Lightning flashovers on the British grid

The frequency of lightning flashovers to overhead lines is calculated on certain assumptions concerning the characteristics of the leader stroke, and is shown to depend on a number of parameters such as line height, shielding angle of earth wires, tower-footing resistance and the insulation level of the line. Results are given in the form of a ratio: the number of flashovers per unit length per year to lightning strokes to earth per unit area per year. To determine absolute values of flashover frequency requires a knowledge of the rate of strokes to earth, a characteristic which is not known precisely, but the performance of different systems operating in a region of uniform lightning activity can be compared; this comparison has been made for the different transmission voltages of the British system. Lightning-fault statistics have been used to estimate the frequency of lightning strokes to earth exceeding 5kA.