I. Overview 10KV overhead distribution line T when connected branch line or the end user an internal malfunction, because the incoming line switching substation protection action time and time with the outgoing switch protection can not reasonably tuning, will result in qualifying substation switch tripping. If the nature of the failure is permanent, the substation will coincide unsuccessful, then a partial pressure of accidents within the user community will make the whole 10KV power distribution lines. It should take place in T or client (demarcation point of responsibility) to install a cutoff switch user (commonly known as the watchdog), after a judge confirmed that occurs in the user interface fault (overcurrent, short circuit, single-phase ground fault), automatic fault isolation, you can ensure that non-fault zone normal use of electricity users. ZW20A-12F user boundary Outdoor vacuum circuit breaker is my company developed a user-boundary switch. It mainly consists of ZW20A-12 type outdoor column vacuum circuit breaker body and FDR-02 fault detection controller and external voltage transformer has three major components, both through the air outlet and outdoor seal control cables for electrical connections. With fault detection, protection and control functions and communication functions, can reliably determine, within the detection boundary and the outer boundary of milliamp current and zero-sequence phase short circuit fault current, automatic removal of single-phase fault and phase fault.