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 MIL-HDBK-1013/1A
steep angle so that the length of the opening will be large enough to
forestall its use as a convenient entry for a saber saw or other cutting
device. Holes should be angled upward and, to the extent practical, contain
sharp turns to prevent the easy introduction of hooks, cables, or explosive
devices. They should by kept to the minimum possible dimensions.
2) Air-Conditioning, Heating, and Ventilation systems.
These
systems include ducts, gravity vents, and exhaust vents.
a) Ducts. Ducts are sheet metal or fiberglass conduits,
round or square, which may vary from 3 inches (75 mm) on a side, or in
diameter up to any required size [e.g., 6 or 8 feet (1,800 or 2,400 mm) on a
side]. Ducts constructed of sheet metal, usually 28 gauge (0.4 mm) through
14 gauge (1.9 mm), can readily be cut with hand tools and light power tools.
These ducts do not present a significant barrier to penetration. Penetration
resistance is, however, sometimes incidentally enhanced by the use of ducts
of less than man-sized cross section and the inclusion of required
appurtenances, turning vanes, dampers, pressure plates, or the final air
distribution fixture. The standard specification for steel air ventilating
grill units for
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