Litigation Defenders

Understanding the concept of arc mapping in a building or vehicle fire is fundamental to eliminating the electrical system as a possible cause for purposes of an insurance claim investigation.

Arc mapping, also known as arc fault circuit analysis, is the examination of an electrical system to assist in an investigator’s reconstruction of a fire scene to determine the area of a fire’s origin and cause.

The first step of arc mapping is to perform an arc survey of the wiring. This survey involves the physical inspection of all the wiring, diagraming the wiring in the fire site, and comparing discovered arcing to ignition points. Starting with an arc survey, a site plan or vehicle wiring diagram as a line drawing is documented. The locations of arcs, branch circuits, the size of branch circuits and conductors, and connections to loads or switchboards are documented within this line drawing.

When examining wiring, the ability to differentiate between melting and arching on wires is a fundamental skill of fire investigation. Typically, a wire that is melted will have the appearance of cheese melted and cooled, with smooth surfaces and little if any material missing. Arced wire typically has notches or beads on the wire. However, it is best practice for melted wire in an area of origin to undergo metallurgical analysis and/or surface chromatography, to rule out arcing hidden under melted wire.

It is not uncommon to locate multiple arcing on a single wire. In such situations, the arc discovered furthest from the power source likely occurred earliest in time. This is because arcing occurring closer to the power source will prevent the flow of current further down the wire. When arcing in the same general area is discovered on multiple wires all receiving power from the same source, the general area of arcing is likely the area of origin. For example, arcing found inside the casing of an electrical appliance is a strong indication the fire originated within that device. 

But why does arc mapping matter to fire investigations? Because arc mapping assists in determining an area or point of fire origin. Most fires expand uniformly in all directions from the point of origin. Therefore, when a fire attacks an energized wire and causes a fault, the point of origin will likely be perpendicular to the fault location. However, several other factors can impact where arcing occurs. These factors include bends in conduit, locations where wires are pressed together and elevation of wires.

Because arc mapping eliminates a possible fire origin cause and narrows the area of origin, it is a fundamental tool for every fire investigator. This is also key information that guides insurance claims professionals who make coverage decisions and valuable evidence for defense counsel to use should a matter proceed to litigation.