Glass vs. Porcelain insulators

Glass and Porcelain Insulators.

Experiments upon the electrical and physical properties of glass and porcelain insulators of European and American manufacture have been made by G. Rebord and described in the Atti dell' Associazione Elettrotecnica Italiana. To clear up some discrepancies on the disruptive voltages obtained with needle points by H. W. Fisher, new experiments were made with points really pointed and not blunt, such as were frequently found in commercial needles. A table, and the curve plotted therefrom, are given, showing the results obtained, which can be expressed by the following formulas.

From 0 to 25 kilovolts, X = 1.3 V, where V is effective pressure in kilovolts and X the sparking distance in millimeters, from 50 to 150 kilovolts, X = 3.2 (V — 25); from 30 to 50 kilovolts, X = 2 (V —10).

The summary of the insulator tests may thus be given. Glass insulators have a superficial resistance not inferior to that of porcelain under various conditions of humidity, and the glass does not retain moisture to any greater extent. The sparking distances over glass and glazed porcelain are from 10 to 30 per cent lower than the sparking distances in air; ebonite, ambroin, and unglazed porcelain show similar behavior. When smooth, the curved surfaces of glass and porcelain are equivalent. The mechanical strength of glass is satisfactory and the surface is not attacked, but glass will not stand sudden changes in temperature as well as porcelain does.