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Arc Mapping - Not Valid for Vehicle / Equipment Fire Origin Determination – Part 1 of 3

Whether or not arc mapping should be utilized in motor vehicle / equipment fire origin and cause investigations has been a contested topic since it was first introduced as a potential method for determining the origin of a fire in the 2001 Edition of NFPA 921 - Guide for Fire and Explosion Investigations. The reasons why it is not an appropriate method for determining the area of fire origin in motor vehicle or equipment fires are summarized here, with additional reference information and the new 2020 NFPA 921 perspective, to be provided next month. Further information and background can be found in our previously published papers. (LINKED HERE)

First, we need to define what arc mapping is. NFPA 921-2017 Edition defines arc mapping as “the systematic evaluation of the electrical circuit configuration, spatial relationship of the circuit components, and identification or electrical arc sites to assist in the identification of the area of origin and analysis of the fire’s spread. (emphasis added) Arc mapping is one of four analysis techniques for fire origin determination that NFPA 921 has recommended in Chapter 18 titled “Origin Determination”. Determination of the area of fire origin is always the critical first step in systematic and scientifically based fire origin and cause investigations.

Historically, the concept of arc mapping was originally presented in a book in 1955, arriving at conclusions based on the study of structural fires. Since that original text, multiple studies and test burns have provided further research on the subject of arc mapping as it relates to structures. Technically speaking, all the research on arc mapping, as a method for determining the origin of a fire, was done in structures wired with typical 120/240-volt alternating current (AC) circuits. This detail is critical.

So, why do we say arc mapping cannot apply to vehicle / equipment fires?

Building structures are critically SUBSTANTIALLY ELECTRICALLY DIFFERENT from motor vehicles and equipment. There is no scientific basis to just extend the validity of a methodology to a environment critically different from the development environment -- without critical analysis and potential verification of the methodology within that new environment.

The development and testing of arc mapping has been done in building structures typically with:

  1. Alternating Current (AC) circuits

  2. 120 / 240 volts

  3. Wired with paired hot and neutral (and ground) wires

  4. Using solid copper wire

  5. Non-electrically conductive wood structures

  6. Circuits somewhat evenly and spatially distributed throughout the structure

  7. Independently run and current-protected (fuses, circuit breakers) circuits

NONE of these seven design features typically exist in vehicles or equipment.

Motor vehicles / equipment typically are comprised of:

  1. Direct Current (DC) circuits - steady voltage, not reversing

  2. 12 / 24 volts (insufficient for true arcing)

  3. A common (conductive) ground (or negative)

  4. Stranded conductors

  5. Surrounded by grounded metallic structures - i.e. the common ground / negative

  6. Multiple (mostly positive) circuit wires packed together in harnesses

  7. Varied forms of circuit protection often interspersed throughout the vehicle or equipment

  8. Some circuits not energized depending on the state of operation

  9. High amperage circuits usually without current protection - starters, alternators

Critical analytical comparison of structures and vehicles / equipment shows the electrical architecture features that (may) allow arc mapping to be valid in structures, simply do not exist in vehicles or equipment and thus Arc Mapping is simply not valid for fire origin determination in vehicles / equipment. Not surprisingly, there have also been no validation studies of Arc Mapping in motor vehicles or equipment.

"Arc mapping may not be an effective technique for determining the origin of a fire in automobiles, trucks, and buses..."- NFPA 921 2017

These differences are the reason that the current NFPA 921-2017 Edition Chapter 27 titled “Motor Vehicle Fires” included the caution note that arc mapping "may not be an effective technique for determining the origin of a fire in automobiles, trucks, and buses." -- FYI, it is NOT!

If you disagree, we sincerely ask that you please contact us to discuss! EMAIL US NOW!

Next month, in Part 2 of this article we will share the latest published literature on Arc Mapping. Part 3, coming in October 2020, will include how the updates in NFPA 921 2021* further clarify that arc mapping is inappropriate for vehicle and equipment fire investigations. Additionally, in the future we will share other recently published literature further questioning the validity of arc mapping, for origin determination, in vehicles and structures.

* NFPA 921 - 2021 currently scheduled for release on August 21, 2020

Click here to read our published papers regarding Arc Mapping


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