Boxoffice (Oct-Dec 1963)

ground conductors, the lightning rod rarely, if ever, fails to safely convey a lightning stroke into the ground, or dissipate it harmlessly in the air. The average number of lightning strikes for any single area across the country is 40 to 60 per square mile per year, based on an average of 40 area thunderstorms annually. If you erect a building in the average location, you are putting up a potential target for about 40 electrical storms per year. Your building may never be struck, of course. But then again it may be hit two times within minutes. Lightning, contrary to an old saw, could — and has — struck twice. LIGHTNING PROTECTION FOR INDOOR-OUTDOOR THEATRES Lightning protection for outdoor theatres and indoor theatre buildings is based on similar principles, but effective installation calls for a specialized knowledge about structural differences, soil and other side conditions, and materials, design and dimensions of the building. Most indoor theatres, particularly the newer ones in suburbia, come under the general rules covering buildings less than 75 feet high. As shown in Fig. 1, a building such as a theatre has air terminals or rods at inter G FOR GROUNDS Fig. 1 vals not exceeding 20 feet around the perimeter. In addition, it has terminals at central points of the roof — placed not over 50 feet apart, all in right angle directions from each other. All roof projections are protected by tiein with the complete system. Chimneys shou'd be protected with air terminals anchored directly to the masonry, so that no outside corner of the chimney is more than two feet from a terminal. A leadcoated metal band may be used to anchor the rods if this is more practicable than direct fastening to masonry. The perimeter rods are placed within two feet of roof edges, and must not be more than the distance from ends of ridges, corners, or edges of dormer roofs. Terminals should project upward at least 10 inches, but not more than 35 inches. If solid copper, they are at least three-eighths inch in diameter, and if solid aluminum, the diameter is at least one-half inch. Tubular copper or aluminum rods must be at least five-eighths inch in diameter, and of 20 gauge thickness. Copper cable conductors in this installation must weigh at least 187.5 pounds per 1,000 feet of conductor, and individual wires of the twisted cable must be at least 17 gauge. If aluminum is used, weight must be 95 pounds for the same length, and individual wires not less than 14 gauge. Standard rules call for bends with angles not less than 90° and radius not less than eight inches; support of conductors at least every three feet when coursed through the air; with maintenance of a horizontal or downward course, free from pockets. Grounds are spaced within 100 feet of each other, and any prominent projection of the building has at least an additional down conductor. Irregular-shaped buildings may require more than the standard number of down conductors and grounds. In good soil, ground rods must be sunk at least ten feet deep. Rocky or sandy soil will call for special ground installations designed according to the amount of electrical resistance. Depth and condition of top soil, moisture content and other factors are taken into consideration and computed in a properly designed grounding. More than half of all American outdoor theatres are protected against lightning, according to the Lightning Protection Institute. Design of the system depends on the screen’s construction. Grounding recommendations are essentially the same as for other buildings. Older theatre screens with wood framing, or with a frame in which metal may not be continuous, are equipped with complete lightning protection systems such as those While "hot-bolt" lightning causes 37 per cent of all destructive fires, "cold bolts" occur twice as frequently and cause high structural or equipment damage, as shown in this picture. described. Newer screens which have metal frames in continuous connection, are fitted with rods or points, but the metal framing often takes the place of down-conducting Continued on following page world’s largest selling X in-a car heater . . . here’s why 7^ /ytozsest $ea(/ EPRAD GOLDEN HOTSHOT HEATE *4* • Building heaters for nearly a decade • Low priced — why pay more? E. M. 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