A new method called the broadband impedance spectroscopy (BIS), which is a kind of frequency domain reflectometry, is being developed as a reliable method for locating a degraded portion of a low voltage cable. The sensitivity of the BIS method is compared to that of the time domain reflectometry (TDR). In the first case, the sheath and insulation of a 50-m-long polyvinyl chloride insulated cable were partially peeled off. In the second case, cables insulated with silicone rubber, flame-retardant ethylene propylene rubber, or cross-linked polyethylene, each about 25 m long, were aged simultaneously by heat and γ-irradiation in air. In both two cases, a clear peak indicating the damaged or aged portion appears in the BIS measurement, while no such a peak appears in the TDR measurement. Therefore, the BIS method is much superior to the TDR method for locating a degraded portion in a cable.