A ramp current circuit is introduced to research interrupting performance of a SF6 blast interrupter.The criterion for its equivalence to a power frequency current is with the same current decay rate approaching current zero. The comparisons between both current conditions are presented. The interrupting performance in term of maximum dv/dt

di/dt

and the electrical conductance of the arc column

a few microseconds prior to current zero are without much difference for di/dt<20A/μs

this corresponds to a nozzle condition without serious clogging during the whole arcing period. It is suggested that a simplified ramp current instead of a power frequency current is justified for the investigation under low di/dt.