Anomalous flashovers on transmission lines

TWO POWER COMPANIES recently have experienced outages caused by lightning far in excess of the rate predicted by usual methods. Furthermore, it has been found that most of these outages are caused by the flashover of top or middle phase insulator strings on towers with very low footing resistance, which represent phenomena contrary to the usual theory. Many of these insulators have lightning-produced glaze burns, which hug the porcelain surface on both sides of the disk, even into the depths of the grooves between petticoats. This characteristic, coupled with laboratory experience in impulse testing, suggests that the lightning voltage pulses which produce these glaze burns have very steep fronts, and that in a fraction of a microsecond they probably reach crest potentials considerably in excess of the usual 1½ × 40-microsecond-wave critical flashover voltage. Marks found on tower tops show where they have been hit by lightning strokes, and it is this observation which rules out the possibility that these anomalous flashovers may be because of shielding failures.