Abstract：Thermal and magnetic protection circuit breakers provided short circuit protection by instantaneous

tripping devices. In the magnetic field formed by the coil with current，the moving armature and stationary core

overcomed the spring force and engage under the influence of electromagnetic force，thereby pushing the circuit

breaker mechanism to operate and causing the product tripping. During the entire process，only when the

electromagnetic force reaches a certain value could be the moving armature and stationary core reliably engage at

the preset current value. In actual engineering design，in order to simplify the design and calculation process，issues such as magnetic leakage may be ignored. Therefore，the theoretical calculated magnetic attraction force，i.e. spring reaction force，will be smaller than the preset target. By analogy calculation method，it is possible to directly eliminate the influence of magnetic leakage and part manufacturing accuracy on the test results during the

manufacturing process，and complete the design of the instantaneous release device once. Finally，practical

application cases were used to illustrate how to accurately and quickly design an instantaneous release devices

through a combination of theoretical calculations and experiments.

      Key words：circuit breaker；instantaneous tripping device；tripping current；detection of instantaneous tripping current；analogous calculation