Current zero flow conditions in a circuit breaker-nozzle

Pressure measurements are presented along the nozzle wall of an SF₆ monoblast circuit breaker for peak alternating currents up to 56 kA. Results are given for a nozzle throat diameter of 35 mm and for both copper and PTFE nozzles. The results are compared with theoretical values determined from both one- and two-dimensional fluid flow theory. It is shown that the flow is profoundly two-dimensional in nature close to current zero so that the normal procedure used predominantly in the literature hitherto of relying on one-dimensional flow calculations, overlooks some important fundamental aspects which govern arc extinction and, hence, current interruption. The implications of the results are therefore important in the design optimisation of commercial circuit breakers.