Reassessment of the Validity of Primary Arc Bead Generation Methods: Ignition Experiments and EBSD Analysis

This study comparatively analyzed the experimental methods used in previous studies to generate primary and secondary arc beads, with particular emphasis on the conductor temperature conditions. On this basis, the suitability of these methods was critically examined, and the conventional method for generating primary arc beads was considered inconsistent with the definition of a primary arc bead, which should be associated with a short circuit that initiates a fire. To verify this hypothesis, two experiments were conducted. In the first experiment, specimens were generated by short circuiting conductors at room temperature using the same method as in previous studies, and the ability of the resulting arc to ignite adjacent combustible materials was evaluated. In the second experiment, which was designed to produce ignition at the time of the short circuit, a poor electrical connection was first introduced at a receptacle to thermally degrade the combustible material and raise the conductor temperature; a short circuit was then intentionally induced to generate the specimens. The specimens produced in both experiments were prepared and their cross-sectional microstructures were analyzed by scanning electron microscopy–electron backscatter diffraction (SEM-EBSD). Misorientation was quantitatively measured and compared. In the first experiment, cotton was the only combustible material ignited by the arc, whereas in the second experiment, the receptacle insulation ignited when the short circuit occurred. In the comparison of low-angle misorientation in the arc beads generated by the two experiments, the mean values for all specimens from the second experiment were higher than those from the first experiment, and the overall difference between the experiments averaged 32.15%. The t-test results showed a statistically significant difference between the two groups. Furthermore, the standardized mean difference between the two groups was substantial, indicating a clear distinction beyond statistical significance. These results raise questions regarding the representativeness of primary arc bead specimens generated using conventional methods and indicate that the experimental method for generating primary arc beads should be re-established to enable reliable arc bead discrimination.