New metallurgical techniques applied to fire investigation: Fid-Bau Portal

New metallurgical techniques applied to fire investigation

Murray, Isabelle

The examination of electrical wiring following a fire may sometimes show evidence of surface damage characterized by depressions or beading that is physically different from the undamaged, initial wire condition. Fire investigators analyze this damage to help establish objective criteria that are related to both the origin and cause of the fire. However, misinterpretation of the nature of this damage is frequent and may lead to erroneous conclusions. This paper investigates new metallurgical techniques that can be applied to enhance the methods used by fire investigators by using analytical methods, such as optical and electron microscopy for the microscopic characterization of wiring surface damage, formation of oxides and their composition, together with an evaluation of the evolution of the microstructure of the damaged or degraded area. A series of samples from controlled laboratory experiments and samples collected from actual fire sites were examined in order to obtain a clear indication of the type of surface damage that can be found after a fire. The damage observed on conductors exposed to intense heat or electrical activity (short-circuit) was carried out by the use of optical and scanning electron microscopy and X-ray diffraction in order to determine the morphology, the microstructure and the composition of the phases that were formed, from the outside through the inside of the specimens. As a result of these analyses, it was possible to distinguish damage related to the heat of the fire (fire induced) from the damage caused by electric faults (arcs or short-circuits). Reactions between different metals used in connecting devices were also studied to reveal characteristics that are typical of this kind of interaction. A novel technique was developed to determine the energy dissipated during a short-circuit event. By monitoring the current-time relation at constant applied voltage, the energy was determined from the area under the current vs time curve. This value was compared and correlated favourably to the amount of metal mass lost by evaporation of the metal conductors that were tested.