Extensive laboratory tests have shown that for external insulation the light-
ning surge flashover voltages are substantially proportional to gap length and
that positive impulses give significantly lower flashover values than negative
ones. In addition, for a particular test arrangement, as the applied impulse crest
is increased the instant of flashover moves from the tail of the wave to the
crest and ultimately to the front of the wave giving an impulse voltage–time
(‘V– T’) characteristic as was discussed in Chapter 5, Fig. 5.45. Figures 8.6
and 8.7 show typical impulse sparkover characteristics for long rod gaps
and suspension insulators obtained by Udo10 at various times to flashover.