HOW PVC-INSULATED CABLES EXPOSED TO RADIANT HEAT FLUXES CAUSE SHORT CIRCUITS AND ARC BEADS

摘要

We found experimentally the mechanism how and when an arcing short or a physical shortrnoccurs on a PVC-insulated cable which was exposed to radiant heat flux. We also considered how thisrnmechanism acts on the size of resulting arc beads. As a physical short usually changes into an arcingrnshort in a moment, it is difficult to determine which type of short circuit occurs for the firstrnphenomenon. To solve this difficulty, a short circuit current was limited to the order of an amperernwhich is large enough to trigger an arcing short, and short circuit tests were conducted with a JapanesernPVC-insulated cable.rnBefore an arcing short, a small amount of leakage current in the order of milliamperes wasrnobserved flowing through PVC insulation, which gradually increased and finally turned into an arc.rnOn the other hand, any leakage current was not observed before a physical short. These results indicaternthe following mechanism. When PVC insulation between conductors in a cable melts due to a heatrnflux applied, conductors often come close to and into contact with each other. In this case, a leakagerncurrent is unrecognizably small because the PVC insulation is not carbonized yet and still has highrnresistivity. Meanwhile, when conductors do not contact each other, PVC insulation is graduallyrncarbonized and a leakage current increases until an arcing-through-char occurs.rnThe same tests without limiting a short circuit current were conducted with a Japanese PVCrnsheathed cable, an American PVC sheathed NM cable and an American Cloth sheathed NM cable. Thernfollowing were found: A Japanese PVC sheathed cable tends to cause a physical short and to form arnlarger arc beads than other cables. American NM cables tend to cause an arcing short. Especially, anrnAmerican cloth-insulated cable tends to be preceded by a larger leakage current and to form a smallerrnarc beads than other cables. This result corresponds to the experimental description in NFPA 921rnconcerning arc beads on a cloth sheathed NM cable.