Publication: Electrical Review and Western Electrician
Chicago, IL, United States
Grading of Suspension Insulators
The suspension type of insulator, consisting of a series of insulating disks between line and ground, has made it possible to insulate lines for voltages of 100,000 to 300,000 with a sufficient margin of safety to make the undertaking a reliable engineering feat, and a safe commercial proposition.
The inventions of Louis Steinberger, Brooklyn, N. Y., relating to "Systems of Insulation for High-Tension Conductors, and High-Tension Insulators", provide means for keeping the line in operative condition even though direct lightning strokes should reach it. A patent was issued February 15, 1910 covering these insulators, which are shown in the accompanying illustration.
In an article entitled, "Some Problems of High-Tension Transmission," which appeared in the Proceedings of the American Institute of Electrical Engineers for March, 1912, Charles P. Steinmetz states:
"With a number of equal insulating disks sharing the voltage between line and ground, the potential difference across the insulators nearest the line is higher, and the potential difference a across the insulators nearest the ground is lower, than the average potential difference per insulator, the more so the greater the number of insulator disks. From this it appears that the use of a large number of small insulator disks is uneconomical at very high voltages, and a few lareg disks of high disruptive strength are preferable. To extend the insulating possibilities of the suspension-insulator type far beyond the voltages now contemplated, therefore, requires a grading of the insulator disks in their capacity, so that the disk nearest the line has the hightest, that nearest the ground the lowest capacity, or the addition of capacity at the surface of the insulator disks, in proportion to their distance from the ground."
This patent covers a number of new and very desirable features. In this patent, in place of disk-type insulators, "thimble-type" insulators are employed. The term "thimble-type" was coined to denote the general thimble form of the insulator body, to distinguish them from disk or hood-type insulators.