. American hydroelectric practice; a compilation of useful data and information on the design, construction and operation of hydroelectric systems, from the penstocks to distribution lines . ., 3^ in. of ice coating, and a wind of 70 miles per hour, astrain of only one-half the ultimate strength of the cables would be pro-duced. The lines are supported on double circuit, four-legged steel towersof an average weight of 4,700 lbs. The height from the earth to the lowestcross-arm was made sufficient to use these towers on a spacing of approxi- LOW, MEDIUM AND HIGH HEAD DEVELOPMENTS 137 mately 750
. American hydroelectric practice; a compilation of useful data and information on the design, construction and operation of hydroelectric systems, from the penstocks to distribution lines . ., 3^ in. of ice coating, and a wind of 70 miles per hour, astrain of only one-half the ultimate strength of the cables would be pro-duced. The lines are supported on double circuit, four-legged steel towersof an average weight of 4,700 lbs. The height from the earth to the lowestcross-arm was made sufficient to use these towers on a spacing of approxi- LOW, MEDIUM AND HIGH HEAD DEVELOPMENTS 137 mately 750 ft. and yet have a clearance above ground in the center of thespan of 25 ft. The vertical spacing of cross arms is 10 ft. with a horizontalspacing between circuits of 15 ft., the middle cross-arm being somewhatlonger than the other two to prevent short circuits due to whipping or iceloads. Six disc strings of the 10 in. corrugated and the 12 in. fiat insulatorswere originally installed on suspension and seven of these discs at the strainpoints. Later an extra disc per string was added to increase the factor ofsafety of these strings on account of the unevenness of matching the units. Fig. 79.—Layout of Generating and Substations and Transmission System of Alabama Power Company in the strings. The lightning storms of Alabama are particularly guard against interruptions from this source two steel wires groundeddirectly at each tower were used over the transmission conductors. Noplates or rods driven into the earth were needed to secure a good con-ducting medium to care for the lightning discharges as the large area ofsteel grillages in the tower footings in fairly moist earth proved of relativelylow resistance. All the 110,000 volt tower lines and 22,000 volt distribu-tion lines are provided with electrolytic lightning arresters in addition tothe ground wires along the lines. 138 HYDROELECTRIC PRACTICE XXXVII. Development of the Tallassee Power Company on theYadkin R
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