On the network of transmission lines between Spier Falls, Schenectady, Albany, and Troy, in the State of New York, the insulators are supported on iron pins of two types. One of these pins, used at corners and where the strain on the wire line is exceptionally heavy, is made up of a wrought-iron bolt 3⁄4-inch in diameter and 161⁄2 inches long over the head, and of a malleable iron casting 83⁄4 inches long. This casting has a flange of 5 by 33⁄4 inches at its lower end that rests on the top of the cross-arm, and the bolt passes from the top of the casting down through it and the cross-arm. Threads are cut on the lower end of the bolt, and a nut and washer secure it in the cross-arm. The total height of this pin above the cross-arm is 91⁄4 inches.
For straight work on this line a pin with stem entirely of malleable iron, and a bolt that comes up through the cross-arm and enters the base of the casting, is used. The cast top of this pin has four vertical webs, and its rectangular base, which rests on the top of the cross-arm, is 31⁄2 by 4 inches. The bolt that comes up through the cross-arm and taps into the base of the casting is 3⁄4-inch in diameter. The cast part of this pin has such a length that the top of its insulator is carried 103⁄4 inches above the cross-arm. For the casting the length is 91⁄4 inches.
Both of the types of iron pins in use on the Spier Falls lines are secured to their insulators with Portland cement poured into the pin hole while liquid when the insulator is upside down and the pin is held centrally in its hole. The top of each casting is smaller in diameter than the hole in the insulator, and is grooved so as to hold the cement.
Fig. 93.—Standard Pin, Toronto and Niagara Line.
On a long line designed for 60,000 volts, and recently completed in California, wooden pins are used with porcelain insulators, each 14 inches in diameter and 121⁄2 inches high. Each of these pins is entirely covered with sheet zinc from the cross-arm to the threaded end, and it is expected that this metal covering will protect the wood of the pin from injury by the leakage current.
CHAPTER XXI.
INSULATORS FOR TRANSMISSION LINES.
Line insulators, pins, and cross-arms all go to make up paths of more or less conductivity between the wires of a transmission circuit. The amount of current flowing along these paths from one conductor to another in any case will depend on the combined resistance of the insulators, pins, and cross-arm at each pole.