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 MIL-HDBK-419A
The zone of protection provided by a grounded vertical rod or mast is conventionally defined as the space
enclosed by a right circular cone with its axis coincident with the mast and its apex at the top of the mast as
illustrated by Figure 3-6(a). Similarly, the zone protected by a grounded horizontal overhead wire is defined as
a triangular prism with its upper edge along the wire as illustrated in Figure 3-6(b). In either case, the zone (or
cone) of protection is expressed as the ratio of the horizontal protected distance, D, to height, H, of the mast
or wire. This ratio is the tangent of the shielding angle,
Some commonly recommended zones of protection
and the associated shielding angles are illustrated in Figure 3-7.
The NFPA Lightning Protection Code (3-9) recommends that a 1:1 zone of protection
be provided in
important areas while a 2:1 zone
is acceptable for less important areas. The British Standard Code of
Practice (3-10) states that a shielding angle of 45 degrees provides an acceptable degree of protection for
ordinary structures, but that for structures with explosive or high flammable contents the shielding angle should
not exceed 30 degrees.
Although the existence of a 1:1 zone of protection does not absolutely guarantee immunity to lightning,
documented cases of the 1:1 zone being violated are very few. Thus for all facilities except those associated
with the storage of explosives or fuels, a 1:1 zone of protection can safely be used as a basis of design of
lightning protection systems. As such, C-E facilities or equipments (antennas, etc.) located entirely within the
1:1 zone of protection generally are not required to have separate air terminals. This does not eliminate the
need to ground metal shelters or to meet the grounding requirements of the subsystems which comprise the
facility ground system. If more than one rod or wire is used, the protected zone is somewhat greater than the
total of all of the 1:1 zones of the rods or wires considered individually. For adjacent structures, the Codes
specify that a 2:1 zone of protection may be assumed for the region between the structures.
Large structures with flat or gently sloping roofs do not lend themselves to the straightforward application of
the 1:1 or 2:1 zone of protection principles. To establish even 2:1 type coverage on large buildings,
exceptionally tall air terminals would be required. Experience, however, shows that extremely tall terminals
are not needed for effective protection. Both the NFPA Lightning Protection Code and UL Master Labeled
Protection System (3-11) specify air terminals that extend from 10 to 36 inches above the object to be
protected. (The British Standard Code of Practice does not require the use of air terminals at all.)
3.6 LIGHTNING EFFECTS.
3.6.1 Flash Parameters.
During the short interval of a lightning flash, several discharges occur. The sequence of events in a multiple-
stroke flash is illustrated in Figure 3-8. The initial path for the discharge is established in 50 microseconds.
Intermediate return stroke currents of about 1 kA follow the initial return stroke and last for a few
milliseconds. Subsequent strokes occur at intervals of 50 to 60 milliseconds. The return stroke interval may
include a continuing current of 100 A or so which flows for several milliseconds or until the start of the next
return stroke.
*The shielding angle is defined as the angle between the surface of the cone and a vertical line through the
apex of the cone, or between the side of the prism and the vertical plane containing the horizontal wire.
3-13
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