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Abstract
Flame spread over wire surface is different from other solid fires as it is usually accompanied by melting and dripping processes. Although the related behaviors at reduced pressure (20–100 kPa) are significant to those fire risk evaluations, very few studies have been undertaken on this matter. Therefore, the thermoplastic dripping and flame spread behaviors of energized polyethylene insulated copper wires were investigated experimentally at reduced pressure. It was known from experimental results that the dripping frequency increases, showing a relatively smooth (linear) and rapid (power) increasing trends under high and low electrical currents, respectively. A short-period flame disappearance was observed during the dripping process, which is unique for the energized wire at reduced pressure. The bright flame can disappear for several seconds and then show again after the dripping. While at 20 kPa or lower, the wire flame would turn to a completed extinguishment after the dripping. A critical dripping point was proposed to show the minimal required electrical current to sustain the flame spearing. The critical current changes smoothly during 100–80 kPa and decreases rapidly at 80–60 kPa. Additionally, the dripping phenomenon can stop or delay the flame spread, partly because of the short-term flame disappearance.
Highlights
Thermoplastic polymer material, such as polyethylene and acrylonitrile butadiene styrene plastic, have attracted considerable attention in research and industry [1,2]
It can be seen that Flame Spread Rate (FSR) is generally higher for the Cu-core wires than those of Fe-core and NiCr-core wires, especially for those energized wires
Several conclusions could be addressed as follows: 1. It was known from experiments that the dripping frequency increased under lower ambient pressure, showing different trends under various electrical currents
Summary
Thermoplastic polymer material, such as polyethylene and acrylonitrile butadiene styrene plastic, have attracted considerable attention in research and industry [1,2]. The dripping of molten thermoplastic can greatly increase the level and risk of a fire as it could result in the ignition of surrounding combustibles and a quick expansion of the burning front and area For those wires under overcurrent, the flame spreads relatively faster along with the energized wire under the molten and dripping matrix [3]. The influencing factors of the relevant fire spreading considered in these studies including inclination angles [5], AC electrical fields [6], oxygen concentration [7], external radiation intensity [8], ambient pressure [9,10,11], and gravity [12,13,14,15] Besides these influencing factors, their ignition behaviors were investigated under various scenarios, such as short-term overcurrent [16,17], external heat flux [18], arc formation [19,20], subatmospheric [21,22], and microgravity [23,24]. A few studies have been carried out on investigating the thermoplastic properties of those melt drips [25,26,27,28]
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