Environmental factors affecting dc resistance to tracking of polyethylene

Resistance to tracking of insulating materials has been studied by many investigators using the test method of IEC Publication 112. This method has now been applied to materials selection. There is no internationally recommended method for determining dc resistance to tracking because the experimental values of the comparative tracking index (CTI) are obtained only under ac voltage. The environmental factors affecting dc resistance to tracking of organic insulating materials are still far from being well understood. This paper, presents a study of the effects of gamma-ray irradiation, atmospheric pressure, and sample temperature on the dc resistance to tracking of polyethylene. Samples were irradiated in air up to 1 MGy with a dose rate of 10 kGy/h using a Co-60 gamma source. The resistance to tracking on applying dc or ac voltage was examined. We found that the dc resistance to tracking is lower than the ac resistance. As the total radiation dose is increased, the dc and ac resistance to tracking both increase. The erosion depth, weight loss and contact angle affected by the radiation have been studied under combined environmental conditions. A nonlinear correlation was observed. As the total irradiation dose was increased, the dc resistance to tracking decreased with decreasing pressure from 100 kPa to 50 kPa. When the samples were above room temperature the dc resistance to tracking decreased.