The DC circuit breaker with type of UR36/40 M is mainly used in large-size and medium-size power plants. Through the analysis of the elements in the control circuit and test data

it is obtained that the design value of the rated action voltage of the main opening coil is 13.8% Ue（Ueis the rated voltage）. But the real action voltage would be low down to 9.3% Uein the minimum opening voltage test

which does not meet the specified value of 30% Uein GB 50150—2006 Commission test of electric equipment installation engineering and DL/T596—1991 Preventative test specification for electric equipment in China. Some man-made mistakes are described in mechanical characteristic test.