Abstract: As an important control center for the operation of high-voltage circuit breaker, the design reliability of circuit breaker control circuit is of great significance to the safe and stable operation of power system.The phenomenon of tripping without outlet pressing plate during the actual inspection of the non-full-phase circuit of the circuit breaker in an EHV substation done by the protection tester is described, the causes of the phenomenon are analyzed, and combined with the operation inspection, the reliability of the non-full-phase circuit design scheme in extreme cases is studied, based on which improvement suggestions are put forward.