The science of fire origin and cause investigation can be a verycomplex subject. No two fires are ever alike and therefore, everyfire investigation has its own particular set of challenges.However, regardless of whether the investigation is for a smallvehicle fire or a large commercial loss, many of the steps used incompetent fire investigation are the same.

NFPA 921, the National Fire Protection Guide for Fire &Explosion Investigations, is a guide that outlines the acceptedapproach and methods used in fire investigation. The basicprinciples incorporated center around the scientific method. Thescientific method involves some very specific steps, beginning withidentifying the problem and concluding with the selection of afinal hypothesis.

It is critical to origin and cause investigations that ahypothesis not be developed until all of the data has beenthoroughly and completely collected and analyzed. Conducting aninvestigation based on any preconceived notions or ideas is a trapthat must be avoided at all costs. You cannot try to make the data“fit” your theory. It can lead to missed evidence, overlookedinformation, or unnoticed facts, and is ultimately likely to bedisproven.

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

1. Witness information is fairly selfexplanatory, 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 assurveillance or security video, fire alarm data, etc.

2. Fire patterns involve the analysis of firedamage throughout the affected space, including burn patternanalysis, 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 andchemistry of ignition, fire growth and spread, and the relationbetween the fire and building or structure. It takes into accountsuch factors as available ventilation, fuel load, ignitiontemperatures of different materials, material properties, etc. Allof these factors must be considered when collecting data anddeveloping a hypothesis.

4. Arc mapping. The principle behind arcmapping is relatively straight forward. It is based on the effectof a spreading fire on energized electrical circuits. Electricalarcing can occur on many different types of circuits, includingpower cords, extension cords, branch circuits, feeder circuits, orequipment wiring. As fire impinges on an energized electricalcircuit, it will degrade the insulation of the conductors.Eventually, the insulation will be degraded to the point thatelectrical arcing can occur. The electrical arcing could be betweentwo conductors in a circuit, between a conductor and a groundedsurface, between conductors of different circuits, conductorswelded together, consumed material, and so on.

Click on “Next” at the bottom right to learn more aboutelectrical 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 electricalactivity within the area being examined can be useful in helpingdetermine the origin area of the fire. Most electrical circuitswill have some form of overcurrent protection upstream of theconductors being examined, such as a circuit breaker or fuse. Dueto the high current levels that result from electrical arcing, theovercurrent protection will typically open the circuit and clearthe fault. Once the overcurrent protection operates, the circuitbeing examined is no longer electrically energized, and no furtherarcing will occur on that circuit. The exception to that would bearcing between a ground or neutral conductor of the de-energizedcircuit to a live conductor of an adjacent circuit that is stillenergized. It is important to note that if the fire burns long andhot enough, portions of the conductors may melt, erasing theevidence that electrical arcing has occurred.

It is also important to note that not all electrical arcing willinstantaneously operate the overcurrent protection. Arcing istypically a short duration event, and the current level or durationmay be insufficient to operate the overcurrent protection.Therefore, it is not uncommon to find arcing at several locationswithin a single circuit. This in itself can be usefulinformation.

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 bebroadly defined. Routed through the entire area were threeextension cords connected end to end across an approximately25-foot length. Evidence of electrical arcing was found at fourseparate areas along the length of the extension cords. At each arclocation, the line (or hot) conductor had arc-severed. Once thearcing had cut the conductor, anything downstream of that pointwould have been de-energized. If the arcing occurred at the arcsite closest upstream (nearest) to the circuit breaker, the rest ofthe circuit would have been de-energized once the conductorsevered, and arcing would not have been possible. Armed with thisinformation, we were able to conclusively determine that the arcingfurthest downstream had to have occurred first. This enabled us tomore narrowly define the origin area, and eventually the cause ofthe fire.

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

Case Study #2: The Luxury ResortKitchen

A fire occurred at a luxury resort in the southeastern UnitedStates. The fire damage was limited primarily to the kitchen areafor the dining room on the second floor of the structure. However,the fire had spread into the ceiling area of the structure causinga partial collapse, and extensive smoke and water damage wassustained throughout the building, making this a very substantialloss. The fire occurred on a Sunday evening, after they had closedfor the day, so nobody was present when the fire ignited. Initialsuspicion of a fire cause was immediately directed toward the largeindustrial ventilation hood above the cooking equipment located atthe center of the kitchen. This initial hypothesis was basedlargely on the fact that the service company responsible formaintaining the ventilation system had been at the site the day ofthe fire performing routine scheduled maintenance.

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

A thorough inspection of the loss site was performed. Thisincluded documentation, photographs, diagrams, sifting throughdebris, etc. Although the focus of the inspection was initiallycentered around the vent hood, other areas showed evidence ofsignificantly greater fire damage. Specifically, the office areaoff the side of the kitchen had much greater damage, and reachedthe point of full involvement (flashover).

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

There was no evidence of arcing or electrical activitiesanywhere near the vent hood that was initially suspected ofcontributing to the cause of the fire. By creating and analyzingthe arc map of the structure wiring, in addition to the use of theinvestigative methods previously mentioned, we were able to clearlydefine the origin area as the office portion of the kitchen, andnot at the ventilation hood. Our conclusions also illustrate theimportance of collecting all of the data before forming ahypothesis, and not “jumping to conclusions” prematurely.

———————————————————————————————————–

As the area being examined is inspected, any location ofelectrical arcing or activity that is found should be identifiedand marked, and the exact location of the activity should beproperly documented, and a diagram of the arc sites can bedeveloped. The usefulness of arc mapping as an effective tool indetermining origin area comes from the spatial relationship betweensites of arcing, as it is useful in determining a sequence ofevents which occurred in the area. Specific areas with higherconcentrations of arc sites found were likely subjected to flameand heat impingement before other areas where no electricalactivity was observed. When used in conjunction with all of theother data collected during the course of the investigation, arcmapping can help more clearly define the origin area of thefire.

There are many tools and techniques available to investigatorsin determining the origin and cause of a specific fire. It is theresponsibility of the individual investigator to use any and alltools at their disposal to conduct a thorough and competentinvestigation. 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 ElectricalEngineer with the ProNet Group, Inc. Mr. Hazelwood's professionalengineering experience includes forensic investigations, electricalutility distribution systems, product failure and constructioninvestigations, electronics, communications and systems failureanalysis, electrocution and electric shock, as well as fire originand cause investigation. Contact him [email protected].

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