Study On Detailed Swelling And Ignition Mechanism Of Flame-retardant Cable In The Annular Heating Condition With Increasing Heat Fluxes

The cables and cords are widely used in industrialized and informationalized society for the transmission of power and information,and massive cables were produced and used annually in our country.The sheaths and insulations of commercial cables are mainly made of organic polymer,thus plenty of fire loads contained in cables.Once the interconnected cables ignited by electric arc,short circuit or other electrical failures,the smokes,gases and heat released from the cable fire will be terrifically harmful for people's health.Nowadays,some high security level places,such as nuclear station,are required to use the flame-retardant cables according to national standard.The typical flame-retardant PVC cables will release lots of gases and swell fiercely when it is heated,and porous char layer will be generated for insulating heat and oxygen from outside.Those make the ignition behaviors of flame-retardant cables more complex.Aiming at cylindrical cables,with the research background of transient heat flux fire environments,the bench-scale experiments for the study of swelling,ignition and flame spread behaviors of 0.1 m-length and 1 m-length cable samples under the transient heat flux(t2 heat resource)were conducted based on a self-made annular heating facility.Also,the effect mechanisms of thermal swelling,crack,phase transition,and the heat transfer between inner layer and outer layer on ignition and flame spread of flame-retardant cables were analyzed in our study.To better understand the detailed decomposition chemistry,swelling and spontaneous ignition process,thermogravimetric(TG)experiments were conducted respectively for the retardant sheathes(PVC)and insulations(XLPE).The results of 0.1 m-length cable experiments show that the thermal deformations of flame-retardant cables contains five stages:inert,fluctuation,swelling,shrinkage and end stable stage,and all these stages have a significant influence on the ignition of multilayer cables.Critical surface temperatures,critical heating conditions and the relationship between the parameters of ignition times(tig)and integral of heat flux(Qig)are discussed in our study based on all of the ignition data.In the lm-length cable experiments,an apparent two-stage burning behavior was found,and with the increasing of electric heating power,the two burning stages merged together gradually.Finally,the decomposition kinetics and parameters of PVC sheath and XLPE insulation were obtained through the TG-DTG analysis.In addition,the results of TG analysis can validate and explain the swelling,ignition and flame spread characteristics with detailed reaction mechanisms.