In order to improve the existing method of electric network reliability evaluation, the basic concepts, for example, the flow through the failure events, the sent ends and received ends of these failure events, the load curtailment region of the received ends and the degree of the load curtailment, are defined. With these concepts, the connotation of LCSN (Load Curtail Strategy by Near) is enriched and algorithms of the load curtailment region are conducted. The theory foundation for further exploring LCSN is created. The results show that the algorithm can quickly and effectively make reliability assessment of power systems.