Effect of insulation melting and dripping on opposed flame spread over laboratory simulated electrical wires

In electrical wires with insulations that bum and melt, the dripping of molten insulation can change the wire fire behavior, ignite nearby objects, and enhance the fire spread. Dripping is a result of gravity and depends on the insulation type of the wire and its orientation. In this work, the opposed flame spread over simulated electrical wires was studied with emphasis on the effect of the core and insulation type, and the melting and dripping of insulation. To facilitate the study, laboratory wires with polyethylene (PE) as insulation, were selected for the experiments. Horizontal and vertical wires of 8- and 9-mm diameter with soild copper (Cu) and hollow stainless steel (SS) cores and two types of PE insulations, low density and high density, were tested. The sizes of the laboratory wires were selected to facilitate the study of the effect of the type of insulation, the ratio of insulation to core thickness, or the thermal properties of the core, on the wire fire behaviors. Experimental results show a strong dependence of wire orientation on molten insulation dripping and flame spread. For horizontal wires, the flame spread is faster with Cu core than SS core because of a larger heat transfer ahead of the flame through the core. For vertical wires, the flame spread rate is dominated by the downward dripping of the molten insulation, but is comparatively not sensitive to the core material. Increasing the opposed flow speed, the flame gets closer to the wire which enhances the heating from Cu core and locally increases the flame spread. The effects of other parameters such as oxygen concentration, wire diameter, and insulation material are also discussed. This work provides important support to a larger project aimed at studying the fire behavior of electrical wires in a spacecraft environment. Without gravity, the dripping of molten material will not occur in a spacecraft, thus, characteristics of the flame spread process over a wire insulation material that melts during the spread of the flame will be drastically different on Earth or in a spacecraft.