Abstract: Wire overheating fault is one of the prevalent intrinsic fault types in electrical circuits, typically triggered by overcurrent, which can induce thermal decomposition and carbonization of insulating materials, thereby precipitating faults such as short-circuit breakdown, open-circuit. Previous studies have predominantly focused on the thermal decomposition characteristics and breakdown behaviors of sheathed wires under high overcurrent conditions, while research on the impact of slight temperature increases on the microscopic physicochemical properties of insulating materials under slight overcurrent conditions is insufficient. In this study, BVVB polyvinyl chloride-insulated copper conductors were selected as the research subject. Slight overcurrent samples were prepared utilizing a self-developed electrical fault simulation experimental setup, and then the relationship between the changes in insulation performance and their microscopic physical and chemical structures under slight overcurrent was analyzed using insulation resistance tester, synchronous thermal analyzer, Fourier transform infrared spectrometers (FTIR) and field emission scanning electron microscopes (FESEM). The results indicate that the short-term overcurrent does not rapidly deteriorate the insulation performance; instead, it enhances the electrical insulation performance in stages. The results of TG, FTIR, and FESEM prove that this phenomenon is due to the formation of conjugated polyene structures. This study demonstrates that slight overcurrent can progressively improve the insulation performance of wires over a short duration. By measuring the changes in the insulation impedance of the wire in a timely manner, it is possible to detect electrical fires earlier and promptly mitigate potential electrical fire risks.

Key words: electrical fire, slight overcurrent, insulation impedance, pyrolysis kinetics, microscopic physical and chemical structure