exceeds 30 m 100 feet, cross-connect down conductors midway between the top
and bottom. Where buried metal pipes enter tank or tower, connect one down
conductor to pipes, approximately 305 mm 1 foot below grade. Ground metal
guy wires or cables set in concrete or attached to buildings or
nonconducting supports to a ground rod driven full length into the ground.
3.2.5.2
Metal or Reinforced-Concrete Tanks and Towers
Make metal or reinforcing steel electrically continuous. Electrical
continuity may be provided by bolting, riveting, or welding metal and tying
or clipping reinforcing bars, unless a specific method is noted on the
drawings. Air terminals and down conductors are required except on bolted,
riveted, or welded 4.75 mm 3/16 inch minimum steel plate tanks. Ground
connections and grounding electrodes are not required on metal tanks that
are electrically continuous with a metallic underground pipe system. On
other structures, provide two ground connections approximately 3.14 rad 180
degrees apart at the base of the structure. Connect each buried metal pipe
entering the tank or tower to one ground connection approximately 305 mm
one foot below finished grade. Ground metal guy wires on tanks and towers.
Metal guy wires or cables attached to steel anchor rods set in earth will
be considered as grounded. Ground metal guy wires or cables set in
concrete or attached to buildings or nonconducting supports to a ground rod
driven full length into the ground.
3.2.6
Stacks
Ground metal guy wires for stacks. Metal guy wires or cables attached to
steel anchor rods set in earth will be considered as sufficiently well
grounded. However, ground metal guy wires or cables attached to anchor
rods set in concrete or attached to buildings or nonconducting supports to
a ground rod driven full length into the ground.
3.2.6.1
Metal Stacks
Make metal smokestacks electrically continuous and to ground. Heavy-duty
metal stacks having a metal thickness of 4.75 mm 3/16 inches or greater do
not require air terminals or down conductors. Otherwise, provide two
[ground rods][grounding plates] driven full length into the earth. Locate
[ground rods][grounds plates] approximately 3.14 rad 180 degrees apart and
set [ground rods] [grounding plates] not less than 915 mm 3 feet nor more
than 2440 mm 8 feet from the nearest point of the stack foundation.
3.2.6.2
Nonmetallic Stacks
On nonmetallic smokestacks constructed of brick, hollow tile, or concrete,
make the air terminals solid copper, copper alloy, stainless steel or Monel
metal. Distribute uniformly about the rim of the stack at intervals not
exceeding 2440 mm 8 feet and extending at least 765 mm 30 inches above the
rim of stack. Electrically connect air terminal together by means of a
metal band or ring to form a closed loop about 2 feet 610 mm below the top
of the stack. Where the stack has a metal crown, connect air terminals to
the metal crown. Where stacks have metal lining extending part way up,
connect lining to air terminal at its upper end and ground at the bottom.
Provide at least two down conductors on opposite sides of the stack leading
from the ring or crown at the top to the ground. When the stack is an
adjunct of building near or touching the building perimeter, extend one of
the conductors directly to a ground connection while the other may be
connected to lightning protection system on the building. On stacks
exceeding 48 m 160 feet in height, cross-connect down conductors
SECTION 13100N
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