Fault detection and localization of an electrical cable are essential to prevent a serious accident of cable system originated from the cable failure. Despite the outstanding performance in fault detection and localization, time-frequency domain refletometry (TFDR) faces an important issue of reliability of the diagnostic result. In this paper, skewness of time-frequency cross-correlation is used as the additional index to examine the existence of the unrevealed fault. In order to verify the validity of the proposed method, simulation is carried out with various types of fault occurrence circumstances. The analytic discussion on the simulation results is presented, and it is found to support effectiveness of the proposed method. It is expected that the proposed method will contribute to enhance the diagnostic performance of TFDR.