The contact force to prevent large area, closed, vacuum interrupter contacts from welding when passing high fault currents of up to 4 seconds duration

An analysis is performed on data from an experiment to determine the closed contact force required to prevent the welding of closed, Cu-Cr contacts inside a vacuum interrupter passing 20kA (r.m.s) from 0.5s to 4s. It will be shown that the data can be explained if these closed contacts have more than one region of contact together with the heating of the contact's body during the passage of the current. The implication of this analysis for vacuum interrupters used in vacuum circuit breakers when required to stay closed during the passage of short circuit currents is discussed.