LAUSR

Summary The mechanism of how and when an arcing short or a physical short occurs on a polyvinyl chloride (PVC)-insulated cable when exposed to radiant heat flux has been found experimentally. How this mechanism acts on the size of the resulting arc beads is also considered. As a physical short usually changes into an arcing short in an instant, it is difficult to determine which type of short circuit occurs as the first phenomenon. To solve this difficulty, a short circuit current was limited to the order of an ampere, which is large enough to trigger an arcing short, and short circuit tests were conducted with a PVC-insulated cable. Before an arcing short, a small amount of leakage current in the order of milliamperes was observed flowing through PVC insulation, which gradually increased and finally turned into an arc. On the other hand, no leakage current was observed before a physical short. These results indicate the following mechanism: When PVC insulation between conductors in a cable melts when a heat flux is applied, conductors often come close to and into contact with each other. In this case, the leakage current is unrecognizably small because the PVC insulation is not yet carbonized and still has a high resistivity. Meanwhile, when conductors do not come into contact with each other, PVC insulation is gradually carbonized and the leakage current increases until an arcing-through-char occurs. Copyright © 2016 John Wiley & Sons, Ltd.