propagating the ionised channel a little further before it is also spent. This process continues a number of times, each step
increasing the channel length by 10 to 200 m. Because of the step like sequence in which this streamer travels to earth, this
process is termed the stepped leader stroke. This process is shown diagrammatically in figure 3.2
รูปที่ 5-4 Propagation of lightning channel
(ที่มา http://www.elect.mrt.ac.lk/HV_Chap3.pdf)
When eventually the stepped leader has approached to within 15 to 50 m of the earth, the field intensity at earth is sufficient
for an upward streamer to develop and bridge the remaining gap. A large neutralising current flows along the ionised path,
produced by the stepped leader, to neutralise the charge. This current flow is termed the return stroke and may carry
currents as high as 200 kA, although the average current is about 20 kA.
The luminescence of the stepped leader decreases towards the cloud and in one instances it appears to vanish some
distance below the cloud. This would suggest that the current is confined to the stepped leader itself. Following the first,
or main stroke and after about 40 ms, a second leader stroke propagates to earth in a continuous and rapid manner and again
a return stroke follows. This second and subsequent leader strokes which travel along the already energised channel are
termed dart leaders.
What appears as a single flash of lightning usually consist of a number of successive strokes, following the same track in
space, at intervals of a few hundredths of a second. The average number of strokes in a multiple stroke is four, but as many
as 40 have been reported. The time interval between strokes ranges from 20 to 700 ms, but is most frequently 40-50 ms.
The average duration of a complete flash being about 250 ms.
The approximate time durations of the various components of a lightning stroke are summarised as follows.
Stepped leader = 10 ms
Return stroke = 40
period between strokes = 40 ms
duration of dart leader = 1 ms
For the purpose of surge calculations, it is only the heavy current flow during the return stroke that is of importance. During
this period it has been found that the waveform can be represented by a double exponential of the form