Sunday, March 18th, 2018

Arc Mapping: A useful tool for discovering a fire’s origin


Arc mapping is a useful tool in helping fire investigators and electrical engineers locate the area of origin of a structure fire. Here are the general steps in generating an arc map:

  1. Conduct an arc survey: Locate all evidence of electrical activity in structure wiring.
  2. Document: Tag, photograph and diagram locations of arcing prior to artifact removal.
  3. Analyze: Examine each suspected site to confirm it is the result of electrical arcing.
  4. Create an arc map: Show the locations of each confirmed electrical arc on a scene diagram.

By combining arc mapping with other fire investigative techniques such as burn pattern analysis, and scene reconstruction, investigators can develop a hypothesis for where the fire began.

What is Arcing?

Arcing is an electrical phenomenon which occurs between two energized conductors or an energized conductor and ground. It happens when the insulation around an energized conductor breaks down and an electrically conductive pathway is formed. It can be seen as a flash of light and a spark jumping across a surface or through the air; not unlike the static discharge that is seen between your hand and a door knob. When an arc occurs, evidence is left behind in the form of a metal loss or deposit. It is usually seen as a small divot on one conductor with a corresponding lump of metal on the other conductor where metal was liquefied during the arc event and transferred from one wire through the arc and deposited on the other wire.

Arcing occurs during a fire when the heat melts and burns away the insulation on electrically energized wiring. An arcing wire can also be the cause of a fire when the insulation around the wire is compromised due to physical damage (such as an improperly driven nail), heat degradation (such as using a 100 watt bulb in a fixture marked for 60 watts max.), or other means.

Conducting an Arc Survey

Arc mapping involves the close examination of the electrical wiring of a building in order to locate arc sites. An arc survey begins by thoroughly documenting the building’s wiring and electrical components. This includes measuring and diagramming the layout of the wiring, tracing circuits back to the electrical panel, and locating and identifying any potential loads on the wiring. The technique for locating arc sites involves feeling along the bare wire conductors with your fingers. Some insulation and char may have to be removed in order to get to the bare wire. An arc site will feel like a small divot or bump on the smooth wire surface. Close visual examination of the site can confirm if the anomaly is the result of electrical arcing. Care should be taken to avoid confusing other wire artifacts, such as melting due to heat and physical damage, with electrical arcing. Known arc sites should be physically tagged and their locations documented on the diagram. Since most building wiring is difficult to examine in situ, an effort to remove and preserve the wiring and circuits in the areas of interest should be made in order to perform a closer examination in the lab.

Utilizing the Arc Map

The result of locating and documenting arcs in the building’s wiring provides the investigator with several important pieces of information. For one, the presence of arcing in the wiring lets the investigator know that the circuit was energized at the time of the fire. Second, spatial analysis of known arc locations provides clues to the fire’s spread and development. These arc locations are visually presented by the investigator in the form of a scene diagram, hence the name arc map. This information combined with other evidence from a fire scene, such as burn patterns, can be used by the investigator to determine the area of origin. Finally, arcs located within energized appliances and equipment can help the investigator determine their role as a candidate for fire causation when located in the area of origin.

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