Location attempt of a degraded portion in a long polymer-insulated cable

The ability of frequency domain reflectometry (FDR) for locating a portion in a cable longer than 1.0 km, where either degradation or abnormality occurred in its insulation, was evaluated by conducting two experiments. First, a 72-m coaxial cable insulated with low-density polyethylene (LDPE) was locally degraded concurrently by heat at 100 °C with gamma irradiation to 312 kGy. Then, this cable was connected to two 1000-m coaxial LDPE cables at either end. Furthermore, a 2-m coaxial lead LDPE cable was connected to a network analyzer, forming a cable of a total length of about 2074 m. The FDR method successfully located the degraded portion and also the connecting points in this long cable. It has also become clear that the FDR method can locate a 30-cm part in a 1000-m coaxial LDPE cable at a position about 900 m from its end when it was heated temporarily using a ribbon heater.