Transmission Line Insulators

Wires, Wiring, and Conduits.

GLASS OR PORCELAIN INSULATORS. -"Jour. Of El'ty," Aug. - A discussion forming part of the proceedings of the second annual convention of the Pacific Coast Electric Transmission Association. Eastwood explained why he had substituted glass for porcelain insulators on the Folsom and Sacramento lines, after having used porcelain for some time. Everything went well during the dry weather, but trouble began with the fogs and storms during the winter, when much difficulty was encountered in the burning off of the pins; a pin almost invariably burnt off at the point where the lower petticoat came into contact with it, and in nearly every case the insulator was found to have been cracked or defective in the outer petticoat; these were replaced by carefully selected ones, the pins having been painted with P. and B. paint, after they were thoroughly boiled in paraffin. He came to the conclusion that the one main point is to have a dry place between the line and then pin, and for this reason he chose a glass insulator; on the new line, on which 10,000 volts are used, with glass insulators, not a single pin has burnt off. The glass insulators were not only cheaper than porcelain but were better in every way; they are sufficiently strong for any strains that will ever come upon them. While porcelain insulators give no trouble in dry weather, they were not safe in wet weather with 19,000 volts. Ensign tested insulators for a 30,000-volt line and he thought the trouble with the porcelain insulators was due to their porosity; he could not find one of glass large enough to stand the potential without the tie-wire being too near the pin, and he was therefore forced to use the porcelain insulators; these measure 6.5 inches across the bottom by about 4 inches from the tie-wire across to the bottom; they are guaranteed not to penetrate with 70,000 volts; he describes a method of testing them by inverting them in a trough containing a liquid connected to one pole and then applying the other pole to a liquid in the interior; 66,000 volts between the tie-wire and the pin in ordinary dry does not show the slightest tendency to jump, and this is four times the strain they will receive. The same insulators tested for rain effects showed a peculiar phenomenon, as the current seems to follow the path of the drop to the cross arms rather than to jump across the shorter path to the pin; with 45,000 volts and the insulator 4 inches above the arm, he could detect absolutely no leakage. In the laboratory test above described, in which the insulators were tested inverted in the liquid, each circuit had a spark gap of one-sixteenth of an inch in circuit; when an insulator broke down a red, flaming arc was produced; some 15 to 17 per cent of those delivered were thus punctured. He found there was no use in testing insulators in this way without having a transformer of suitable capacity; moreover the voltage must be raised gradually. While he is satisfied with the insulator which he uses, he would prefer glass if he could get good ones large enough and free from cracks. At Telluride it was found that if the plant was shut down during a rain storm it would not start up again.