Research On Flame Spread Of Wire And Cable Under Internal And External Heat Source

As one of the important infrastructures for people’s life and industrial production,wire and cable are widely used in various fields.However,the fire accidents caused by wire and cable occur frequently,which seriously restricts the development of the national economy and threatens people’s life.Moreover,in a real fire scene,it happens often that a lot of materials burn at the same time and place,so wire and cable flame spread is usually exposed to radiation from external heat source.Besides,wire and cable are the main carriers of power transmission.In fire,the current releases Joule heat and acts as an internal heat source to promote the wire and cable flame spread.Therefore,the research on flame spread of electric wire and cable under external radiation is carried out to explore the effect of internal and external heat sources,which has a great practical significance.Firstly,the polyethylene insulated copper wire was selected as the research object.The small-scale flame spread experiment of the electric wire under external radiation and internal current was carried out.The effects of external radiation and internal current on flame size,flame spread rate and melt dripping characteristics were investigated.The experimental results show that both external radiation and internal current can cause the equilibrium temperature of the insulating layer to rise,resulting in changes in the flame shape,flame spread rate and melt dripping during flame spread process.When the internal current or external radiation increases,the melting rate of the insulating layer is quicker.When the current is too high,a flowing flame is formed,resulting in a significant reduction in the flame height.When the current exceeds a certain value,the flame spread rate increases rapidly.In addition,by using the measured drop mass increase rate and drop frequency,the mass of a single molten droplet is obtained.Based on the measurement of the drop mass and the calculation method of the drop volume method,a predictive model of the dripping frequency of the molten insulating layer was obtained,and the results of verifying the model through the experimental measurement value show that the accuracy of the model is good.In the comparison of the experimental results of the external radiation effect and the internal current effect,it is found that they can preheat the wire to almost the same equilibrium temperature,and the flame height and flame spread rate are also relatively close.A flowing flame is formed in the current of 50 A,but no flowing flame in formed under the action of the corresponding radiation of 8.1 k W.Other current values are approximately the same as the drop frequency of their corresponding radiation.Therefore,on the whole,the influence of internal and external heat sources on the flame spread of wire is the same.Then,a PVC flame-retardant cable was selected.The combustion behavior experiment of the electric cable under external radiation and internal current was carried ou.The change of the flame shape under the effect of radiation and current was mainly analyzed.It is found that the equilibrium temperature of the cable is still proportional to the square of the current.When internal and external heat sources work together,the equilibrium temperatures of cables and wires can be predicted from each other.When the current is less than 30 A,the cable flame height is continuously reduced.When the current is greater than 30 A,the flame height appears in three stages: falling,rising and falling.With the increase of the external radiation,the flame height falling phenomenon caused by the withdrawal of the igniter gradually disappeared,and the rising rate continued to increase.When the current is too high,the cable flame separates,and the separated flames spread to both sides.Due to the complex pyrolysis and combustion process of the cable,the flame spread behavior of the cable presents a multi-stage spread phenomenon.During the flame combustion process of the cable,the phenomenon of dripping still occurs,and the change of the flame shape caused by the dripping is the same as that of the wire.