Lightning Investigation on Transmission Lines---III

The field data for 1931 and 1932 are reviewed, with especial reference to lightning stroke currents. When direct strokes take place to towers or ground wires, flashover occurs if the product of current and tower footing resistance is higher than the fiashover strength of the insulators. Flashover seldom occurs if ground wires are present and the towers have low footing resistance. Three years´ records on four systems indicate tower currents varying from 1,000 to 150,000 amperes. Tripouts correlate almost exclusively with high tower leg currents of negative polarity, that is, the tower top is negative with respect to the base. This indicates that the severe lightning strokes come from negatively charged clouds. A great amount of experimental data, relating to flashover between electrodes of dissimilar shapes, are shown to be in accord with these field observations.