For these lightning strokes, the shielding failure rates were 1.80 and
0.98% for UHV-designed and 500 kV transmission lines, respectively, whereas the lightning
outage rates due to direct lightning strokes (flashover) were almost equal at 0.37 and 0.39%
for UHV-designed and 500 kV transmission lines, respectively. This is due to the balance
between the shielding effect associated with the insulation clearance and the flashover voltage
between arcing horns. Conversely, the lightning outage rate due to back flashover was mainly
a factor of the distance between arcing horns and was 0.05% for UHV-designed transmission
lines, which was very low compared with 0.59% for 500 kV transmission lines. Based on these
results, since a comparatively high number of lightning outages were caused by direct
lightning strokes on large-sized transmission lines, e.g. UHV class, the need for insulation
design to tackle such cases was indicated.