The effect of transient voltages on dielectrics II: The effect of lightning voltages on arrester gaps, insulators and bushings on transmission lines

This paper treats of some of the practical applications resulting from an investigation of the effect of lightning voltages on insulators, bushings and protective gaps. There is a great difference in the relative lightning spark-over voltages of various gaps as well as a great difference in the setings imposed by operating conditions. Both of these factors must be considered in comparing the relative protective values. A gap must be set so that the normal line voltage does not cause it to spark-over. Gaps are generally used out of doors. Rain lowers the 60-cycle spark-over voltage of all uncovered gaps and thus imposes a greatly increased setting and decreased protective value since the lightning spark-over voltage is not changed by rain. The covered sphere gives the maximum protection. The protective value is constant under all conditions. The sphere-horn, having electrodes of points, horns and spheres, gives very good protection over the whole range of frequency and wave front. The spheres discharge the very steep waves, the horns the moderate ones, and the points continuous high-frequency waves of slanting front and static. The protective value of selective gaps varies with the wave front. Its protective value is a minimum for very steep wave fronts and for waves of slanting front. Over a certain range its protective value is very good. The relative protective values of various gaps for steep and slanting wave fronts and high frequency are shown graphically in Figs. 14, 15, 16 and 17. The relative protective values are approximately independent of the point on the 60-cycle wave at which the discharge occurs. Data are given on the steepness of lightning waves actually occurring on transmission lines in practise. Bushings and insulators with equal 60-cycle spark-over voltages may have entirely different lightning spark-over voltages. A bushing should be designed for a high lightning spark-over voltage. The lightning wet spark-over voltage of a bushing or insulator is the same as the dry spark-over voltage.