Long-term field experience with RTV coated porcelain insulators

This paper presents results from a long-term study of two different room temperature vulcanizing (RTV) silicone rubber (SIR) coatings. The RTVs were applied to identical longrod porcelain insulators and energized under natural outdoor conditions for a period of about nine years. In addition to being energized with AC and DC voltage, each type of coating was also kept non-energized outdoors and indoors, respectively. The surfaces of the insulators were analyzed using electron spectroscopy for chemical analysis (ESCA), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and scanning electron microscopy (SEM). In 1997 the insulators were dismounted from the test line and studied in the laboratory. The indoor investigation encompassed surface resistance measurements under clean fog conditions and voltage-withstand testing under rain. The material analyzes indicated that one of the coating materials was filled with alumina trihydrate (ATH) whereas the other was not. Both materials showed significant signs of surface oxidation and loss of hydrophobicity. The coating without the ATH seemed the most affected by the outdoor exposure. However, both types of coatings maintained low leakage currents and a very good performance during the nine-year-long outdoor study.