It is important to note that more intensive dry band
arching and in one case a flashover of a single disk in string
was observed just on the cleanest disks in pre wetted
naturally contaminated insulator string (Fig. 11). This can be
explained by the fact that contaminated insulator disk’s
surface resistance is lower than on clean disk. Inequality of
disks’ resistances affects the voltage dividing’s between
disks in string. Highest voltage drop is applied to insulator
which has highest resistance. Higher voltage drop with same
current means higher dissipation power. Higher dissipation
power dries less contaminated disk faster than more contaminated
disks, which causes further increase of resistance,
voltage and dissipation power on less contaminated disk.
When the voltage drop increase to less contaminated disk
exceeds the disk’s low frequency withstand voltage, the
flashover of a single disk will occur. Dry band arching or
flashovers of single disks in string will need sufficient leak-