Investigation of porcelain insulators at high altitudes

The use of high voltage apparatus in military aircraft made it necessary to obtain flashover data on capacitor bushings at altitudes up to 50,000 feet. Some of the more pertinent data are presented in this paper which gives a-c and d-c flashover voltages for porcelain bushings having spacings of 5/8 inch to 2 7/8 inches from cap to ring. The tests were made at both normal atmospheric and reduced pressures, but no attempt was made to simulate the frosting conditions as encountered in flying. Correlations are shown between bushing spacings and the effect of pressure change. A single correction curve for pressurechanges cannot be used, but a family of curves is required for various spacings of insulators and electrostatic field configuration. This paper shows that the practical use of porcelain insulators is limited to altitudes and voltages well below maximum values proposed for aircraft use. It is recommended that solid insulation be used in such a manner as to obtain the benefit of its puncture strength rather than its flashover at the more extreme conditions of altitude and voltage.