This paper introduces the differential protection’s new principle’s appliance and the principle of RCS -915 numerical bus-bar protection. In fact, when the fault occurs at dead zone ,the voltage of the bus-bar is normal ,while there is some problem in voltage fuse of secondary of PT, so, secondary voltage can not reflect the primary , as a result , the result is not true, to expend the outage dimension. By analysis , the paper proposes a new program, which can avoid the default and reduce the outage dimension.