To secure a service continuity is one of the most important mission in the ower distribution system operation. In this paper the necessary and sufficient condition to guarantee 100% service restoration capability for any fault on the system is reported. An evaluation methodology of the restoration capability(or restorability) is developed based on the developed restoration conditions. Applications of the developed concept to the system operation in the normal and emergency states, that would enhance the supply reliability of the system are described. They include enhancement of restoration capability adapting to load change, identification of best open switch and supervised switch positions.