For the optimization design of air circuit breaker (ACB), it is important and necessary to calculate the electro-dynamic repulsion force acting on the movable contact. A method based on 3-D FEM with the equations that describe the relationships among current, magnetic field and repulsion force, which takes the ferromagnet into account, is adopted to calculate the electro-dynamic repulsion force. The method enables one to analyze the factors that affect the electro-dynamic repulsion force, including the number of the movable conductor parallel branches as well as the location of the axis and the shape of the flexible connection. The discussion of the calculation results is also presented in this paper.

To one double-breaker model, experimental investigation on blow open force was carried out. It demonstrates that the ratio between the emerging blow open force and arc power FB/ui decreases with the arcing time, the contact gap has less effect on FB/ui, and the characteristics of the blow open force are similar when the peak value of the short circuit current is beyond 4 kA. Then, according to the experimental data and conclusions, considering the influence of blow open force, the interruption process of molded case circuit breakers (MCCBs) was investigated. It demonstrates the blow open force has significant influence on interruption process and the proposed method is effective to evaluate new design of MCCBs.