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 ML-HDBK-419A
c.
Distribute ground connections symmetrically about the circumference of the structure rather than
grouping to one side.
Interconnect all metal objects close to the discharge path to prevent side flashes. (Representative
d.
interconnections are shown in Figure 1-15.)
Make certain that the mechanical construction of the air terminal system is strong and that the
e.
materials used offer high resistance to corrosion.
1.3.2.1 Air Terminals. Air terminals (lightning rods) must intercept, or divert to themselves, any lightning
stroke that might otherwise strike the building or structure being protected. Antennas and their associated
transmission lines/supporting structures shall be protected by air terminals meeting the requirements of
1.3.2.1.1.a rather than be dependent upon transient protection/suppression devices described in 1.3.3.5.22.
1.3.2.1.1 Size and Materials. To keep from exploding, igniting, or otherwise being destroyed, air terminals
should be made of copper, aluminum, brass, or bronze. The minimum sizes are 1.27 cm (1/2 inch) in diameter
for solid copper, brass, or bronze rods and 1.6 cm (5/8 inch) in diameter for solid aluminum rods.
Air terminals must extend at least 25.4 cm (10 inches) directly above the object being protected and
a.
be of sufficient height so as to provide a 1:1 zone of protection for adjacent objects (antennas and associated
support/control towers, etc). Rather than choosing the shortest terminal which will provide this minimum
height, all parts of the structure must be checked graphically or analytically in the manner described in the
next section to deter-mine if the zone of protection provided by the terminal is adequate. Where taller
terminals are required to provide complete protection, adequate support and bracing as specified by ANSI-C5.1
(2.1.15) must be provided.
Where air terminals are mounted on or very near (less than 1.5 meters (5 feet)) to vents or stacks
b.
which emit potentially explosive or ignitable dusts, vapors, or gases, provide additional clearance.
(1) Over hooded vents emitting explosive substances under natural draft, the air terminals should
extend at least 1.5 meters (5 feet) above the opening.
(2) Above open stacks emitting explosive substances under forced drafts, air terminals should
extend at least 4.5 meters (15 feet) above the opening.
1.3.2.1.2 Location.
Locate air terminals along the ridges of gable, gambrel, and hip roofs in the manner illustrated in
a.
Figure 1-16.
b.
Place them on the corners and along the edges of gently sloping roofs as shown in Figure 1-17.
Gently sloping roofs are defined as (1) having a span of 40 feet or less with a rise-to-run ratio, i.e., pitch, of
one-eighth or less or (2) having a span greater than 40 feet and a rise-to-run ratio of one-quarter or less.
1-24
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