MECHANISMS AND MODES FOR IGNITION OF LOW-VOLTAGE PVC WIRES, CABLES, AND CORDS

Abstract

PVC is the most common insulation material used for wiring in low-voltage (LV) service. “Low-voltage,” in the context of this paper, is taken to be 120 – 240 VAC. “Wiring” is taken to include not only insulated wires, cables, or cords, but also the appurtenant termination devices, e.g., male plugs or female taps. Well-known factors leading to the ignition of PVC wiring include: (a) manufacturing defects; (b) grossly excessive current; (c) over-insulation, sometimes augmented by overcurrent; (d) localized heating due to strand breakage; (e) localized heating due to mechanical strand severing by staples or nails; and (f) localized heating due to failed terminations. Other failure modes are known but have received only limited study. These include (i) excessive force and creep; (ii) chemical interaction effects; and (iii) breakdown under voltage surge conditions. Additional research is needed in these areas. The proximate cause of ignition involved with many of the above mechanisms is arc tracking (arcing across a carbonized path). In turn, it is shown that PVC is especially susceptible to becoming charred, it requiring only approximately 160oC for the material to become semiconducting during short-term exposure (around 10 h), while longer-term exposure (around 1 month) may cause failures at temperatures as low as 110oC. Some limited data exist which suggest that standard UL and IEC temperature classifications are unduly optimistic, as applied to PVC. Mains-connected electrical appliances need to be designed to resist 6000 V surge voltages, even though this is not mandated in most of the current UL standards. Data are presented showing that the IEC 60112 wet-tracking test gives especially misleading results for PVC and should be improved or abrogated.