FIRE HAZARD CAUSED BY THERMAL DEGRADATION OF ORGANIC INSULATING MATERIALS AT PLUG AND RECEPTACLE CONNECTION

摘要

In cases of fires from plug and receptacle connection, it has been generally thought that the main cause of fire is arc tracking, which results from the pollution on the surface of a plug. But insulating materials of plugs and receptacles has changed from low tracking resistance materials such as Bakelite resin into PVC and urea resin that has high tracking resistance. Tracking resistance of the insulating materials is so high that an arc tracking on the surface cannot easily occur. If an overcurrent flows through a loose connection of a plug and a receptacle, heat may be generated. We suppose that the heat may degrade insulating materials of plugs and receptacles. Actually we have frequently found thermally deformed and discolored plugs in ordinary dwelling houses. In these cases, we think the insulation performance and the tracking resistance deteriorates and the fire hazard at plug and receptacle connection increase. In this paper, we discussed the fire hazard caused by thermal degradation of organic insulating materials at plug and receptacle connection. We researched into the temperature rise at various loose connections of plug and receptacle with an overcurrent and tracking resistance of plug insulating materials degraded thermally by heating in an electrical furnace. Conclusions are as follows: (1) The temperature of loose connection of plug and receptacle exceeded 200℃ with an overcurrent and the average temperature was more than 150℃. (2) Though PVC and urea resin had high tracking resistance before thermal degradation, by heating for a short time at 150 up to 200℃ tracking resistance easily deteriorated. (3) It is clarified that the fire hazard increases by thermal degradation of insulating materials.