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Post arc current characteristics of a vacuum interrupter have been examined under a condition of a large current interruption. To measure post arc currents, a new measurement technique was developed in which a shunt for measuring post arc current was set in a synthetic test circuit voltage source. To use this method, post arc current flowing after a 20-57kA current interruption were measured. Their magnitude was greatly influenced by the rate of the current decline. Residual resistance between electrodes was calculated from the measured values of recovery voltage arid post arc current. The recovery speed of residual resistance was very quick immediately after current zero and become slow later. Imomc!rIoNVacuum circuit breakers are now widely applied to medium voltage switchgears in distribution system because of their excellent feature namely high breaking performance, compactness, long life, no maintenance and so on. The success of vacuum circuit breaker business has resulted from great efforts paid by many scientists and engineers. Attempts to understand the interruption phenomenon of vacuum circuit breaker were started in early days of vacuum circuit breaker development. Mitchell et.al [l ] first revealed that behavior of vacuum arc change according to the magnitude of interruption current. Heberlein et.al [ 2 ] also classified the modes of the arc behavior.Concerning to the interruption mechanism, phenomena near current zero are very important. The interruption failure of vacuum circuit breaker seems to be caused by a dielectrical break down, and the break down may concern the post arc current. Studies on the post arc currents are important for improvement in breaking performance. Reece 131 has proposed a mathematical model for post arc current and measured post arc current flowing after a 1.6kA(500Hz) interruption. childs and Greenwood [ 4 ] have developed a theoretical model €or current interruption in a diffuse vacuum arc with dc current commutation and measured the post arc current after commutating current of a few kA dc. For ac vacuum arc, Yanabu et.al [ 51 have measured the post arc current which flowed after interrupting a high arc current (36kA maximum) by a post arc current measuring shunt to which a circuit breaker was connected in parallel to protect the shunt. And relations between interruping current and post arc current and between electrode materials and post arc current were shown wlt interrupting currents of some vacuum circuit breakers are larger and it is very interesting to study the post arc currents flowing after large current interruptions. For measuring then, the authors have a7 SM 582-0 A paper recommended and approved by the IEEE Switchgear Committee of the IEEE Power Engineering Society for presentation at the I E E E / PES 1987 Summer Meeting,