Arc Mapping: A Critical Review

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.

Appendix 1: Analysis of Carey’s (2009) Data

Due to the large number of tests reported (22 room tests with single fire ignition), a statistical analysis is possible of Carey’s data; this is shown in Table 2. Note that, apart from ‘Elsewhere,’ the categories are not exclusive. For example, an arc bead may occur both near the fire origin and near a ventilation source. Thus, for any given experiment, the totals may add up to more than 100%.

Table 2 Arc Beads Found by Carey in the Experiments Having Only a Single Fire Origin

The following conclusions may be drawn from these data:

• 1. The dominant factor determining the likelihood that an arc bead will be found at a given locale is fuel load.

• 2. Ventilation is the second most likely cause for arc bead formation

• 3. The likelihood that an arc bead will be found at a given location due to this being the location of fire origin is only 1/3 to ${\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 2}}\right.\kern-0pt} \!\lower0.7ex\hbox{$2$}}$ that due fuel load or ventilation reasons.

• 4. The likelihood is very small that an arc bead will be formed at locales not near heavy fuel, not near ventilation inflow path, and not near the fire origin.

• 5. The data do not support the hypothesis that the most likely reason for finding an arc bead at a given locale is that the location is near the fire origin.