TM 5-811-3/AFM 88-9, Chap. 3
CHAPTER 2
LIGHTNING PROTECTION
2-1. Discussion
ward earth in a succession of steps, pulsing forward
with an additional step every 50 microseconds
a. Lightning phenomena. The planet earth is simi-
creating a faintly luminous trail called the initial or
lar to a huge battery continuously losing electrons to
stepped leader. As the leader nears the ground, its
the atmosphere. These electrons could be lost in less
effects create an ionized streamer which rises to
than an hour unless the supply is continually replen-
meet the advancing leader. When the two join, the
ished. It is widely agreed among physicists and scien-
i o n i z e d air path between cloud and earth is
tists that thunderstorms occurring thousands of
completed, and the leader blazes a faint trail to
times daily around the earth return electrons to
earth. Immediately a deluge of electrons pour from
earth to maintain normal magnitude of electrons at or
this lightning discharge channel creating the brilliant
near the surface of- the earth. The rate of electron
main or return stroke that produces most of the light
loss from earth, called the "air-earth ionic current",
we see, The motions of the leader and the main or re-
has been calculated to be 9 microampere for every
turn stroke appear to move in opposite directions,
square mile of earth's surface. Thunderstorms supply
but lightning is not an alternating current, since the
electrons back to earth by an opposite electron poten-
transferred electrical recharge current moves back to
tial gradient of perhaps 10 kilovolts per meter within
earth.
a thundercloud. This feedback forms a potential dif-
b. Nonconventional systems. Nonconventional and
ference of from 10 to 100 megavolts in a single dis-
unacceptable systems include the so-called dissipa-
charge between the center of a cloud and earth.
tion array, and those using radioactive lightning
These lightning discharges carry currents varying
rods, Radioactive lightning rods have been proven
from 10 to 345 kiloamperes to earth at an average
less effective than passive air terminals in storm situ-
rate of 100 times per second with duration of less
ations. These systems have not been recognized by
than second per flash. Each flash consists of up to
NFPA or UL. Use of these systems will not be
40 separate strokes, Each stroke of lightning lasting
permitted unless specifically approved by the appro-
for this brief instant releases about 250 kilowatt-
priate using agency. Dissipation arrays consist of two
hours of energy-enough to operate a 100-watt light
types:
bulb continuously for more than three months at the
(1) A high tower with top-mounted dissipation
rated voltage of the lamp. Lightning discharges do
suppressor, and radial guy wire array. This type is
not always bring electrons to earth, because so-called
used on isolated high towers, antenna structures and
positive ground-to-cloud strokes consist of low power
offshore facilities.
energy transmissions from earth to small negative
(2) A series of high towers located beyond a
charge pockets in a thunder cloud. However, magni-
given area to be protected and supported by a num-
tudes of discharge voltages and currents are approxi-
ber of sharp pointed strands of barbed wire for the
mately the same from cloud to earth, and all occur
protection array.
within the same discharge timeframes. Just before
the lightning flash, the ground within a radius of sev-
c. Code applicability. NFPA No. 78 is intended to
eral miles below the cloud becomes deficient in elec-
apply to the protection of ordinary buildings, special
trons. Repelled by the army of electrons in the cloud
occupancies, stacks, and facilities housing flammable
base, many of the free electrons on the ground are
liquids and gases. The lightning protection code will
pushed away. The result is that the ground beneath
be utilized where lightning damage to buildings and
the cloud base becomes more positively charged. As
structures would cause large economic loss or would
the cloud moves, the positive charge region below
prevent activities essential to the Department of De-
moves like its shadow. As the cloud charge balloons,
fense. NFPA No, 78 does not relate to the protection
the pressure becomes so great that a chain reaction
of explosives manufacturing or storage facilities. Pro-
of ionized air occurs. Ionization is the process of
tection for these facilities will be in accordance with
separating air molecules into positive ions and nega-
paragraph 29. Since NFPA No. 78 does not pre-
tive electrons. This air which is normally a good elec-
scribe a comprehensive coverage pattern for each
trical insulator becomes a good conductor and allows
type of facility required by the military departments
the cloud electrons to pierce the faulted insulation
of the government, additional guidance is given in
and descend this newly created ionized air path be-
this chapter. Temporary DOD storage facilities and
tween cloud and earth. The lightning flash starts
structure housing operations not regularly conducted
when a quantity of electrons from the cloud heads to-
at a fixed location and other facilities specifically ex-
2-1
Previous Page
