Spray and fog tests on 220-kv. insulators

To determine insulation for an outdoor 220-kv. station on the coast subject to ocean spray, an insulator test rack was installed at Redondo, California. Ten types of insulator, including widely different designs, were tested continuously for two years and a half at 150 kv. to ground. Comparative results were obtained by adding or subtracting units in suspension strings until an equality against arc-over was approximated. Ninety arc-overs occurred. The surface leakage resistance was found to be a fair index of the resistance to arc-over under salt spray conditions. The shape of the insulator made no difference as long as the total surface resistance of the string remained the same. The surface resistance is that calculated upon the assumption of a uniform conducting coating upon all the exposed surface of the insulator and is the line integral of distance divided by circumference along the shortest surface path from cap to pin. Accidental differences of conditions are such that one insulator string would not consistently arc-over in preference to another unless its surface resistance were less than 80 per cent of the other. Suspension strings having a total surface resistance of 11.0, using inch units, were found satisfactory for a steady 150 kv. to ground under the conditions at Redondo. A spray method of cleaning insulators while energized was devised.