The problem of the relative importance of direct and indirect lightning surges on high-voltage distribution lines has not previously been solved quantitatively. From existing knowledge of the lightning discharge, the wave shapes and amplitudes of electrostatically induced surges on an overhead conductor are derived. These are found to vary with the amplitude of the inducing lightning current, with the distance between the lightning stroke and the line, with the type of object struck and with the construction of the line involved. Frequency distribution curves are established of the amplitudes both of indirect and direct lightning surges on typical 11-kv distribution lines. The probable numbers of flashovers are evaluated for different types of line construction, for different insulation levels, and for lines surrounded by open ground as compared with lines surrounded by buildings or trees. The corresponding numbers of surges produced on these lines without causing flashover of the line insulation are also determined. The theoretical results obtained are found to be in good agreement with such field observations as are available for comparison.
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