Insulators for Electric Power Transmissions

[Trade Journal]

Publication: The Journal of Electricity

San Francisco, CA, United States
vol. 4, no. 6, p. 114-115, col. 2, 1-2




(A paper read before the Santa Cruz Convention of the Pacific Coast Electric Transmission Association, August 17, 1897.)


The advent of electric transmission of power into the electrical field has made the question of line insulation considerable of a study. The possibilities of a shut down, especially in instances where the plant is delivering a twenty-four hour service, are worthy of serious consideration and the object of every manager is to reduce these possibilities to a minimum. The high voltages at which transmission lines are now being run and the still higher voltages at which they will soon be operated, makes the insulator a very important factor in the successful insulation and operation of electrical transmission plants.

The climatic conditions must be considered in every Case. In countries where snow and ice are unknown, an insulator might stand heavy rains, but it might not stand the heavy strains to be brought upon it by the atmospheric rigors of other localities. Where snow and ice are prevalent for four or five months of the year, the question of proper line insulation becomes a serious problem, for the insulator has not only to resist the passage of. current through the ground, but must be sufficiently homogenious to withstand the extra weight of ice, sleet and snow that would adhere to the wire. The heavy snow and- sleet: storms prevelent in the East instance the great strain that insulators might be subjected to.

In California these conditions do not exist except in cases where the source of power is located above the snow line. This is a condition to be considered, as the future of the transmission of power in California lies to a certain extent in the utilization of streams that have their sources well up in the mountains where snows prevail each winter. In localities subject to heavy rain, it is necessary to use an insulator that will shed the rain quickly, and at the same time have an outside petticoat sufficiently large to protect the pin from moisture.

On July 16th, 1894, the Sacramento Electric, Gas & Railway Company, with which the writer was connected, commenced transmitting electric power from. Folsom to Sacramento, a distance of twenty-two and one-half mile. During the succeeding winter the rains were quite heavy and some difficulty was experienced from punctured insulators and burned pins. The line was equipped with two styles of porcelain insulators, viz: A triple, vertical petticoat, and a later type consisting of a triple flare petticoat. The former gave us all the trouble the first year and the latter most of the trouble the second year of operation.

In most of the cases the pin was burned at the lower edge of the inner petticoat and only in a very few cases was the pin burned at the top. Sometimes the insulator was punctured at the top, but in most of the insulators the trouble that developed seemed to be at the lower edge of the thinner petticoat close to the pin where the glazing had been removed in baking.

In examining the insulators taken off the line I found in some cases that although the puncture showed at the top it did not appear to carry all the way down. Generally cracks were shown at the top of the insulator which followed all the way dawn to the bottom, thus showing a lack of vitrification in moulding the insulator, as the cracks were, in some cases, simply covered by the glazing. Invariably the glazing was off where the insulator rested when baking, and this was also in evidence on many of the new insulators. Both styles of these insulators are now in use on the transmission line between Folsom and Sacramento. This experience appears to show that it is not so much in the shape of the insulator as it is in the manner in which it is made. A dozen china Locke insulators were placed on the line last winter and so far as is known they