Ignition of plastics from electric faults

by Dr. Vytenis Babrauskas, Fire Science and Technology Inc.

“This plastic is V-0, VW-1, etc. rated–How come it burned in a fire caused by an electric fault?” This question is commonly asked. The plastic in question can be PVC wire insulation, or the plastic parts of an outlet, wall plate, or various other insulating materials which are in close proximity to current-carrying parts. We can assume the rating of the plastic part was correctly determined in a laboratory. Better yet, a person can conduct an ad hoc small-flame test and find that, indeed, the item burns only very little and certainly does not spread fire. So why did it burn unexpectedly in a real incident or spread flame?

To understand the situation requires that we realize one important aspect of UL 94 and various other small-flame tests: All of these tests are conducted on specimens which are at room temperature. A different situation can happen in the case of an electric fault. The electric fault will preheat the plastic to a temperature much higher than room temperature. When a preheated specimen is confronted with an igniting flame, rapid flame spread can occur. At least two electric fault categories can cause this type of behavior:
(1) a poor connection, for example, at a screw terminal of an outlet or at a place where a wire is crimped onto the prong of a male plug.
(2) broken wires in a flexible cord. Substantial heating may occur if only a single strand remains.
Probably other fault modes also exist that can lead to the same outcome, but these two have received at least some scientific study (although nowhere near enough!).

There is nowhere near enough research to comfortably quantify this phenomenon, but at least sufficient research exists so that the basic concepts are validated:

[1] Lupton, E. C., Tahlmore, C. D., and Obsasnik, J., Some Differences Noted in the Flammability of Wire Constructions between Testing at Room Temperature and at Elevated Conductor Temperatures, pp. 1-3 in Proc. of 24th Intl. Wire and Cable Symp.,U.S. Army Electronic Command, Cherry Hill, NJ (1975). NTIS No. ADA 017 787. This study, although brief, documents that substantially worse fire propagation results are obtained if specimens are preheated, as compared to their room-temperature performance.

[2] Sletback, J., Kristensen, R., Sundklakk, H., Navik, G., and Munde, R., Glowing Contact Areas in Loose Copper Wire Connections, pp. 244-248 in Proc. 37th IEEE Holm Conf. on Electrical Contacts, IEEE (1991). This study documents that temperatures of around 1200°C can be found at glowing screw connections.

[3] Oda, S. J., Progress Report–Fire Initiation Potential of Failing Electrical Receptacles (Report 78-92-K), Ontario Hydro, Toronto (1978). This study documents ignitions of combustibles due to glowing connections.

[4] Nagata, M., Firing Current and Energy Input of Polyvinyl Chloride Covered Cords Having Disconnected Element Wires [in Japanese], Bull. Japanese Assn. Fire Science and Engrg. 33:1, 1-7 (1983). This study documents the mechanism by which PVC-insulated cords ignite when all strands are broken except for the final strand.