When a fault occurs on a tapped branch line or at the end user of a 12kV overhead distribution line, and the protection coordination between the incoming switch and the substation outlet switch cannot be reasonably set, it may cause the substation outlet switch to trip. If the fault is permanent and the substation reclosing fails, a local incident within a medium-voltage user boundary could result in a complete outage of the entire 12kV distribution line. By installing an outdoor boundary circuit breaker at the tapping point or user end (demarcation point of responsibility), which can automatically isolate the fault after confirming issues within the user boundary (such as overcurrent, short circuit, single-phase grounding, etc.), normal power supply for non-fault areas can be ensured. The application of the ZW32BF-12 outdoor boundary high-voltage vacuum circuit breaker on 12kV feeder overhead lines helps avoid collateral outage incidents on fault-free lines, reduces the scope of fault-related outages, shortens user outage durations, and protects the safe operation of the main grid. It also features fault detection, protection control, and communication functions, enabling automatic clearance of single-phase grounding faults and automatic isolation of phase-to-phase short circuit faults. The installation points are suitable for the responsibility demarcation points of user incoming lines on 12kV distribution lines, as well as compliant branch line connections.