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 In accordance with para. 3, Appendix A, determine actual D1 distance from the
center-line of the building or complex to row of masts on the diagonal axis.
With C = 25 ft, D is equal to 82 ft.
After the height and location of the masts have been determined,
proceed as follows:
a) Draw a plan of the building to scale. Locate the masts at
distances which are determined by the method shown in Appendix A, A-3,
Example 1, Figure 1.
b) Draw an elevation of the building to scale. Locate the masts at
distances which are determined by the method shown in Appendix A, A-3. Draw a
100 ft striking distance radius from the apex of the masts to the center-line
of the building or complex as shown. Draw two arcs (the normal axis and the
diagonal axis clearance arcs) as shown in Appendix A, A-4, from the point of
intersection with the center-line of the building. These arcs must clear the
salient plane.
c) Draw a lengthwise scale elevation of the building. Locate the
masts as shown in Appendix A, A-5 (refer to para. 4b, Appendix A). No part of
the protected building should lie outside the zone-of-protection. Appendix A,
A-6 through A-11 provide examples which illustrate application of this
principle.
5.
Facilities Other Than Structures.
The following criteria apply for
facilities other than structures.
a) This type of facility can be protected by using overhead ground
cable spanned between two metal towers or poles which are effectively grounded
and meet criteria in para. 3, Appendix A.
b) In this system, the overhead cable will intercept lightning
strikes and the resulted electrical current will be safely carried to ground
via the cable and supporting towers or poles.
c) The design method establishes the height of the horizontal
ground cable at mid-span to provide the required zone-of-protection for a
specific facility.
d) The mid-span sag of the overhead cable must be included when the
height of the supporting towers or poles is determined (see Appendix A, A-2).
e) Selection of cables is based on mechanical strength rather than
electrical considerations. The overhead ground cable should be stranded,
non-corrosive, copper coated steel wire. Minimum size shall be 3/8 in.
(9.5 mm) high strength (7 No. 8 strands).
f) The construction of the towers and their structural members or
the class of poles should be based on considerations of mechanical and wind
loading stresses.
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