PRIMARY LIGHTNING PROTECTION FOR
ORDNANCE HANDLING FACILITIES
Scope. The following design method will provide adequate primary
lightning protection for ordnance handling buildings with vertical masts, or
overhead aerial wiring, in the vicinity of the building to be protected. This
method can also be used to provide protection for a group of closely
associated structures or complexes.
General. Experiments have indicated that under certain assumed test
conditions, a vertical conductor will generally divert to itself direct
lightning strikes which might otherwise fall within a cone shape or wedge
shape space zone-of-protection space in which the apex is the top of the
vertical mast, or the overhead horizontal ground wiring of the wedge. In this
case, the base is approximately two times the height of the mast or the
overhead horizontal ground cable.
Lightning Protection System. The lightning protection system
employed herein is based on the zone-of-protection as determined by clearance
arcs and a 100 ft (30.5 m) lightning striking distance. All masts and
overhead ground wiring that is used for the protection of a structure must be
adequately grounded. If the structure being protected is of metal, grounding
must also be bonded to the structure. The ground resistance should not be
over 10 ohms. A sufficient number of masts or overhead ground wires must be
used so that the entire structure is covered by their zone-of-protection.
Application of Criteria.
The following criteria apply to primary
a) Basic requirements are an adequate design for lightning
protection and economical cost of the system provided.
b) All three dimensions, the length, the width, and the height, of
a structure to be protected by a primary lightning protection system are of
major importance in determining the height, number, and location of the masts
or the overhead aerial wiring which will be used to protect the structure.
c) The spacing of the masts along the length of the structure shall
be a minimum of 1-1/2 times the height of the masts and a maximum of 200 ft
d) The distance "C" of the masts from the structure shall be one-
half the height of the assumed salient plane (S/2) but never less than 6 ft
(1.8 m) or more than 25 ft (7.6 m). In cases where a minor readjustment may
be necessary to accommodate road clearance, an exception shall not grossly
exceed the limits.