SF 6 self blast circuit breakers use the flow back of high pressure gas from the expansion chamber, which is generated by ablation of a PTFE-nozzle and the arc radiation to interrupt the arc. The interruption capability of SF 6 self blast circuit breakers depends on the pressure inside the switching chamber at current zero. During each fault current interruption, the arc radiation induces contact and nozzle ablation. The nozzle widening causes reduction of the pressure build-up and thus results in the reduction of the interruption capability. The objective of this work is to use a non-invasive method to assess the degree of nozzle ablation without opening the interrupter and to estimate the degradation of the interruption capability. In this work, the frequency response analysis is applied to detect the change in capacitance due to the change of the geometry inside the circuit breaker interrupter.