Lightning and the long spark; the significance of leader-stroke velocity

Abstract

The velocities of stepped leader-strokes of lightning from cloud to ground, or vice versa, vary by a factor greater than 20:1. The lowest velocities are almost the same as those of the leader-strokes between electrodes spaced 1–10 m apart when stressed with impulse voltages just sufficient to cause sparkover. Laboratory-produced leader-strokes can be increased in velocity by applying an over-voltage; it is suggested that the faster lightning leaders are effectively driven by an over-voltage probably created by discharges in the cloud before the visible part of the leader emerges below the cloud. The essential features of the positive and negative leader-strokes between two electrodes such as a rod/plane or rod/rod do not change as the gap is increased from 1 to 10 m, so tests on relatively small models of buildings and transmission-lines based on a maximum striking distance of 1 m can apparently be as informative as tests done on larger models. A variety of electrode arrangements have therefore been tested with minimum impulse sparkover voltage and with over-voltages up to a maximum of 1000 kV to observe the differences in behaviour as the leaderstroke velocities are increased. The faster leaders travel more directly between electrodes, and the leaders to a plane electrode are less deflected by secondary leaders rising from ‘buildings’ or irregularities on the plane, and in this sense their behaviour may be compared with the behaviour of some lightning flashes, but whether these were lightning leaders of high velocity is not known. Tests with the minimum impulse sparkover voltage and with overvoltages applied to model transmission lines have shown a behaviour which agrees with the behaviour of a 400 kV line struck by what was considered to be a small lightning current flash, and the tests suggest that field observations of lightning currents ought to be more closely linked with observations on leader-stroke velocities. Space-charge from the ground electrode has been shown to influence the path of the leader-stroke.