The science of fire origin and cause investigation can be a very complex subject. No two fires are ever alike and therefore, every fire investigation has its own particular set of challenges. However, regardless of whether the investigation is for a small vehicle fire or a large commercial loss, many of the steps used in competent fire investigation are the same.

NFPA 921, the National Fire Protection Guide for Fire & Explosion Investigations, is a guide that outlines the accepted approach and methods used in fire investigation. The basic principles incorporated center around the scientific method. The scientific method involves some very specific steps, beginning with identifying the problem and concluding with the selection of a final hypothesis.

It is critical to origin and cause investigations that a hypothesis not be developed until all of the data has been thoroughly and completely collected and analyzed. Conducting an investigation based on any preconceived notions or ideas is a trap that must be avoided at all costs. You cannot try to make the data “fit” your theory. It can lead to missed evidence, overlooked information, or unnoticed facts, and is ultimately likely to be disproven.

NFPA 921 describes four basic methodologies used in determining the origin area of a fire: Witness Information, Fire Patterns, Fire Dynamics, and Arc Mapping, which is the focus of this article.

1. Witness information is fairly self explanatory, as in who saw what, and when did they see it. This can include homeowners or business employees, neighbors, passers by, and so on. It can also include other tools, such as surveillance or security video, fire alarm data, etc.

2. Fire patterns involve the analysis of fire damage throughout the affected space, including burn pattern analysis, the varying levels of fire damage throughout the area, consumed materials such as wood framing members in the space, etc.

3. Fire dynamics describes the physics and chemistry of ignition, fire growth and spread, and the relation between the fire and building or structure. It takes into account such factors as available ventilation, fuel load, ignition temperatures of different materials, material properties, etc. All of these factors must be considered when collecting data and developing a hypothesis.

4. Arc mapping. The principle behind arc mapping is relatively straight forward. It is based on the effect of a spreading fire on energized electrical circuits. Electrical arcing can occur on many different types of circuits, including power cords, extension cords, branch circuits, feeder circuits, or equipment wiring. As fire impinges on an energized electrical circuit, it will degrade the insulation of the conductors. Eventually, the insulation will be degraded to the point that electrical arcing can occur. The electrical arcing could be between two conductors in a circuit, between a conductor and a grounded surface, between conductors of different circuits, conductors welded together, consumed material, and so on.

Click on “Next” at the bottom right to learn more about electrical arcing.

The Arcing Event

Electrical arcing will often leave evidence of the arcing event, such as beads of melted conductors, notches in the conductors, severed conductors, etc. The location of arcing and electrical activity within the area being examined can be useful in helping determine the origin area of the fire. Most electrical circuits will have some form of overcurrent protection upstream of the conductors being examined, such as a circuit breaker or fuse. Due to the high current levels that result from electrical arcing, the overcurrent protection will typically open the circuit and clear the fault. Once the overcurrent protection operates, the circuit being examined is no longer electrically energized, and no further arcing will occur on that circuit. The exception to that would be arcing between a ground or neutral conductor of the de-energized circuit to a live conductor of an adjacent circuit that is still energized. It is important to note that if the fire burns long and hot enough, portions of the conductors may melt, erasing the evidence that electrical arcing has occurred.

It is also important to note that not all electrical arcing will instantaneously operate the overcurrent protection. Arcing is typically a short duration event, and the current level or duration may be insufficient to operate the overcurrent protection. Therefore, it is not uncommon to find arcing at several locations within a single circuit. This in itself can be useful information.

To help illustrate, please consider these two cases studies:

Case Study #1: The Doctor's Office

Due to the amount of fire damage, the origin area could only be broadly defined. Routed through the entire area were three extension cords connected end to end across an approximately 25-foot length. Evidence of electrical arcing was found at four separate areas along the length of the extension cords. At each arc location, the line (or hot) conductor had arc-severed. Once the arcing had cut the conductor, anything downstream of that point would have been de-energized. If the arcing occurred at the arc site closest upstream (nearest) to the circuit breaker, the rest of the circuit would have been de-energized once the conductor severed, and arcing would not have been possible. Armed with this information, we were able to conclusively determine that the arcing furthest downstream had to have occurred first. This enabled us to more narrowly define the origin area, and eventually the cause of the fire.

Click on “Next” at the bottom right to read another case study.

Case Study #2: The Luxury Resort Kitchen

A fire occurred at a luxury resort in the southeastern United States. The fire damage was limited primarily to the kitchen area for the dining room on the second floor of the structure. However, the fire had spread into the ceiling area of the structure causing a partial collapse, and extensive smoke and water damage was sustained throughout the building, making this a very substantial loss. The fire occurred on a Sunday evening, after they had closed for the day, so nobody was present when the fire ignited. Initial suspicion of a fire cause was immediately directed toward the large industrial ventilation hood above the cooking equipment located at the center of the kitchen. This initial hypothesis was based largely on the fact that the service company responsible for maintaining the ventilation system had been at the site the day of the fire performing routine scheduled maintenance.

A large multiple party inspection of the loss site was conducted and lasted several days. Various parties were placed on notice, including vendors, contractors, manufacturers, etc. ProNet Group was retained to represent the interests of the ventilation hood service company.

A thorough inspection of the loss site was performed. This included documentation, photographs, diagrams, sifting through debris, etc. Although the focus of the inspection was initially centered around the vent hood, other areas showed evidence of significantly greater fire damage. Specifically, the office area off the side of the kitchen had much greater damage, and reached the point of full involvement (flashover).

Eventually, the decision was made to conduct an arc mapping survey of all of the electrical wiring throughout the kitchen area. It was a substantial task considering the large amount of electrical wiring in the area, with most of it located at ceiling level. All of the wiring was carefully inspected and examined, and any points of electrical arcing were noted, marked, and diagrammed, and a map of the arcing activity was created. Throughout all of the wiring in the entire kitchen area, the only area where electrical activity was found was above the office area off to the side of the kitchen, where I had observed the greatest fire damage.

There was no evidence of arcing or electrical activities anywhere near the vent hood that was initially suspected of contributing to the cause of the fire. By creating and analyzing the arc map of the structure wiring, in addition to the use of the investigative methods previously mentioned, we were able to clearly define the origin area as the office portion of the kitchen, and not at the ventilation hood. Our conclusions also illustrate the importance of collecting all of the data before forming a hypothesis, and not “jumping to conclusions” prematurely.

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As the area being examined is inspected, any location of electrical arcing or activity that is found should be identified and marked, and the exact location of the activity should be properly documented, and a diagram of the arc sites can be developed. The usefulness of arc mapping as an effective tool in determining origin area comes from the spatial relationship between sites of arcing, as it is useful in determining a sequence of events which occurred in the area. Specific areas with higher concentrations of arc sites found were likely subjected to flame and heat impingement before other areas where no electrical activity was observed. When used in conjunction with all of the other data collected during the course of the investigation, arc mapping can help more clearly define the origin area of the fire.

There are many tools and techniques available to investigators in determining the origin and cause of a specific fire. It is the responsibility of the individual investigator to use any and all tools at their disposal to conduct a thorough and competent investigation. Electrical arc mapping is just one of those tools, but can be a very effective one.

Timothy A. Hazelwood, P.E., C.F.E.I. is a Senior Electrical Engineer with the ProNet Group, Inc. Mr. Hazelwood's professional engineering experience includes forensic investigations, electrical utility distribution systems, product failure and construction investigations, electronics, communications and systems failure analysis, electrocution and electric shock, as well as fire origin and cause investigation. Contact him at thazelwood@pronetgroup.com.

Reference – NFPA921, the Guide for Fire and Explosion Investigation, National Fire Protection Association, 2011 Edition

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The Effectiveness of Arc Mapping To Determine Fire Origin Area

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