This paper deals with the optimal use of the ELB (Electric Leakage circuit Breaks) for the protection of low voltage distribution system. Since the ELB is recently used together for grounding fault while only the ground resistance was used in the past, the analysis of ELB characteristics is indispensible. By the quantitative analysis and the setting of limit the ground resistance and the ELB is derived and then the optimal use of the ELB is expected to contribute to the system protection.