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Summary It has been discovered through experimentation the mechanism of how and when an arcing short or a physical short occurs on a cable when exposed to a radiant heat flux. It is then considered how this mechanism acts on the size of the resulting arc beads. Another discovery was that a leakage current precedes a short circuit. Subsequently, it was found that arcing shorts and physical shorts can be distinguished based on the amount of leakage current that precedes the short circuit. In the case of an arcing short, the leakage current is large and the arc bead is small. The opposite is true for a physical short. In the case of a physical short, the leakage current is small and the arc beads are large. Tests were conducted with a Japanese polyvinyl chloride (PVC)–insulated sheathed cable, an American PVC–sheathed non‐metallic (NM) cable, and an American cloth–sheathed NM cable. The following were found: A Japanese PVC–insulated sheathed cable tends to cause a physical short and to form larger arc beads than other cables. American NM cables tend to cause an arcing short. Especially, an American cloth–insulated cable tends to be preceded by a larger leakage current and to form smaller arc beads than other cables. This result corresponds to the experimental description in NFPA 921 concerning arc beads on a cloth‐sheathed NM cable.