Electric Transmission of Water Power

by Alton D. Adams

Fig. 90.—Glass Insulator and Sleeve on 50,000-volt Line
Between Cañon Ferry and Butte, Mont.

When any suitable insulator is dry and clean it offers all necessary resistance to the leakage of current over its surface, and any resistance in the pin that carries the insulator is of small importance. If the resistance of an insulator needs to be reinforced by that of its pin in any case, it is when the surface of the insulator is wet or dirty. Unfortunately, however, the same weather conditions that deposit dirt or moisture on an insulator make similar deposits on its pin, and the resistance of the pin is lowered much more than that of the insulator by such deposits. The increase of current leakage over the surface of an insulator during rains and fogs usually does no damage to the insulator itself, but such leakage over the wet pin soon develops a surface layer of carbon that continues to act as a good conductor after the moisture that temporarily lowered the resistance has gone. Reasons like these have led some engineers to prefer iron pins with insulators that offer all of the resistance necessary for the voltage employed on the line.

It may be suggested that the use of iron pins will transfer the charring and burning to the wooden cross-arms, but this does not seem to be a necessary result. The comparative freedom of cross-arms from charring and burning where wooden pins are used seems to be due to the larger surface and lower resistance of the cross-arms. With iron pins having a shank of small diameter, so that the area of contact surface between the pin and the wooden cross-arm is relatively small, there may be some charring of the wood at this contact surface. Should it be thought desirable to guard against any trouble of this sort, the surface of the iron pin in contact with the cross-arm may be made ample by the use of large washers, or by giving each pin a greater diameter at the shank than elsewhere.

It may be noted that the pins with a central iron bolt only half an inch in diameter, that were used on the 33,000-volt Santa Ana line, were said to have caused no burning of their cross-arms in those cases in which the wooden threads about the top of the central bolt were burned through.

Another possible trouble with iron pins is that they, by their greater rate of expansion than glass or porcelain, will break their insulators. Such results can readily be avoided by cementing each iron pin into its insulator, instead of screwing the insulator onto the pin. Iron pins will, no doubt, cost somewhat more than those of wood, but this cost will in any event be only a small percentage of the total investment in a transmission line. Considering the cost of the renewals of wooden pins, there seems little doubt that on a line where the voltage and other conditions are such as to result in frequent burning, iron pins would be cheaper in the end.

Iron pins have already been adopted on a number of high-voltage lines. Not only iron pins, but even iron cross-arms and iron poles are in use on a number of transmission lines. On a long line now under construction in Mexico, iron towers, placed as much as 400 feet apart, are used instead of wooden poles, and both the pins and cross-arms are also of iron. The 75-mile line from Niagara Falls to Toronto is carried entirely on steel towers.

The Vancouver Power Company, Vancouver, British Columbia, use a pin that consists of a steel bolt about 12 inches long fitted with a sleeve of cast iron 4¹⁄₂ inches long to enter the cross-arm, and a lead thread to screw into the insulator. On the 111-mile line of the Washington Power Company, of Spokane, which was designed to operate at 60,000 volts and runs to the Standard and Hecla mines, a pin consisting of a steel bar 1¹⁄₈ inches in diameter, with a cast-iron shank 2¹⁄₁₆ inches in diameter to enter the cross-arm, and with the lead threads for the insulator, is used.

Fig. 92.—Iron Pins on Spier Falls Line.