Abstract: Mechanical damage such as animal gnawing and squeezing may cause partial loss of insulation layer and core of the wire, resulting in increased local resistance and heating, leading to electrical fires. A specialized electrical fire fault simulation platform was established, to systematically conduct research on heating and ignition. The resistance increment is controlled by cutting the ZR-BVR multi-strand copper wire core by core. The representative combustible material was powdered pine wood. Infrared thermography, high-speed videography, and metallographic microscopy were used to analyze the thermal behavior, ignition boundaries, and morphology of melting marks. The results indicate that two necessary conditions must be met for ignition at defective wire sections: sufficient release of flammable pyrolysis gases from the combustible material, and the formation of a breaking arc upon wire fusing. Excessive or insufficient defect severity failed to trigger ignition. Combustion occurred primarily under the conditions of 2 strands at 2 cm, 4 strands at 8~10 cm, and 5 strands at 10 cm. Melt mark morphology was correlated with electrode polarity: spherical at the anode and pointed at the cathode. When ignition occurred, the metallographic structure exhibited equiaxed crystals, large pores, and indistinct transition zones. Without ignition, the structure appeared as fine dendrites with well-defined grain boundaries. Local strand defects in energized copper wires pose a significant potential fire risk. Accurate localization of defects, combined with microstructural analysis of melting marks, can provide robust evidence for determining fire causation and fault attribution in electrical fire investigations.

Key words: fire investigation, electrical fire, wire defect, metallographic structure