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 MIL-HDBK-419A
actually the nearest objects to point P, use P as a center and swing an arc of radius X through the tips of the
terminals. Let the value of this radius X be 100 feet, since 100 feet represents the shortest length usually
associated with a stepped leader (see Volume I, Section 3.2). Because of the large differences between the
height of typical terminals and the striking distance X, graphical determination of the protected zone will
usually be awkward. For greater accuracy, calculate the critical distances through the use of the following
equation:
(1-2)
which is valid for S < 2X. In this equation, G is the minimum height between the terminals that is completely
protected; H is the height of the terminals, S is the spacing between terminals, and X is the radius of the arc.
Sample calculation. To illustrate the application of this method, suppose it is necessary to determine the
minimum spacing between 3-foot air terminals that will guarantee that all parts of a flat roof remain in the
protected zone. In other. words, what value of S corresponds to G = 0 in Equation l-2? To perform the
calculation, first set G = 0:
Rearranging to be
and squaring both sides produces
Eliminating X2 and changing signs on both sides of the equation yields
or
Substituting H = 3 feet and X = 100 feet in this last equation shows that S must equal 48.6 feet or less to
guarantee that all parts of the roof remain within the protected zone.
1-28
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