Masts. Lightning masts shall be tapered metal, self-supporting
type, (single section design or multisection design) with slip joints, as
dictated by total height of mast. The cross section shall be circular or
polygonal, symmetrical about the longitudinal axis, and uniform in
configuration throughout the entire length. Wood masts may be used only when
heights and structural strength permit and shall be electrical pole line
embedded type, topped with a lightning rod or metal cap and with two bare
copper ground wires not less than No. 1/0 AWG (53.5 mm2), run down each side
of the pole to the ground system. The ground wires shall have a protective
molding extending from grade level to a point at least 10 ft (3.05 m) above
Material. Metal material shall be corrosion-resistant steel,
noncorrosion-resistant steel with hot-dipped galvanized finish, or aluminum
for single section or multiple section masts with anchor bolt mounting. No
combination of materials shall be used that form an electrolytic couple which,
in the presence of moisture, causes accelerated corrosion.
Top Point. Mast top shall be fitted with a copper or bronze air
terminal or metal cap to take the lightning stroke. The point shall be
included in the determined overall height of the mast.
Height and Location. Refer to Appendix A for determination of mast
height and for location as related to structure being protected. Masts of
heights up to 40 ft (12.2 m) shall be of single section design. Masts 40 to
70 ft (12.2 to 21.34 m) in height should be single-section design if delivery
to the site is practical. Design for masts in excess of 150 ft (45 m) receive
Zone of Protection. A lightning mast system establishes a
zone-of- protection. The-zone-of-protection is described by an arc having a
radius not greater than 100 ft (30.5 m). To prevent sideflashes, each mast
shall be separated from the structure by not less than one-half of the height
of the assumed salient plane, but never less than 6 ft (2 m). The maximum
distance from the structure shall be 25 ft (7.6 m).
Slip-joint design shall meet the following
Assure overall structural integrity of the mast.
b) Include field assembly requirement to assure a snug fit, so that
joints of the mast will not loosen when subjected to vibrational modes caused
by wind or other means after erection.
c) Be compatible with field erection requirements to assure ease of
installation at the site.
d) Have good metal-to-metal contact, so that electrical
conductivity will be equal to or better than the parent metal used.