Precise location of the excessive temperature points in polymer insulated cables

This paper demonstrates that it is possible to locate very precisely a point showing a high temperature in a polymer insulated cable. The method is based on frequency domain reflectometry and inverse fast Fourier transform. The cables tested are a coaxial communication cable with a length of 32 m insulated by low density polyethylene and a flat in-house electric cord with a length of 21 m insulated by polyvinyl chloride. The cable or cord was heated at different positions for different lengths. The ratio between the powers of electromagnetic waves incident to and reflected from the cable was measured using a network analyzer in a frequency range from one to several hundred MHz or 1.5 GHz. The spectra obtained by the measurements were then analyzed by inverse Fourier transform. As a result, the position exhibiting a temperature higher than the adjacent points can be located with a spatial resolution as short as 2.5 cm. It was also confirmed that the sensitivity or spatial resolution can be improved by an increase of the highest measurement frequency.