The application and operating mode of the reactor in EHV electric power system are described in brief. The working conditions of a circuit breaker for breaking shunt reactor current and the simulated test circuit in laboratory have been analyzed. In accordance with the date of Jin-Liao power transmission line

the means of the choosing test circuit element values on source side and line side are shown in this paper

while the auxiliary break of C.B. KW5-500 is tested in the vibrating circuit. The tested results are also given. The results show that chopping current and overvoltage had gread disp ersion. Chopping current is the main reason which causes oveIvoltages.