Reference
Beads - Telling the Past from Electrical Arcing Events
By Mark C. Hughes, P.Eng.
© 2005 Sintra Engineering Inc.
Most fire investigators know that beaded wiring is indicative of some type of electrical arcing as, in general, the heat from a fire is insufficient to melt copper wiring. What is of more interest to the investigator looking at beaded wiring in the area of origin, wasÖ ìCan I tell if this arcing was the cause of the fire, or a victim of the fire?î
As wiring in a house is energized to provide a ready source of electricity, when a fire occurs, the heat from the fire often will melt the insulation on these powered conductors. Once the insulation is gone, there is nothing preventing the hot and the neutral or ground wire from touching and causing an arc and the corresponding arc bead. Arcing is indicative of the intense heat from electrical contact and the victim arcing may or may not allow sufficient current to flow and cause the over-current protection (breakers or fuses) to trip.
Similarly if there is mechanical damage to the insulation on a wire, providing intermittent contact between conductors, this can also generate arcing between the conductors. If conditions are correct, the arcing can sometimes produce sufficient heat to start a fire. After the fire develops significantly, the damage to the wires (the beaded wiring) will look the same as the instance where the fire caused damage to the insulation. Is it possible to tell one type of arc bead from the other?
The answer is an unequivocalÖ..maybe. From just the appearances of the beads, there is no qualitative difference between the two types of bead. In other words, you cannot tell whether an arc bead is a victim or cause of a fire just by looking at the bead. However, there is a theory out there that it is possible to tell by looking closely at the bead surface. It is, however, controversial.
For a number of years now there is a gentleman out of California that has been professing that it is possible to tell whether the bead was there before the fire or after. This should, in theory, tell you if the electrical arcing was the cause or the victim of the fire. The gentleman's name is Dr. Robert N. Anderson, of RNA Consulting, Inc. Dr. Anderson has put forward a theory regarding the use of a type of metallurgical equipment called an Auger (pronounced ìOh-jayî) Electron Spectroscopy as a method to examine the surface of a beaded wire to determine the whether the arced bead was the cause or the result of the fire.
The theory is that like many other metals, copper can absorb the gaseous elements present in the environment when heated. As temperatures increase, such as in a fire, hot gases will break down into their respective atoms, which can make it easier for them to be absorbed in the copper. The hotter the temperatures, the more the gases are absorbed. During electrical arcing the temperatures are very hot, and as the arcing wires cool, copper bead traps the elements present in the environment, taking a ìsnap shotî of the conditions present at the time of the arcing.
Therefore, if a fire caused the arcing, the environmental gases ó principally carbon monoxide and carbon dioxide ó 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 looks at a small area of a copper bead and shaves a small depth of the bead (measured in angstroms which are units of 10-9m). This process looks at the elements present on the surface of an object, but to make things complicated, arc beads are usually contaminated after formation by fire suppression activities and handling. As a consequence, it is necessary to repeatedly etch the surface to obtain an elemental profile with depth.
The theory sounds reasonable, and may one day produce good results. Right now the problems are simple: Firstly, the AES analysis looks at a very small area and the properties at one end of the electrical arc bead can be very different from other areas of the bead. Secondly, because of the need to repeatedly etch the surface to find uncontaminated copper on the bead, the process is a little selective, which is never a particularly good way to approach a scientific problem. Finally, there has been only limited verification of the process and a number of authors have completely disputed the validity of the technique. All of these problems make the use of the technique of very limited benefit to most fire investigators.
Until proper documented and repeatable tests are conducted with the results published in a peer reviewed journal, the concept of identifying electrical beading from AES testing will need to still remain in the realm of more fiction than fact. It is more like television's ìCSIî where they can tell what who started the fire by the type of sulphur used with the matches (sounds good, makes for an interesting story but bears no resemblance to reality).
Mark Hughes, P. Eng. is a principal with Sintra Engineering, a Forensic Engineering firm.
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