The evidence left after any building fire can often be misleading. Melted copper wires can suggest electrical arcing was at fault, or they can just be casualties of the fire. In either case, arc beading on the wires can reveal whether the fire was electrical in origin. Sometimes when a fire wipes out an entire building and leaves few clues behind, new methods in arc bead examination can be crucial to uncovering the source.

The extensive destruction caused by a recent building fire made it difficult to reconstruct the events that led to the event. In a more limited fire, the area of origin is easier to identify, making it less complicated to pinpoint the source, whether it's electrical-, hot work-, or smoking-related. When you know where the fire began, other clues begin to fall into place. For example, arc beading in the area of origin is suspicious. Elsewhere, it isn't. In most cases, failed insulation and short circuits cause arc beading. Sometimes, however, extreme heat can cause conductors to bead, creating the appearance of arc beading. As a result, determining how the bead formed when the melted copper solidifies can explain a lot about the fire's origin.

If a fire causes a conductor to overheat, the resulting arc bead will trap products of combustion, proving the heated conductor was not the cause of the fire. If the insulation should fail due to a short, products of combustion will not be trapped in the arc bead. However, this does not prove outright the conductor is the cause of the fire. Further analysis using Auger (pronounced “oh-jay”) Electron Spectroscopy (AES) will offer a clearer picture of what happened.

At this burned-out facility, the state fire marshal found beaded copper wiring throughout the structure. Therefore, the beads needed to be a focus of any examination.

In the case of complete destruction, the reports from fire personnel and witnesses usually help pinpoint the area of the fire's origin. At this facility, the origin appeared to be the area where the electrical distribution panels were located. Despite the condition of the building, investigators still found several pieces of evidence that led them to believe the fire was electrical-related. Wiring in the ceiling did not appear to have been contained within conduit. An electrical junction box that showed traces of arcing was attached to a charred frame, and conductors were still attached to it. Although the fire was fueled by nearby combustibles, it appeared to have started in the junction box.

The building's insurance company hired our firm to review the fire investigator's work and determine whether the electrical contractor or roofer working in the area was at fault. I obtained the junction box and using AES, tested the arc beads present on the copper conductors still attached to the box.

Like many other metals, copper can absorb the gaseous elements present in the environment when heated. As temperatures increase, gases break down into their respective atoms, making them easier to be absorbed. The hotter the temperatures, the more gases are absorbed. As they cool, electrical arc residues trap the elements present in the environment, taking a “snap shot” of the conditions present at the time of the arcing.

If a fire caused the arcing, the environmental gases — principally carbon monoxide and carbon dioxide — collected in the arc bead should contain significant combustion products. Conversely, if the arc caused the fire, the bead should not contain a noticeable amount of combustion products. An AES analysis identifies the elements present on the surface of an object, but arc beads are usually contaminated after formation by fire suppression activities and handling. As a result, it is necessary to repeatedly etch the surface to obtain an elemental profile with depth.

The surface of the bead we received was contaminated by carbon. However, the metal below the surface was free of carbon or other combustion products, so the arcing must have occurred before the fire started. Our tests confirmed the initial investigator's suspicions — the electrical contractor was at fault. After we reported the findings to our client, the insurance company sued the contractor, which resulted in an out-of-court settlement.

Although it has primarily been successful in determining the cause of several building fires, the AES technique is helpful in ascertaining the cause of fires in aircraft and cars, too. However, it is only useful if the fire investigator finds electrical activity in the area of the fire's origin. In the past, if investigators could not determine the cause of a fire with any certainty, they would blame faulty wiring. However, AES analysis shows only one-third of arc beads show signs of arcing prior to a fire. Forensic analysts hope this new technique will make determining the cause of fires a more accurate science.

Anderson is president of RNA Consulting Inc., Los Altos Hills, Calif.