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 serious problems. We have been trying to monitor the degradation of cable insulation by measuring the magnitude and phase angle of the 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 partially damaged by peeling off their insulation layers, or partially aged by heat and γ-rays. The impedance and phase angle were measured from a terminal of the cable. The difference in impedance between the damaged and sound cables is made clear by fast Fourier transform analyses, from which the damaged portion can be located. It can be clearly shown that this method has the potential ability to detect the degradation of cable insulation induced by physical damage, γ-ray irradiation, and thermal aging.