A computational study of the thermal interruption process of an auto-expansion circuit breaker with different arc durations (12.25 ms, 15.25 ms and 18.62 ms) has been carried out. While the pressure rise in the expansion volume depends mainly on the final half-cycle of the current, the thermal interruption capability, in terms of its critical rate of rise of recovery voltage (RRRV), is severely affected by the contact separation at current zero, thus the effective flow cross sectional area of the main nozzle. The short arc duration case has the lowest RRRV.