Electron probe micro analysis and surface resistance measurement investigation of copper chrome coatings on vacuum circuit breaker ceramic surfaces following switching operations

Abstract

Vacuum Interrupters (VIs) are designed for a long operating life. Even after a large number of switching operations they should still provide good arc extinction and be able to withstand high voltages. Switching operations produce substantial masses of metal vapor that condense on the inner component parts of the VI. The Vis are equipped with shields to protect the ceramic surfaces from being coated by metal vapor. Since the first VI was put into service, a large number of shield designs have been developed. If the shield design consists of a floating shield with two end-shields, there are two gaps between these shields. These gaps are necessary for insulation when the VI is in the open position, but during the switching operation metal vapor can flow through these gaps and coat the ceramics. If the deposited area on the alumina ceramics reaches a critical value, the dielectric performance of the VI can become reduced. Standard VIs have been tested with a floating shield and two end shields, that did not overlap the floating shield. The VIs were assembled at the Institute's high power test facility to perform switching operations at different current intensities. After opening the VIs, the coated ceramics were examined. It became clear that, the more switching operations were performed, the darker the deposited layers became. Electron Probe Micro Analysis (EPMA) is a method for surface investigations which was used here to obtain a very detailed view of the thickness of the deposited layers. With EPMA it is possible to detect very thin layers in the range of nanometers. Several coated ceramic surfaces of VIs that had been switched with different current intensities were investigated. In order to provide a better understanding of how strongly different deposited layers can affect the dielectric performance of the VI, surface resistance measurements were carried outdone. These were performed with a high resistance meter, which is able to measure up to 1016Ω using a 1 kV voltage source.