Breakdown and field-emission behaviour of differently prestressed vacuum interrupter copper contacts

To obtain more information about the dielectric strength of open vacuum interrupters under practical conditions, the breakdown behaviour of differently prestressed contacts was determined with test voltages (alternating and pulse voltages with various rates of rise). Also, the corresponding surface micro- structure was examined by recording the field-emission current as a function of the applied voltage; the Fowler-Nordheim equation was used to calculate the microscopic field-enhancement factor and the emission area. A large number of tests were performed using a computer-controlled test and registration system to obtain statistical reliability. Additionally, the influence of long-time voltage application (~¿¿ min) and the influence of surface gas layers were examined. The results of the investigation show clearly that high-voltage testing of vacuum interrupters yields results not representative for the behaviour of the interrupter at service conditions, if the influence of switching operations on the dielectric strength is neglected. The lowest breakdown voltages are caused by no-load opening of welded contacts, not by high-current interruption