If a flash hits an overhead line, the current injected into the conductor is divided at the
strike point, giving rise to two voltage waves that propagate in opposite directions. The
prospective magnitude of these voltages can be estimated by multiplying the current
that flows in each direction (half of the stroke current) by the characteristic impedance
of the line, which is normally in the range 400–550 V. Therefore, for a line characteristic
impedance of 400 Vand a stroke current of 10 kA, whose probability of being
exceeded is larger than 90 per cent, the corresponding overvoltage is 2 000 kV,
which is far beyond the line insulation level. As a consequence, multiple flashovers
occur between all the conductors and also to earth in various points of the line,
causing many current and voltage reflections as well as reduction of the effective
earth impedance