Boxoffice (Oct-Dec 1963)

LIGHTNING PROTECTION NEED GROWING By MARVIN M. FRYDENLUND Three modern-day factors are increasing America’s property toll to lightning at a rate of nearly 10 per cent per year. They are (1) greater vulnerability, ^2) greater exposure, and (3) increased valuation. Outdoor theatres, new indoor theatres and new homes all are in the thick of these factors, making knowledge about proper lightning protection a more vital need. Lightning starts more than 30,000 fires annually in the U.S., is blamed for 12 per cent or $150,000,000 worth of the nation’s total fire loss, and “cold bolts” add millions of dollars to the over-all toll. The three factors responsible for the rise in losses are these: 1. Increased exposure — As homes, commercial structures and other buildings take the place of barns and trees in suburbia and the country, they become more exposed to lightning. In large cities, skyscrapers, towering chimneys, and other tall structures offer a “cone of protection.” In outlying areas, “hot-bolt” lightning causes 37 per cent of all destruc tive fires, and the twice as frequent “cold bolts” also take a correspondingly higher toll. 2. Greater vulnerability — Covering more area, housing much more electrical equipment and other metallic bodies of conductance, topped by radio and TV antennas, modern buildings present more opportunity for damage by lightning and, essentially, make more likely targets. 3. Each lightning strike on a building today finds a target that’s more expensive to repair or replace. “Cold bolts,” which do not cause fire but do cause structural or equipment damage, are a greater factor for this reason. CAUSE AND EFFECT Lightning is a gigantic electric spark containing tens to hundreds of millions of volts and sometimes terrific amperage — from 1,000 to 340,000 amperes. “Cold” lightning is high voltage with low amperage and strikes and dissipates at a ‘Executive secretary of the Lightning Protection Institute. speed of about 1/10,000 of a second, seldom causing fire, but the tons of pressure of its expanding gases will explode parts of the building or other target. “Hot” lightning — low voltage with high amperage — is the fire-setting variety. Its high-current, long-duration core path reaches a temperature up to several thousand degrees. In either case, lightning is the result of an overpowering attraction between opposite electrical charges in the ground and in storm clouds. Here’s what happens: When storm clouds gather, humid air condenses to raindrops, water turns to ice crystals inside violent air currents, and there is a resulting separation of electrical charges. Usually, negative charges accumulate in the lower part of the cloud, while positive charges build up in the earth and in the upper part of the cloud. When the attraction between these opposite charges becomes strong enough, they leap across the gap of non-conducting air between, and there you have lightning! The lightning rod, invented by Benjamin Franklin after the famous kite and key test, has been described as the world’s most perfect invention. Correctly installed and connected to properly designed roof and Basic Lightning Protection System (or Homes Also Applies to Theatre Buildings Modern lightning protection system includes web of conducting cable, conveying any bolt harmlessly from air terminal to groupd rod. Here are details of concealed installation: 0-Ai r terminal is pushed up through hole cut in ridge at each location. Rod is secured to ridge board while cable is attached to terminal base. ©-The TV aerial is bonced to the main conductor cable of protection system. Lightning arrester is fastened on the lead-in wire, tape or cable; grounded midway on the mast. Q-Chimney conductor may be coursed down between masonry and flue, or if this is not possible, cable may extend down the outside to the roof and then be brought inside for attachment to main cable. JTo ground system, the conductor might be (1) carried down along wall to a rod beneath floor slab, (2) brought on outside to grade-level trench and then out 2-ft. to ground rod or (3) coursed down inside the wall. In a semi-concealed system, slender and neutral-colored air terminals are fixed to conductors which are inconspicuously attached to outside of the house. If it is possible, cables are concealed along downspouts or other objects on exterior. 54 The MODERN THEATRE SECTION