Mechanisms and modes for ignition of low-voltage, PVC-insulated electrotechnical products

Abstract

SUMMARYPVC 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. The electrotechnical products considered include insulated wires, cables and cords, and also appurtenant termination devices, e.g. male plugs or female taps. Well-known factors leading to the ignition of PVC-insulated wiring and related products 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 1608C for the material to become semiconducting during shortterm exposure (around 10 h), while longer-term exposure (around 1 month) may cause failures at temperatures as low as 1108C. Some limited data exist which suggest that standard UL and IEC temperature classifications are unduly optimistic, as applied to PVC. Fire can originate if wiring or equipment cannot withstand a powerline surge. 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 and IEC standards. Data are presented showing that the IEC 60112 wet-tracking test gives especially misleading results for PVC and should be improved or abrogated.

Research on Electrical Fires: The State of the Art

“…Perhaps the topmost issue which has received no useful study is the breakdown voltage of plastic insulation materials at sub-kV voltages. Even for the most common low-voltage insulator, PVC, there are no published data [14]. The situation is so poor here that even the functional form of the governing relation (electrode gap distance vs. breakdown voltage) is unclear.…”

“…Much of the published research has been conducted using standardized International Electrotechnical Commission (IEC) or ASTM tests [9,10] tests. Unfortunately, what occurs in these tests has almost no relation to performance of real cables [14] and the tests, especially IEC 60112 [9], are misleading. Fig.…”

“…Perhaps the most unusual electrical phenomenon identified by Japanese researchers has been the propensity for PVC-insulated devices to fail by means of self-induced wet-tracking, while located in an atmosphere which is at very low humidity [14]. This is due to the fact that calcium carbonate is almost universally used as a filler in wire and cable grades of PVC.…”

“…Thus, the question arose: Are there aspects of the dielectric breakdown of PVC which need to be understood to explain these fires? The result of this investigation was a recently (2006) published paper [14]; it is striking that this was the first-ever paper in the literature to endeavor to examine in detail the mechanisms of ignition of PVC-insulated wiring. Two important issues were documented in this study: (a) voltage surges (Fig.…”

Effects induced by gamma irradiation on free‐volumes, mechanical, and thermal properties of flame‐ and non flame‐retardant polyvinylchloride

“…Polyvinylchloride (PVC) is an important material in cable insulation and jacketing, because it is considered itself as nonflammable due to the presence of chlorine in its structure. 1 The flame-retardancy of PVC can be improved by incorporating flame-retardant chemical elements as additives or as reactive which may significantly influence mechanical, thermal and electrical properties. 2,3 Radiation cross-linking of wire and cable insulation was first introduced by Ray Chem Corp., USA.…”