Masayuki Ishiwa

The cause of tracking-induced abnormal drop in the surface resistance of rubber-made electrical components

It is widely known that the tracking deterioration of high-voltage electric equipment occurs in heavily polluted environments, including the seaside where equipment faces injury from salt. Development of materials always takes these environments into account. This report deals with cases in which rubber-made electrical components that have been used in enclosed environments free of any contamination including injury from salt suffer a rapid drop in their surface insulation resistances, leading to tracking deterioration. It attempts to guess the cause of the deterioration and states measures to improve the rubber composition. Deteriorated components featured an overall wet condition with partial incineration. The reduced insulation resistances on the surface did not recover even after the components were left inside a room for several months, suggesting that the deterioration had not been caused simply by the wet condition. As the most likely cause, attention was paid to possible influence of active gases that are generated by partial discharge. For confirmation, insulation rubber sheets were exposed to active gases generated by ozonizer using air as raw material. Measurement after the exposure revealed a rapid drop in the surface resistances, which did not recover even after the sheets were returned into normal atmosphere. Then, electric wire of small size whose surface resistances had dropped after being exposed to active gas was subjected to a power application test in a highly humid room. In the test, glow-like discharges that were stable in position could be observed continuously. Next, power was applied to untreated pieces of electric wire in an environment in which such discharges were occurring. At the beginning nothing happened, but then the wire began to generate discharge and a drop was observed in its surface resistance. Examination of the compounding ingredients of the rubber composites whose surface resistances had dropped revealed that the drop had been caused by calcium carbonate used as filler and by zinc oxide used as activator. It was assumed that the drop in the surface insulation resistance was caused by the high deliquescent property of the nitrate that was generated in a reaction between the NOx created by discharge and inorganic compounding ingredients blended into the rubber composition. These results showed that in an enclosed wet environment, even when the rubber-made electric component is not polluted at all, discharge caused by some outside factor except the rubber component sometimes lowers the insulation resistance on the rubber surface rapidly, leading to a tracking deterioration in a short period of time. In this case, some kinds of in organic fillers can accelerate the deterioration instead of adding to the tracking resistance. In preventing deterioration of the rubber products used in the said environments. Therefore, it is effective to restrain the points that generate discharge in enclosed environment. And when the countermeasure is taken in the form of using proper insulation rubber, selection of compounding ingredients must be made in consideration of the influence of active gases on them.