Investigations of Copper Chrome Coatings on Vacuum Circuit Breaker Ceramics by Electron Probe Microanalysis and Electric Field Simulation

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. After performing switching operations, the coated ceramics were examined. Electron probe microanalysis (EPMA) 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. To provide a better understanding of how strongly different deposited layers can affect the dielectric performance of the VI, surface resistance measurements were carried out. These were performed with a high-resistance meter, which is able to measure up to $10^{16}~\Omega$ using a 1-kV voltage source. Electric field simulation was carried out to know how the coatings do affect the electric field inside the VI and why they can cause breakdowns during lightning impulse voltage tests.