Analysis of electrical activity associated with inclined-plane tracking and erosion of insulating materials

Surface electrical activity and physical changes were measured and compared for several types of outdoor insulation materials, using a standard test method. The leakage current activities during the inclined-plane test (ASTM D2303) were measured by using four representative materials: a silicone with high (>70% by weight) loading of alumina trihydrate (ATH), a silicone with no ATH, a poly ethylene vinyl acetate (EVA), and a glazed porcelain. Quantitatively, the severity of the inclined-plane test was defined by detailed leakage current measurements. Qualitatively, it is observed that the test was severe enough to damage the glaze on porcelain. The study enabled us to compare polymers with porcelain, silicones containing no ATH vs. high loading of ATH, and silicones with EVA. The analysis showed that electrical activity, particularly the average leakage current and the distribution of peak current, depended on the surface wettability of the materials by the contaminant solution. The compound formulation is more important than the generic polymer types. It was demonstrated that silicone with no ATH, and EVA with only a medium level of ATH, exhibited excellent tracking and erosion resistance, comparable to the silicone highly filled with ATH. The presence of ATH is not absolutely necessary to achieve the superior tracking and erosion resistance of a silicone elastomer.