Inverse-Time Over Current (ITOC) protection is widely used in medium and low voltage distribution networks because its protection operation time varies with the fault current. With ITOC protection, operation speed is easily affected by system operation modes, fault types and line series. In addition, the wide access of Distributed Generation (DG) has a negative impact on the selectivity and speed of ITOC protection. The basic principle of ITOC protection and the problem of long operation delay are expounded, and the impacts of DG on ITOC protection are analyzed. Considering the measured impedance at different fault locations, a novel Impedance Correction based Inverse-Time Over Current (ICITOC) protection scheme is proposed. The percentage of measured impedance and impedance correction index are used to modify the inverse time characteristic curve. This can accelerate the protection operation speed to the greatest extent while the coordination relationship between protections can be guaranteed. Theoretical analysis and PSCAD-based simulation results verify the correctness and effectiveness of the proposed ICITOC protection scheme. This work is supported by National Natural Science Foundation of China (No. 51477090).