Location feasibility of degradation in cable through Fourier transform analysis of broadband impedance spectra

Since electric cables play important roles such as power supply and information transmission, their degradation may cause a serious problem. We have been trying to monitor the degradation of cable insulation by measuring the magnitude and phase angle of impedance as a function of frequency in a very wide frequency range. The cables tested were insulated with flame-retardant ethylene propylene rubber or special heat-resistant polyvinyl chloride. They were damaged partially by peeling off their insulation layers, or aged partially by heat and γ-rays. Impedance and phase angle were measured from a terminal of the cable. The difference in impedance between the damaged and sound cables becomes clear by fast Fourier transform analyses, from which the damaged portion can be located. It can be clearly shown that this method has a potential ability to detect the degradation of cable insulation induced by physical damage, γ-ray irradiation, and thermal aging.