed buildings with rooftop towers
shall be equipped with a lightning protection system, as outlined in NFPA 780-2004 (ANSI T1.313-
2003, section 10.3.3). See “Lightning Activity and Exposure” on page 4-3 for information regarding
lightning exposure to buildings and towers.
NOTE: Consult the building engineer or manager to determine information about any existing building
grounding (earthing) electrode systems. The building engineer should also be informed before
attempting to weld or drill on the building rooftop.
An engineering firm specializing in the design and installation of lightning protection systems should be
consulted for proper design and installation of the building lightning protection system. A licensed
contractor specializing in the installation of lightning protection systems should be used. The lightning
protection system shall be constructed of only listed components. The lightning protection system shall
meet the requirements of BS 6651:1999, IEC 61024-1-2, NFPA 780-2004, or other standard in effect
and recognized by the local authority having jurisdiction. As a minimum the lightning protection system
shall contain the following:
• The lightning protection system shall contain a grounding conductor installed around the roof
perimeter to form the main roof perimeter lightning protection ring. The main roof perimeter
lightning protection ring 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)).
• Strike termination devices, also known as air terminals, are typically installed along the length of
the main roof perimeter lightning protection ring, typically every 6.1 m (20 ft.) or as otherwise
required by the standard in effect and recognized by the local authority having jurisdiction.
• The main roof perimeter lightning protection ring shall contain at least two down conductors
connected to the grounding electrode system. The down conductors shall be physically separated
from one another as much as practical. The down conductors shall be sized as follows:
• The conductor shall be a 35 mm2 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 mm2 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)).
NOTE: Available effectively grounded structural building steel can typically be used as a grounding downconductor.
The conductor used to bond the main roof perimeter lightning protection ring to the
structural building steel shall be sized as required in this section for down-conductors. See Figure 4-53
on page 4-75 for an example of acceptable structural building steel bonding connections.
• Structures exceeding 76 m (250 ft.) in perimeter shall have a down conductor for every 30.5 m
(100 ft) of perimeter or fraction thereof (NFPA 780-2004, section 4.9.10.1).
68P81089E50-B 9/1/05 4-79