Resistance increase of vacuum interrupters due to high-current interruptions

The resistance of vacuum interrupters can be calculated from the geometry and resistivity of current carrying parts and the additional resistance of the contact points between movable and fixed contacts. Since vacuum interrupter contacts are designed as flat contacts facing each other, the resistance is mainly determined by contact force, hardness and resistivity of the contact material. It is known that the contact material changes consistency and structure during short-circuit interruptions within melting depth. Indeed, the overall resistance of a vacuum interrupter has been observed to increase by up to 60% after short-circuit making and breaking tests. Since the resistance increase across the switching device is considered by IEC and IEEE standards as one of the acceptance criteria for the integrity of the interrupter after tests, it is essential to understand the origin of this increase. Different causes are discussed, among them the change of grain structure, resistivity and hardness of the molten top layer of the contact, and flatness of the contact surface. An increase of hardness and a different position of contact points seem to be the main factors responsible for the increased resistance.