3.1.1
Air Terminals
Air terminal design and support conforming to NFPA 780. Rigidly connect
terminals to, and make electrically continuous with, roof conductors by
means of pressure connectors or crimped joints of T-shaped malleable metal.
Provide pressure connector or crimped joint with a dowel or threaded
fitting to connect ground rod conductor with air terminal. Set air
terminals at ends of structures not more than 610 mm 2 feetfrom ends of
ridges and corners of roofs. Do not exceed 7620 mm 25 feet in spacing of
610 mm 2 foot high air terminals on ridges, parapets, and around perimeter
of building with flat roofs. When necessary to exceed this spacing,
increase specified height of air terminals not less than 50 mm 2 inches for
each 305 mm one foot of increase over 7620 mm 25 feet. On large flat, or
gently sloping roofs, as defined in NFPA 780, place air terminals at points
of the intersection of imaginary lines dividing the surface into rectangles
having sides not exceeding 15 m 50 feet in length. Secure air terminals
against overturning either by attachment to the object to be protected or
by means of a substantial tripod or other braces which are permanently and
rigidly attached to the building or structure. Metal projections and metal
parts of buildings such as smokestacks and other metal objects that are at
least 3/16 inch 4.763 mm thick and that do not contain hazardous materials,
need not be provided with air terminals. However, bond these metal objects
to a lightning conductor through a metal conductor of the same unit weight
per length as the main conductor. [Where metal ventilators are installed,
mount air terminals thereon, where practical. Bond air terminals, erected
by necessity adjacent to a metal ventilator, to the ventilator near the top
and bottom.] Where nonmetallic spires, steeples, or ventilators are
present, mount air terminals to the side. In addition, where spires or
steeples project more than 3050 mm 10 feet above the building, continue
conductor from air terminal to nearest down conductor securely connect
thereto.
3.1.2
Roof Conductors
Connect roof conductors directly to the roof or ridge roll. Avoid sharp
bends or turns in conductors. Do not make turns of less than 205 mm 8
inches. Preserve horizontal or downward course on conductors. Rigidly
fasten conductors every 915 mm 3 feet along the roof and down the building
to the ground. Rigidly connect metal ventilators to the roof conductor at
three places. Make connections electrically continuous. Course roof
conductors along contours of flat roofs, ridges, parapets, and edges; and
where necessary, over flat surfaces, in such a way as to join each air
terminal to all the rest. Connect roof conductors surrounding tank tops,
decks, flat surfaces, and flat roofs to form a closed loop.
3.1.3
Down Conductors
Make down conductors electrically continuous from air terminals and roof
conductors to grounding electrodes. Course down conductors over outer
extreme portions of the building, such as corners, with consideration given
to location of ground connections and air terminals. Provide each building
or structure not less than two down conductors located as widely separated
as practicable, such as at diagonally opposite corners. [Rectangular
structures having gable, hip, or gambrel roofs more than 33 m 110 feet
long, provide at least one additional down conductor for each additional 15
m 50 feet of length or fraction thereof.] [Rectangular structures having
French, flat, or sawtooth roofs exceeding 76 m 250 feet in perimeter,
provide at least one additional down conductor for each 30 m 100 feet of
perimeter or fraction thereof.] [L- or T-shaped structure, provide at
SECTION 13100N
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