Influence of eccentrically mounted vapor shields on the dielectric strength of vacuum interrupters
The breakdown mechanism in vacuum is primarily a process that is related to the electrode surfaces. The theoretical critical field value for a vacuum breakdown is much higher than the practically determined values. Reason is that the critical field strength consists of the geometric and the microscopic field. This fact is represented by a field enhancement factor. Its value is often discussed, but there are no usable values for real applications. The question is how small deviations of the field strength, caused by ir-regularities of the geometry, affect the dielectric behaviour of the interrupter. Are these deviations the cause of the dielectric interrupter performance worse than expected, or is the breakdown caused by the surface treatment/surface condition? In this contribution the dielectric behaviour is investigated with focus on the local field enhancement, caused by eccentrically mounted vapour shields. Eccentricity is usually not an intended feature of an interrupter but may unintentionally originate from a non-perfect production process. How-ever, for the purpose of these investigations, special test samples were prepared with in-tended, well controlled eccentricity of the shields. The surface condition of the breakdown location will be investigated, and the electrical field in this area will be estimated. Based on the simulation and the measurement results it will be decided if the dielectric perfor-mance is depending on the geometric field strength, on the eccentricity of the vapour shields, or on the surface condition of the contact material. The result may help the de-veloper of vacuum interrupters to focus on the parameters of major importance.