Abstract: TN-C-S system is widely used in China. Due to the large inherent residual current and the difficulty in distinguishing the residual current mutations caused by load switching or faults, level 1 and 2 leakage protection cannot be put into operation. Serious safety hazards exist in TN-C-S systems. To solve this problem, a leakage protection method based on the ratio of the residual and unbalanced current mutation vectors is proposed. First, the residual current loop is analyzed, and it is concluded that the ratio of residual current and unbalanced current mutation vectors changes with the grounding faults but not with the load switching. Then, the mutation of the fault current is calculated based on the stabilization of the neutral line impedance and the system grounding impedance and leakage faults are detected with the criterion that the calculated fault current exceeds certain threshold. Simulations and tests prove that the proposed method can effectively distinguish the residual current variation caused by load switching and leakage fault, and realize the leakage fault detection on the condition of large inherent residual current. The detection sensitivity in single-branch and multi-branch systems is 70 and 150 mA, respectively, satisfying the sensitivity requirements for the multi-level leakage protection of TN-C-S systems.