The surface resistivity and charging of lexan
Linzey, William George
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The surface resistivity of the polymer insulator Lexan™ (polycarbonate) in a vacuum environment has been investigated both experimentally and theoretically. The term surface resistivity is often used to describe the electrical properties of insulators at low fields; however, the standard method of measuring it is not applicable at the high value of the resistivity encountered under these conditions and will return a lower than tme value for the surface resistivity. While very little work has been done in this area, the presence of surface conduction can be an important factor in the field distribution and electrical aging of high voltage insulators. In the investigation, the current resulting from the application of a step voltage was measured as a function of time for up to 30 hours, at which time the magnitude of the current was in the femtoampere range. The build-up of the surface voltage was also measured by translating a voltage probe above the insulator surface. The voltage and field distributions were calculated using a numerical technique because the geometry used in this experimental Investigation included a dielectric mismatch along a plane of asymmetry. This information was used in a computer simulation that attempted to correlate the current measurements with a model that assumed a constant surface resistivity. The results did not indicate the presence of a constant surface resistivity, and we conclude that if Lexan does have a surface resistivity it must be greater than 5 X102 0 Q/sq which is 5 orders of magnitude higher than the value obtained using the standard method. Lexan™ is a trademark of General Electric Incorporated.