Experience With Preventive Lightning Protection on Transmission Lines

Footing resistances for transmission line towers must be low for good operation, but it appears that no benefit results from reducing them below a certain limiting value characteristic of a given construction. This is because flashover can apparently be caused not only by excessive footing resistance drop, but also by the reactive drop accompanying rapid rates of current rise when tower currents are large. The records seem to show that overhead ground wires provide better shielding than has been believed. Overhead ground wires and tower footing resistances co-ordinated with line insulation provide preventive protection which is highly efficient. Operating experience indicates that lightning outages can be predicted fairly well from simple probability calculations.