When a fault occurs in the T-connected branch line or the end user of a 12kV overhead distribution line, the outgoing switch of the substation will trip if the protection action time limit of its incoming switch cannot be reasonably adjusted with the protection time limit of the outgoing switch of the substation. If the nature of the fault is permanent, the reclosing of the substation will fail, and a local accident in the boundary of a medium-voltage user will cause the entire 12kV distribution line to be shut down. If an outdoor boundary circuit breaker is installed at the T-connection or the user end (responsibility boundary point), after judging and confirming that a fault (over-current, short circuit, single-phase grounding, etc.) has occurred in the user boundary, the fault is automatically isolated, and normal power supply can be ensured for users in non-fault areas. The application of ZW32-12F outdoor boundary circuit breaker on 12kV feeder overhead lines can avoid joint accident power outages of fault-free lines, reduce the scope of fault power outages, shorten the power outage time of users, and protect the safe operation of the main network. It also has fault detection function, protection control function and communication function, which can realize automatic removal of single-phase grounding faults and automatic isolation of inter-phase short circuit faults. The installation point is applicable to the responsibility dividing point of the user incoming line of the 12kV distribution line, and can also be applied to the branch line connection point that meets the requirements.