Arc mapping

Abstract: Arc mapping is a method for graphically documenting a fire pattern which comprises arc marks on the wiring of a structure due to the effects of fire, with the goal of using this information to assist in establishing the area of origin of the fire. The concept was initially presented in 1955, but without any published science (experimental, theoretical, or modeling) basis. By the 1990s, the method was being widely used by fire investigators. Meanwhile, the first paper containing substantive research did not appear until 2005. In this first-ever critical review of the subject, careful consideration of engineering principles and large-scale experimental studies on the subject indicate that the relevance and prominence of arc mapping as a leading indicator of fire origin have been notably overstated. The technique is valid and applicable only in some very limited scenarios. Yet it has seen increased use in recent years by investigators preparing fire reports. In many cases, such attempted use of arc mapping is based on incorrect and invalid hypotheses, which are often implicitly assumed to be true instead of being explicitly stated. Fire patterns are subdivided into directionality (movement) patterns and intensity patterns. Analysis of the research indicates that valid conditions can be expected for use of arc mapping as a directionality indicator in less than 1% of building wiring circuits which sustain arcing. For intensity patterns created by arc sites, propensity is governed by three main variables: fuel loading, ventilation, and burning duration. Only the last is potentially associated with a location being the area of fire origin. But experiments show that fuel and ventilation effects are likely to dominate, instead. In the best-documented study so far, only 23% of arc beads were found to be located near the area of origin, while 61% were found at areas of heavy fuel concentration. This indicates that, in the general case, arc mapping results cannot be used to draw conclusions as to the fire origin. Only in rare cases where it might be demonstrated that fuel concentration or ventilation effects were not governing, would it be possible to use arc mapping results as pointers to the area of fire origin. Since arc mapping is used almost exclusively for forensic purposes, it must be emphasized that methods should not be used, unless it can be demonstrated that they are reliable indicators of what is claimed, and that they are being used properly.