Influence of the Microstructure of Cu-Cr Alloy Electrode on Vacuum Arc

Vacuum circuit breakers (VCBs) are widely used in the medium voltage class. Vacuum arcs are categorized into several modes. As the current increases, the diffuse mode transits to the anode spot type 1 and type 2, where the surface of anode is damaged. Thus, many VCBs are designed so that the diffuse mode arc is formed. The arc discharge may cause a scatter of small particles from the electrode. It has been reported that the particles often lead to a re-strike when they stay between the electrodes after current interruption. In VCBs, Cu-Cr alloys are widely used for electrodes. Cu-Cr alloys have a different microstructure depending on the manufacturing process. However, it has not been studied how the microstructure affects the performance of VCBs. In this study, we investigate the effect of the microstructure of the electrode alloy on anode modes and the number of scattered particles. Two kinds of Cu-Cr electrodes, whose grain sizes are approximately $100- 200 \mu m$ and $3- 5 \mu m$ respectively, are compared. It turns out that both two kinds of electrodes have similar properties in terms of threshold current where the anode-spot mode type 1 starts to appear and the number of scattered particles. It is also revealed that the particles were observed only when anode-spot mode type 2 (type 2 mode) occurred, and the number of particles and the current under type 2 mode have a positive correlation regardless of the microstructure at least for the two kinds of electrodes.