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.

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