When lightning occurs in the summer, a negative charge develops in the upper part of the lightning cloud.
When a power transmission line and a communication line cable exist under this lightning cloud, equilateral electric charges collect on the cable and high voltage occurs. Electrostatic induction surge occurs when a positive charge on a cable develops near a negative charge from a lightning cloud. When the cloud releases its charge to another cloud or the ground, the charge on the cable is released and travels in a wave, advancing in either direction.

When electric discharge (lightning) occurs between a lightning cloud and the ground, a severe electric current drifts and the electromagnetic induction surge produces an electromagnetic field. If a power transmission line and the cable of a communication line exist near a lighting strike, they act as an antenna and electromagnetic induction causes abnormal voltage.
When an electric discharge occurs between lightning clouds or between a lightning cloud and the ground, abnormal voltage occurs by electrostatic induction and electromagnetic induction to the neighboring power transmission line or communication line. This is what is called Induced Surge as illustrated in fig.2 (a) and (b).