GB643814A - Improvements in or relating to electric insulators having semi-conducting glazes - Google Patents

Abstract

643,814. Insulators. BULLERS, Ltd., and PERRINS, G. April 5, 1948, No. 9510. [Class 36] An electric insulator having a semi-conducting glaze is characterized in that the form factor of the insulator and the effective radii of the electrodes are such that the maximum voltage gradient in the glaze does not exceed a value of the order of 1.0 to 1.5 kv. per inch at the normal operating voltage. The voltage gradient (G) at any point on the surface of the insulator is obtained from the formula where V is the operating voltage, r the radial distance of the point from the axis of the insulator and Y the form factor of the insulation. The form factor is given by the formula R=Yp where R is the surface resistance and p the surface resistivity. The maximum voltage gradient on the surface of the insulator occurs at the point where r is a minimum, i.e. at the periphery of the electrode having the smaller radius, where it contacts the surface of the insulator. By increasing the form factor and the radius of the smaller electrode the voltage gradient is reduced to the desired value. In the case of a cap and pin type insulator the former may be effected by increasing the depth of flanges 21 by a suitable amount, and the latter by increasing the radius of the pin electrode 14 to a suitable value by surrounding the pin with a metal collar 18, or by forming the pin with an enlarged portion. Alternatively the pin may be secured by conductive cement 15, the under surface and bore 16 of the insulator being provided with a metal coating. Specification 527,357 is referred to.