The effects of external heat radiation on combustion and toxic gas release characteristics of flame retardant cables, which were XLPE insulated, flame retardant PVC sheathed and steel armoured cables, were investigated. The combustion characteristics of the level A and the level C flame retardant cables were explored by the cone calorimeter. For the level C cables, heat release rate (HRR) and CO concentration in cable fires increased and the ignition time decreased with increase of the external radiation heat flux. For level A cables, the HRR and CO concentration showed two-stage variations with the external radiation heat flux. When the external radiation heat flux was smaller than 35 kW/m2, the cable self-extinguished quickly after the ignition. When the external radiation heat flux was larger than 50 kW/m2, the cables showed continuous burning phenomena after the ignition. The level A cable had smaller HRRs compared with that of the level C cable under the same external radiation heat flux. However, the CO concentration of level A cable was remarkably higher than that of the level C cable in the present study. The high CO release rate of cable with well flame retardant ability under large external radiation heat flux requires more caution in the cable fires.

To evaluate the fire performance of the cable in service, the ignition time, heat release rate and insulation failure parameters of the accelerated thermal aging cable on stimulated fire condition were studied. The results show that, the extended ignition time and decreasing peak of heat release rate of the cable in the early stage of aging are the addictive including the lubricant, plasticizer and anti-oxygen. In the aggravating of thermal aging, labile materials with low heating value will be exhausted, while the thermal stability of insulation sheath material decreases, its corresponding ignition time will start to shorten and initial peak of heat release rate will start to rise. The insulation failure temperature of the cable is related with the decomposition temperature of insulation materials of the cable by heating. In each aging stage, the decomposition temperatures of insulation materials by heating are all lower than the insulation failure temperature of the cable for about 10°C. The reason for the insulation failure of the cable is that the ambient heat , gives rise to the decomposition by heating of internal insulation materials of the cable through sheath materials finally under the thermal transmission effect.