An SF6 autoexpansion breaker: the correlation between magnetic arc control and critical current

It is possible to design SF₆ autoexpansion breakers which are free from critical currents. In these devices, which combine arc rotation in a magnetic field and thermal expansion of gas, critical currents can effectively occur if these effects are incorrectly created and combined. An analytic and experimental method optimizing the current at which rotation should start is presented. The study of the gas flow then ensures coupling with expansion. These investigations lead to representation of a thermal time constant directly related to interrupting capacity