Experimental study on the fire spread behavior of downward-bending cables

Due to the height differences during the power transmission process, the bending installation of cables is a common method. The stress in the bending section of cables is usually relatively concentrated and more susceptible to damage, leading to a greater fire hazard. According to the different bending forms, the bending cables can be divided into upward-bending cables and downward-bending cables.

An experimental study was conducted to investigate the effect of the bending angle and number of cables on the flame spread behavior of downward-bending cables.

Results show that: (1) the peak temperature on the cable surface of downward-bending cables gradually increases, as the number of cables increases. The temperature peak of 5 downward-bending with 60°bending angle was about 782.3℃, which was 1.8 times higher than that of a single cable at the same angle. This is mainly due to the fact that combustion of multiple cables laid side by side produces more combustible pyrolysis gases, while the flame has a stronger preheating effect on the cables. (2) As the bending angle increases the flame spread time of downward-bending cables is shortened and the average flame spread rate increases. Under the ignition condition of five 90°downward-bending cables, the average flame spread rate of the cables was 5.4 cm/min, which was 1.9 times of that of five 0° downward-bending cables; under the ignition condition of five 90°downward-bending cables, the temperature peak reached 868.3℃, which was about 1.4 times of that of that of five 0°downward-bending cables. This is mainly due to the larger bending angle, which was mainly due to the difference in the role of the flow of melt drippings on the cable under different bending angles, when the flame spread in the inclined section of the cables, the flow of high-temperature melt drippings was an important driving force to ignite the cables in the unburned section. As the bending angle increased, the force of gravity increased in the direction of the cables, and the drippings were more likely to flow downwards under the combined effect of the gravitational component force, surface tension, friction of the melt drippings. The melt drippings had a more significant effect on the preheating of the unburnt section of the cables, which in turn shortens the ignition time of the cable and increased the average flame spread rate. In addition, as the bending angle increased, the "flame attached" effect was more significant, which increased the thermal convection and thermal radiation in the unburned section of the cables.

The peak temperature on the cable surface of downward-bending cables are positively related to the number of cables and the bending angle. The average flame spread rate increases as the number of cables and the bending angle increase. Regressivity analyses between flame temperature and the number of cables were carried out to analyze the flow mechanism of melt drippings in inclined sections of cables in this work, these correlations are well described by physically based models for all the experimental results.