TM 5-811-3/AFM 889, Chap. 3
Economic and operational considerations will be
empted by the responsible using agency are not gov-
made in determining the need for lightning protec-
erned by the lightning protection code.
tion system, unless otherwise directed by the using
d. Effects of lightning discharges.
agency. Unless lightning frequency at the project
(1) General. When any building or structure is
site averages five or less thunderstorms per year, as
located within a radius of several hundred feet from
indicated in figure 2-1, lightning protection will be
the point where a lightning discharge will enter the
provided for buildings and structures as follows:
surface of the earth, the lightning discharge current
(1) Buildings of four floors having elevator or
becomes so high that any building or structure within
stairwell penthouses or other similar projections
this radius becomes vulnerable to immediate damage.
above roof.
(2) Nature of damage. Damage may range from
(2) Buildings of five floors or more with or with-
minor defacement to the building to serious founda-
out projections above roof.
tion upheaval, fire and personnel casualties. Damage
(3) Structures such as steel towers, aluminum
control can be effective dependent on extent of fire-
and reinforced concrete towers, and flagpoles with-
proofing and lightning protection incorporated into
out inherent grounding, and smoke-stacks and stee-
the project design. Although lightning strokes gener-
ples of 50-foot elevation or more above lowest point
ate static discharges in the form of radio noise, it is
of contact with finished grade.
generally accepted that these cause only an instant of
interference to manmade electronic systems. In-
b. Other applications. Special consideration will be
creased heating effects are also a factor since a light-
given in determining need for lightning protection as
ning bolt increases the temperature of the lightning
follows:
channel to about 15,000 degrees C. This sudden in-
(1) Whether building is manned, and there is in-
crease in temperature and pressure causes such an
herent hazard to personnel.
abrupt expansion of air that any hazard type of at-
(2) Whether building contains explosive or haz-
mosphere which comes within the ionized air path of
ardous areas or rooms, weapons systems technical
the lightning bolt becomes explosive. The explosive
equipment, or security communication equipment.
nature of the air expansion of bolt channels can cause
(3) If an unprotected building is destroyed by
physical disruption of structures located near the
lightning, the length of outage which can be tolerated
lightning stroke. Lightning discharges below the
until replacement is made. This includes the restora-
earth surface sometimes fuse sand into fulgurates
tion of high priority facilities such as water supply,
which appear like glass tubes. Trees of 40 feet or
weapons systems, police and security intelligence
more in height are especially vulnerable targets for
communications, strategic communication system op-
attraction of lightning discharges, and are susceptible
erating components.
to being totally destroyed.
(4) Replacement of building contents and value
thereof.
e. Effective resistance to ground.
(1) The lightning protection system will be de-
23. Air terminals. The purpose of air terminals is
signed to provide an electrical path to ground from
to intercept lightning discharges above facilities. Air
any point in the system, and that point will be of con-
terminals will be in accordance with UL 96, and 96A,
siderably lower resistance than that otherwise avail-
NFPA No. 78 or MILHDB-419. Where building
able by use of the unprotected facility.
roof is not metal and building construction includes
(2) Low resistance to ground is desirable for any
steel framing, air terminal connection assemblies will
lightning protection system but not essential. This is
conform generally to figure 2-2.
in conformance with NFPA No. 78 and MI LHDBK
419. Where low resistance to ground is mandatory,
2-4. Grounding
grounding electrode patterns as described herein and
a. General. Grounding generally will conform to
MIL-HDBK-419 will furnish ample length of electri-
NFPA No. 78, except as required by this manual or
cal path in contact with earth to dissipate each light-
by the using agency. Guidance for grounding for pur-
ning discharge without damage to the protected
poses, such as electromagnetic pulse (EMP), electro-
facility.
magnetic interference shielding, NASA and HQDCA
22. Limitations in use of lightning protection
electronic facility grounding, are subjects of other en-
gineering manuals which govern grounding require-
a. General. Lightning protection will be installed
ments. Those grounding systems will also serve as
as part of the initial construction project, particularly
grounding of the lightning protection system. Where
in view of long replacement time and high cost of
separate systems are installed such systems will be
structures. Installation cost of lightning protection
bonded below grade to any other independently in-
systems during project construction is small when
stalled exterior grounding system such as for electro-
compared to the cost of the installation as a whole.
2-2
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