Process: Melt Bead Analysis
Arcing or Fire First?

Short circuiting of an energized electrical conductor can result in high temperature electrical arcing that creates a melt bead. Fire investigators often question if a short circuit or arc was the cause or result of fire. A short circuit can obviously cause of fire; a fire can also melt wiring insulation and cause a short circuit. One investigator (Robert Anderson) has proposed a controversial method by which the melt bead may be analyzed to answer this question. His methodology is to examine a copper melt bead using an Auger Electron Spectroscope (AES). Auger can provide detailed elemental analysis from a very shallow surface layer. The instrument is also set up with ion milling, where charged ions are used to erode away successive layers of metal surface, which are then subject to Auger analysis. Anderson proposes that the trends of elemental concentration within the melt bead can be related to whether the bead was formed during a normal atmosphere, or an atmosphere already engulfed in combustion products. He mainly examines carbon content trends. There are numerous qualified materials experts who have cast doubts on  Anderson’s methodology, based on metallurgical theory of materials solubility and diffusion, and on the ability to reproducibly perform the intricate analysis required. At this writing there has been no blind testing conducted of a statistically meaningful number of samples to prove that the method is viable.

References

1. Anderson, R. N., “Which Came First, the Arcing or the Fire,” Fire and Arson Investigator, March 1996.
2. Anderson, R. N., letter to editor in reply to Dr. Bernard Beland’s June 1994 article on arc bead analysis, Fire and Arson Investigator, December 1994.
3. Beland, B., “Examination of Arc Beads,” Fire and Arson Investigator, June 1994.