Abstract: In order to better solve the selectivity of small-current ground fault protection, a scheme for protection of small-current ground faults based on inverse time-limit characteristics is proposed. This paper first analyzes the characteristics of transient zero-sequence currents in small-current grounding faults, and explains the problem that conventional time-invariant over-current protection cannot meet the time-level difference at the same time coefficient. For the characteristics of the line parameters, find an action time equation that is suitable for this line. After a series of processing such as ROUND function selection, the operation time of the switch is obtained. This time meets the coordination of the switching time difference in the line, and is suitable for lines of different lengths. The feasibility of this scheme was verified by ATP simulation, satisfying the requirements of selectivity and quickness.