DC local discharge on the insulator surface is apt to continue for a long time due to no-polarity alternation. Partial arcs easily bridge over the tips of adjacent ribs in the case of AC Fog type.
From the extensive investigation at NGK High Voltage Laboratory, optimum shed profile with alternate long and short ribs, keeping long distance between rib tips and long leakage distance have developed.
NGK High Voltage Laboratory is equipped with "750kV DC Contamination Test Facilities" and can make DC contamination test on actual scale insulator string.
Please apply with the contact form and give us your comments or questions.