This phenomena is observed on Contacts and Nozzles in circuit breakers at service voltage.Contacts of circuit breaker deteriorate by arcing during current interruption. The surface of contact material melts due to the high temperature of the arc and electrical wear of the contact progresses cumulatively with repetitive current interruptions. Electrical wear of contacts in oil circuit breaker s are comparatively severe, since they generally show longer interruption duration than other types of circuit breakers. On the other hand, less electrical wear can be achieved in gas CBs by shorter duration of current interruption due to excellent arc quenching properties of SF6. Nozzles in gas CB also show the same degradation process, electrical wear by arcing. Because of the mechanism of these ageing processes, their progress shows a strong dependency on interruption current, duration of current interruption and number of current interruptions
In left pic we can see electrical wear on fix contact and in right pic electrical wear on nozzles of gas circuit breakers
