Fire & Forensic Investigative Updates


arc mapping
An Introduction to Arc Mapping

This article was submitted by David M. Brani, P.E., Ph.D. David is a Senior Forensic Engineer with Applied Technical Services, Inc.

For the insurance professional concerned with fire investigations, it is important to effectively choose and communicate with those you rely on for a proper investigation. This article is intended to better inform you of a tool known as Arc Mapping. The goal is to provide a brief introduction to the subject matter to keep in perspective how this tool can be beneficial and the roles investigative professionals can play.

What is Arc Mapping? NFPA 921 section 3.3.8 (for this article, the 2014 edition is always cited) states the following:

Arc Mapping: The systematic evaluation of the electrical circuit configuration, spatial relationship of the circuit components, and identification of electrical arc sites to assist in the identification of the area of origin and analysis of the fire’s spread.

At face value, the investigator draws a map (sketch) depicting all of the electrical circuits in the fire zones of interest—wiring, outlets and appliances etc. Add to the map, locations where electrical activity (arcing) is present. Next, add spots where wiring has suffered melt damage. Finally, make note of the breaker condition for the circuits of interest. Investigators then combine this knowledge with the other tools of fire investigation (witness information, burn patterns, fuel sources and ignition sources) for added insight as to the area of origin and cause of a fire.

On a good day, Arc Mapping can provide key information with regard to the area of fire origin; and on some occasions even assist with the determination of the cause of the fire. At other times the mapped activity does not bear fruit. Like any tool, at times it is indispensible and at other times it’s like using a hammer to tighten a bolt. As an insurance professional, be wary of an investigator that is overly smitten with this procedure or overly dismissive.

To provide some perspective, an electrical arc (also referred to as electrical activity) may occur when two conductive elements of differing electrical potential are brought in close proximity such that the combination of voltage difference, gap distance and conductivity of the intervening gap enables electrical current to flow across the gap. The insulation separating a pair of wires in the extension cord you are using for the family room lamp breaks down allowing the wires to almost touch. If the cord is plugged into an energized wall outlet, an arc can sometimes take place.

For those that have not lived an overly sheltered life, you produced an electrical arc when you walked across carpeting in your socks and purposely zapped another. Your electric potential changed as you were walking such that when your finger almost touched the person of differing potential, a momentary flow of electricity passed between the two of you. In addition to feeling the arc you may have actually seen the arc (spark).

The manufacturers of extension cords, appliance cords and general electrical wiring fortunately insulate the wires to prevent arcing between them. This layer of insulation between the copper wiring does a great job of preventing arcs. But when these same electrical conductors are attacked by fire, the fire’s heat and flames break down this insulation such that at spot locations the only thing separating the wires is an air gap. To make matters worse, this fire damaged insulation can produce "char" that encourages electrical activity across these air gaps. When the ingredients are just right (spacing, char and voltage difference), an electrical arc ensues.

When an arc occurs between the cord wires, the results are much more dramatic than when you gave that “carpet shock” to the unsuspecting “friend” (cat, significant other, annoying child). Not only is the spark more vivid, this electrical activity physically damages both of the almost touching copper wires leaving forensic clues for the investigator to discover. The heat generated at an arc site can momentarily raise the wire’s temperature many thousands of °F. Permanent damage takes place in the form of copper beading, craters formed in the copper wire and copper splatter on the wires or nearby surfaces. Each electrical arc is brief but very violent.

So what is so special about the discovery of arc damage? One does not need to find evidence of arcing to know that an electrical conductor had been attacked by fire. There is plenty of evidence left behind in the form of degraded and missing wire insulation for starters.

The discovery of arc damage signifies to the investigator that the electrical cord at that localized area was energized at the time of the fire attack. The science associated with Arc Mapping is founded upon that one simple truth. Arc Mapping can provide a time dimension to the other forms of evidence which includes witness statements, burn patterns, ignition sources and fuel sources.

For example, suppose a kitchen fire has heavily damaged the appliances including a toaster. The homeowner claims that the toaster had been unplugged prior to their leaving the house. But the investigator discovers arc damage on the toaster’s cord. This contradictory physical evidence adds value to the overall analysis.

In the previous example, at first glance, it appears the home owner was either lying or forgot the toaster was still plugged in. After all, if the toaster was not plugged in, how could there have been an arc if no electrical current was flowing? Well … suppose during the fire the power cord for a nearby coffee maker cord had been draped over the toaster cord. While both cords were attacked by fire, their insulation broke down such that electrical current in the coffee maker cord energized the unplugged toaster cord enabling an arc to occur in this cord.

As with any technique, the investigator must be aware of the subtle nuances associated with the supporting science. Some other complications (not discussed in this article) include direct versus alternating current, copper versus aluminum wiring and 120 voltage versus 240 and 480 voltage. There are numerous texts and published papers addressing this topic. NFPA 921 devotes eight pages just informing how the investigator is to identify and distinguish arc damage from other fire related damage.

So who should you turn to for technical guidance? According to NFPA 921, a fire investigator well versed in the methods of their guidelines. NFPA 921 section 9.11.7.2 states the following:

An electrical engineer is not required to perform Arc Mapping. Arc Mapping is fundamentally pattern recognition, which fire investigators routinely perform in other areas of fire investigation.

However, a good investigator knows there are instances when the services of an electrical engineer or metallurgist are warranted.

In summary, Arc Mapping is a tool based on a simplistic truth that an electrical conductor must have been energized at the time of arc damage. However, the underlying science is both subtle and complex. Arc Mapping is an important technique to be considered for fire investigations involving electrical power; but is not a cure-all solution.

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