German company adding metal bowls on suspension insulator

Turin International Electrical Congress — II.

Abstracts of only part of the eleven papers dealing with construction, central station, switchboard and distribution, forming Section II of the Turin International Electrical Congress, were printed in the Oct. 28 issue, owing to pressure on our columns. The remaining papers of the section are given in abstract below.

PROTECTION OF INSULATORS FROM POWER ARCS.

Mr. W. Weicker gave an account of recent developments in insulator design and showed a number of photographs of insulators under the arcing test.

Fig. 1 — Insulators Covered by Metallic Cap.

Insulators should be designed with as large a margin as practicable between the arcing-over voltage and the puncture voltage and yet they must thoroughly insulate the normal potentials. Of course, it would be desirable to avoid arcing over, but practical experience has shown that this is impossible of attainment and, therefore, means of harmlessly dissipating the energy of the arc must be devised.

The arcing rings described by Mr. L. C. Nicholson before the A. I. E. E. (1910) and in the Electrical World, March 23, 1911, were referred to and photographs shown of such rings in operation, both on pin and on suspension insulators. They provide effective protection from the arc and have the advantage of being easily attached to any form of insulator.

The author points out that the metallic umbrella type of insulator recently introduced by the Porzellanfabrik, Hermsdorf, affords an even greater degree of protection from arcing over and has the advantage of equalizing the dielectric stress. This metallic umbrella principle has been extended to fulfil the requirements of the suspension-type insulator and as a result of experimental research a quite new form of insulator has been produced. Each unit consists of a porcelain insulator covered by a metallic cap shaped like a bowl that comes down over the sides and almost completely hides the insulator. This metallic cap serves the following useful purposes: It equalizes the dielectric stress; it provides a path for arcs, allowing a harmless dissipation of their energy; it protects the porcelain from mechanical injury during installation, or from missiles; it shields the insulator from rain and snow. Tests showed that the relation between arcing distance and arcing voltage was practically uniform under dry and rain conditions alike and that the arcing voltage increased in practically direct proportion with the number of units.

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