Ignition-to-spread transition of externally heated electrical wire

Ignition of electrical wires by external heating is investigated in order to gain a better understanding of the initiation of electrical-wire fires. An ignition-to-spread model is developed to systematically explain ignition and the following transition to spread. The model predicts that for a higher-conductance wire it is more difficult to achieve ignition and the weak flame may extinguish during the transition phase because of a large conductive heat loss along the wire core. An experimental study was performed using several sample wires with different core metals, diameters and coating thicknesses of polyethylene. A coil heater was adopted as the ignition source, and both the heat flux and heating time were selected as the main parameters to identify the flashpoint and spread point of wire fires. Experiments show that additional heating times after flash are required in order to fully pass the transition and achieve a spreading flame, agreeing with model predictions. Finally, the effects of different heating lengths, environmental pressures, and oxygen concentration on wire ignition are discussed, which may be useful for upgrading the design and standards of future fire-safe wires.