Abstract: According to the connecting rod transformation characteristics and static characteristics of the moulded case circuit breaker(MCCB) operation mechanism,the mechanism sketches of each key state were drawn,and the movement principle of the operation mechanism was elaborated in detail.On the other hand,the force analysis of the links of the operation mechanism in some stable and specific transient positions was carried out by graphic method,and the key mechanical parameters of MCCB were calculated by using the principle of mechanical balance and the theorem of positive rotation.These five mechanical parameters are the re-buckling force of the handle,the closing force of the handle,the main spring force,the final contact pressure and the release force.It provides a theoretical reference was provided for the force optimization of the existing MCCB operating mechanism and the design of a new operating mechanism.

Key words: moulded case circuit breaker(MCCB), operating mechanism, force analysis, graphic method, mechanical balance, positive rotation theorem