Publication: Scientific American
New York, NY, United States
INSULATOR FOR LINES CARRYING CURRENTS OF
BY W. R. GREENWOOD.
Tests made near Santa Monica, Cal., have demonstrated the utility of a device designed to maintain the insulation of long-distance high-voltage electric currents. The United Electric Gas and Power Company, from its central power house located at Santa Monica supplies current for lighting and power purposes to Long Beach, San Pedro, Terminal Island and Redondo. The total length of the circuit is forty-five miles. The current is generated by direct-connected units at a pressure of 2300 volts. By means of transformers this pressure is raised to 22,000 volts and transmitted over the circuit to the different towns, and by means of step-down transformers it is lowered again to 2300 volts.
Ever since the installation of this system eighteen months ago the company has experienced the greatest difficulty in keeping the current from "slopping over" and burning off the pins. That is on account of fogs along the coast. The insulators used are of the types known as "No. 1 Provo" and "Lock," [sic] "Locke," both of 60,000-volt glass. It was found that the leakage was not due to any fault of the insulation of the glass, but to the action of the fog. This was demonstrated by the fact that the line worked perfectly in wet weather. In dry weather dust would accumulate under the bell in time of fog the damp atmosphere as it moved past the insulator would deposit moisture with the dust and form a sort of paste, which appeared to establish a good conductor for the high-tension currents to flash across. Within a short time the pin would be burnt off. The wire, dropping on to the cross arm, would burn it off and, in almost all cases, would next swing in against the pole and burn it off.
After having tried almost every conceivable scheme to overcome this serious trouble the company hit upon the novel device for housing or fencing in the pin and glass. The new arrangement, which has been shown by tests to have completely overcome the leakage and to have thereby prevented the burning off of the pins, is a box made of 1-inch wood 12 inches square and 4 inches thick. The box has on its upper side a round hole 10 inches in diameter. The device is-placed so that the 7-inch bell of the insulator projects down a little into the hole. The box is previously treated with gas tar and has holes in its bottom sufficient for drainage. By preserving the static condition of the air under the bell of the insulator the deposit of moisture there is prevented.
Incidentally the box protects the insulator from damage by shot or other missiles.
The model of the device was perfected by Superintendent J. J. Davis, of the United Electric Gas and Power Company. The company, satisfied as to its utility, is installing the boxes along its transmission line and already has about ten miles of the circuit so equipped.