Investigation on Fire Damage Caused by Breakage of Shielding Layer in Extruded Three-layer 6.6kV XLPE Cables

Breakage of shielding layers is one of the factors for degradation of extruded three-layer (E-E type) 6.6kV cross-linked polyethylene (XLPE) cables. When a shielding layer breaks, where a charging current flows through an outer semiconducting layer, it is possible that a potential difference would arise at the gap of the shielding layer and temperature of the outer semiconducting layer increases, which may result in a fire accident of the cable. To investigate its mechanism, we applied an AC voltage as the potential difference to the gap between copper tapes which simulated a breakage of a shielding layer of an E-E type 6.6kV XLPE cable, and measured a current flowing through an outer semiconducting layer and temperature between the gap until the outer semiconducting layer caught fire. As a result, it was found that the temperature of the outer semiconducting layer between the gap exceeded 150°C immediately after the voltage application, and increased up to 250°C and higher just before the fire damage occurred. This result suggests that the temperature reached the ignition point of the outer semiconducting layer.