Effect of altitude on the spark-over voltages of bushings, leads and insulators

The dielectric strength of air decreases with decreasing pressure and increasing temperature; that is, with the density. Therefore, at high altitudes where the barometric pressure is low, brush discharge starts and spark-over takes place at lower voltages than at sea level. The effect of air density on corona, and spark-over between spheres, etc., has already been given. In the following investigation the effect of altitude and temperature on the surface spark-over of leads and insulators was studied by placing them in a large wooden cask and gradually exhausting the air. Correction factors are given for various standard types. The spark-over voltage decreases almost directly with the air density.