Ignition of Veld Grass by Hot Aluminium Particles Ejected from Clashing Overhead Transmission Lines

It is common cause that the arcing of high voltage electrical conductors produces a shower of sparks. The flame temperature of aluminium burning in air is reported as having been measured as 3800 K (3527°C) [1]. After ejection from the point of arcing, a globule of aluminium is likely to be at or above its boiling point (2700 K or 2427°C) and surrounded by an aluminium vapour cloud which is promptly ignited and burns with a diffusion flame. If during the luminous period a spark was to fall onto dry veld grass then, provided it had sufficient energy not to be quenched, it would almost certainly cause ignition. Should the flame around a boiling globule be extinguished while it was still of finite size and were such a particle to fall into dry veld grass while its temperature was between 300 and 400°C or above it could cause ignition of the grass. In the present paper, ignition of the veld grass is determined by the temperature-trajectory history of hot particles of different sizes and initial conditions after their projection into a high velocity gusting airstream. This paper complements that of Mills and Xu Hang on trajectories of sparks from arcing aluminum cables [2] and that of Fernandez-Pello and Tse on the flight paths of metal particles and embers generated by power lines in high winds—a potential source of wildland fires [3]. The present study was commissioned in an attempt to establish the likelihood, or otherwise, that the occurrence of a particular veld fire could have been due to hot aluminium particles ejected from the clashing of high voltage overhead transmission lines. From in situ measurements, the fire started some 10.7 m horizontally from the clashing of the overhead conductors. Sets of graphs were obtained by computational experiment with the objective of verifying the starting point of the fire. These sets comprised: Time versus Particle Temperature, Time versus Vertical Penetration, Time versus Horizontal Penetration and Horizontal Penetration versus Vertical Penetration. Not all of the set of graphs satisfied the requirement of starting a fire at the given point. Sets H, I and J provided the definitive clue that the fire would have started by hot aluminium particles. The investigation was done in phases corresponding to the different set of graphs. Two models were subsequently elaborated called herein less rigorous and more rigorous. The effects of downward, upward and negative horizontal ejection velocities were also examined. The aim of the paper was to reconstruct the conditions prevailing on the day of the fire. The results obtained show that the probability the fire having been started by the hot aluminium particles ejected by clashing overhead conductors was high.