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Arc mapping is the process of finding the location of energized electrical events, or arcs, within a defined area to “map” an area of origin. Technically, the basic assumption is that an electrical arc which occurs furthest from the power source, usually occurred first. Used logically and interpreted appropriately, this method is a powerful analytical tool. Results are strong, often definitive, and can be used when investigating any type of fire. Unfortunately, although an excellent tool, it is one that is not commonly used. In SAMAC’s case, Dave Hines, our electrical expert, routinely uses arc mapping in his investigations.

The basic premise in arc mapping is to locate, mark, and systematically record all electrical events within a defined area, whether or not you can identify them as arcs. Be aware that melted or heat-beaded wiring is often confused with arcing. Each event is recorded using photographs or video and detailed note taking is crucial. Once all arcs are marked and recorded, the location of each event is referenced on a blueprint or drawing. Knowing the basic architecture of the electrical system, it is then usually possible to evaluate the timeline of the events.

Logical evaluation of each electrical event should proceed as follows:

  1. Is the event an electrical arc? If it is not clear on first examination, additional scene investigation or metallurgical analysis should be performed.
  2. After verifying and eliminating all non-arc events, each arc event is evaluated assuming arcs further from the power source usually occurred first. However, even though an arc may have occurred first, it sometimes does not mean that the arc affected electrical current further away. If the wire was not completely severed, a fuse was not ‘blown’, or a breaker not tripped by the arc event, arc mapping may be limited in its use.
  3. The arc event data is then compared to circuit data, witness accounts (contractors, maintenance personnel), operational records and other fire data.

If properly performed, the evaluation of each electrical event will give you a map indicating the origin of the fire. Arc mapping, in conjunction with other investigative tools, will greatly assist in proving or disproving the origin of the fire, present a clearer sequence of events, and possibly provide the cause of the fire. At the very least it will provide clients with a thorough and complete investigation.