Megger and other tests on suspension insulators

This paper gives the results of megger tests made on disk insulators on a 66,000-volt transmission line in central Massachusetts after 2.5 years´ operation. In one lot of 4410 insulators tested 9.77 per cent tested less than 2000 megohms. The percentage of failures in different positions in the string is given on both strain and suspension towers. The actual cost of making these tests under different conditions of weather and of service requirements is given per insulator on the line, per bad insulator and per tower. The cost per insulator on the line, of testing only, varied from 7.3 cents to 11 cents. The cost of replacing bad insulators was 74.5 cents per bad insulator, not including the cost of the replacing insulator. Laboratory tests made on 30 of the bad insulators taken from the line showed that those which measured very low by the megger fail on 60-cycle tests much below spark-over value while those with medium high resistance may reach flash-over value and then puncture within one minute. Some insulators which show infinite resistance by the megger, and which withstand arc-over potential at 60 cycles, will fail under high-frequency test immediately.