Arc Mapping

A fire’s area of origin can be difficult to determine when it has been burning for a considerable time or if it has involved a large area. Patterns of fire damage used by investigators following very severe fires might point to the seat of the fire, but equally, they might indicate areas where the fire has vented the building causing severe damage or where a greater load of fuel was available. Our research involving compartment fires explored the reliability of a technique called “arc mapping” which uses the electrical wiring in a building to establish the origin of a fire. The analysis of three-dimensional data collected during the research indicated a high probability that localised melting on electrical conductors known as ‘arcing damage’ occurred in close proximity to the fire’s area of origin. 

What is electrical arcing?

Arcing is defined as “a high temperature luminous electric discharge across a gap or through a medium such as charred insulation”. Figure 1 details a demonstration of arcing by the author when two wires (conductors) were shorted together at mains voltage. The copper conductor melts and forms a bead(s) and/or notch(es) on the wire’s surface.

Figure 1 (left) - The author demonstrating electrical arcing between live conductors.

The beads and notches known as “arcing damage” can be useful to a fire investigator because they provide reliable information to assist in establishing a fire’s area of origin. Arcing damage usually shows the point where a fire first attacked a live electrical circuit. This can be of particular importance when the fire has reached full involvement or flashover. When flashover occurs in a room the patterns of fire damage, often relied upon by fire investigators, are overprinted and confused, rendering them less reliable. Arcing damage is sometimes the only way of establishing where a fire started.

One area of potential confusion in using arcing damage is that during very severe fires, the high temperatures reached can melt copper conductor(s). This effect can create evidence which appears to the inexperienced eye, to be similar to arcing or it can mask arcing created earlier in the development of the fire. 

What is arc mapping?

The majority of arcing damage observed at a fire scene away from connections is generally a result of the effects of the fire attacking electrical wiring, rather than being the cause of the fire. The position of the arcing damage on conductors can help to establish the area where the fire first attacked energised cables.

Figure 2 (above left) – a view of an experiment fire during full involvement (‘flashover’) conditions. Figure 3 (above right) - a post-experiment fire compartment before the identification of arcing.

Generally, arcing will cause the circuit protection device (fuse or circuit breaker) to operate so that no further arcing can occur on that circuit. By finding the location of arcing damage on several cables, the fire’s area of origin can be progressively reduced in size. If arcing damage is found inside the casing of an electrical appliance, it is usually a good indicator that the fire started within that appliance.

Prior to this research, the reliability of arc mapping had not been confirmed, so a series of full-scale experimental fires were conducted at our GATR fire test facility in Essex, where different compartment scenarios were repeated. 42 fully-furnished compartments were wired with four electrical circuits (for example Figures 2 and 3), which led to the generation of 141 examples of arcing on copper conductors. Figures 4 to 13 detail nine separate categories of arcing damage, following the experiments.  


Figure 4 – Two conductors welded together.Figure 5 – Severed conductor with a bead on one end and a notch at the other end.

Figure 6 – Arcing between two conductors. One severed with beading at each end. The other conductor is notched.Figure 7 – All three conductors of this cable were involved. The arcing had occurred at a fixing screw. All are notched with beading on the notch edges.

Figure 8 – A severed conductor with a ”bull-nose” bead. The demarcation between the melted and un-melted conductor is visible.Figure 9 – A bead within a notch on the conductor. The demarcation between the melted and un-melted conductor metal is visible.

Figure 10 – Conductor severed, with beading and it is welded to a zinc plated steel cable fixing screw.Figure 11 – A raised bead on a conductor that was visible at the fire scene without magnification.

Figure 12 – Arcing through char affecting two conductors over a length of 14mm. The scale in the image is in mm.Figure 13 – A close-up of a section of Figure 11 showing the arcing through carbonised cable insulation in more detail.

These tests established clearly the relationship between the fires area of origin and the location of arcing damage. The results identified the reliability of using arc mapping at fire scenes to establish where a fire started.

Conclusions

Analysis of the three-dimensional data recorded during 42 experiments showed that there is a high probability of arcing damage affecting electrical conductors located close to the origin of a fire. The research highlighted the value of finding arcing damage during a scene investigation particularly when the fire has developed to flashover or involves a large area.

Dr Nick Carey joined Hawkins in June 2012 after a 27 year career with the London Fire Brigade. During the last 16 years of his fire service career, he was a full-time Fire Investigation Officer. In 2009, he was awarded a PhD from the University of Strathclyde for his research into the interaction of fire and electrical circuits titled “Developing a Reliable Systematic Analysis for Arc Fault Mapping”, the subject of this article. Nick is a qualified electrician and has undertaken a large number of fire and electrical examinations, including marine losses, in the UK and overseas. He has also delivered electrical and fire investigation training throughout the UK and overseas.

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