Investigations on Metallographic Structure of Sparks formed as the Severe Overload Melts a Stranded Copper Conductor

Abstract

If the overload is severe, the conductor may become hot enough to ignite fuels in contact with it as insulation melts off. The sleeved insulation is the typical effects indicating the overload causes the fire. However, it may be difficult to identify the overload in the circuit because the sleeved insulation disappears easily in fire. Compared to the sleeved insulation, the re-solidified copper sparks are less easily to be damaged for locating lower in fire. Those sparks can assist in identifying that some circuit, which became hot and was melted by a large overload before the fire. In this paper, the 2.5mm2 PVC stranded copper conductors, as the most common in Chinese residential electrical systems, was melted by large overload at different current in order to produce overload sparks. Through the analysis of internal metallographic structure of overload sparks, there are significant differences from sparks formed by short circuit and globules exposed to laboratory flame testing. The results reveal that when the current through the conductor is greater than 5Ie(Ie=34A), the conductor would be melted in two and produce a parting arc throwing some sparks; the larger current through the conductor was, the more luminous the parting arc was, and the more sparks produced; In the internal metallographic structure of sparks caused by large overload, there are more dendritic segregations than that of globules caused by flame testing and less dendritic segregations than that of short-circuit beads. The larger the current flows in a conductor, the more serious dendritic segregations in internal structure of sparks are. Those metallographic structure features can be used to identify the sparks formed by the large overload as opposed to other globules and sparks in fire investigations.