Chapter 17 Fire Cause Determination

Chapter 17 Fire Cause Determination

Knowledge Objectives (1 of 2) Explain the roles that source and form of heat of ignition play in fire cause. Discuss identification of the first fuel ignited. Identify the potential oxidizing agents at a fire scene. Discuss identification of the ignition sequence.

Knowledge Objectives (2 of 2) Explain the use of the scientific method in cause determination. Describe the process of elimination and negative corpus. Discuss levels of certainty in investigative opinions.

Additional NFPA Reference NFPA 921, Guide for Fire and Explosion Investigations

Introduction Fire cause determination identifies: – Ignition sources – First fuel ignited – Oxidizing agent – Human actions Also considers: – Circumstances, conditions, agencies that brought the fuel, ignition source, and oxidizer together

Ignition Source (1 of 3) At point of origin when the fire started – Postfire physical evidence of it may remain. – It may also have been destroyed or removed. FirePhoto/Alamy

Ignition Source (2 of 3) Any identified ignition source must be competent. – It has sufficient energy to ignite the first fuel. – It is capable of transferring that energy to the fuel long enough to bring it to ignition temperature.

Ignition Source (3 of 3) Three steps: – Generation – Transmission – Heating The ignition source for ignitable vapors can be difficult to identify.

First Fuel Ignited Identify the first fuel that sustains combustion beyond ignition source. Consider configuration of fuel. – With like fuels, one with a higher surface-tomass ratio will require less thermal energy to ignite. – A pile of shavings requires less energy to ignite than a wood block.

Oxidizing Agent Generally just the oxygen in the air Fire intensity can be enhanced by the addition of other oxidants. – Medical oxygen cylinders, pool sanitizers Residue of chemical oxidants may remain after the fire. – Collect, test, document. – Consider their role in the fire.

Ignition Sequence Johner Images/Getty Images. Identify events that brought ignition source and fuel together to establish cause. Be prepared to describe the way fire spread from the first fuel.

The Scientific Method in Cause Determination (1 of 2) Determining cause follows the accurate identification of origin. Data must be collected and analyzed. Cause hypotheses can then be developed and tested for each individual heat source.

The Scientific Method in Cause Determination (2 of 2) Appropriate way to apply scientific method is to attempt to disprove each hypothesis Hypotheses that cannot be disproved may be possible or probable.

Data Collection Essential data – Identification of fuel packages that sustained combustion beyond ignition (fuel analysis) – Potential ignition sources (source analysis) – Unusual oxidants in area of origin Information from persons with knowledge of the area and prefire photos can be helpful. Consider ignition sources that do not correspond to a physical item on scene, such as static electricity.

Data Analysis (1 of 4) Data analysis can be done once all potential initial fuels, ignition sources, oxidants, and relevant circumstances have been identified. Investigator must not jump to conclusions. A separate hypothesis must be tested for each fuel/ignition source/oxidant combination.

Data Analysis (2 of 4) Ignition sequence must be identified. – Ignition source can sometimes be logically inferred when physical evidence is lacking. – Testing of alternative hypotheses can help lead to the one sequence that is consistent with all known facts.

Data Analysis (3 of 4) Certain evidence supports the inference of a reliable ignition sequence. – Diffuse fuel ignitions – Multiple origins – Trailers – Witnessed ignitions – Presence of ignitable liquids

Data Analysis (4 of 4) Factors to consider – How and when initial fuel came to origin – How and when oxidant came to be present – How and when competent ignition source came to be present – How and when competent ignition source transferred heat to initial fuel – How safety devices (failed to) operate(d) – What acts or omissions brought together the fuel, oxidizer, and ignition source – How initial fuel ignited subsequent fuels

Cause Hypotheses Develop and test alternate hypotheses of cause to identify the one that is consistent with all known facts. Hypothesis testing may include reference to scientific and trade literature. You may do your own physical tests or experiments.

Process of Elimination and Negative Corpus (1 of 2) The process of elimination is essential to the analysis of cause and to the investigator’s efforts to test and evaluate hypotheses. In many investigations, the investigator will be successful in testing and disproving a range of hypotheses, to the point that one remains that has not been able to be refuted.

Process of Elimination and Negative Corpus (2 of 2) The remaining hypothesis must not be adopted as the investigator’s final opinion simply because it is the last one standing. – This is an improper technique known as negative corpus. Base all hypotheses and determinations on the analysis of facts, derived from observations, evidence, science, calculations, and experiments.

Levels of Certainty Using scientific method the investigator should establish a level of certainty to opinions formed. – Probable—more likely true than not – Possible—feasible but not probable Do not confuse these terms with the term “probable cause” used in criminal investigations.

Summary (1 of 5) Determining cause is not limited to identifying the ignition sources, the first fuel ignited, the oxidizing agent, or the human actions. It also involves identification of the circumstances, conditions, or agencies that brought the fuel, ignition source, and oxidizer together and the conditions under which the fire was able to start and propagate.

Summary (2 of 5) Many times the source of the ignition or its remains will still be at the point of origin. Identification of the first fuel ignited is an important part of the hypothesis testing. Depending on the properties of the ignition source, physical evidence of it may remain at the point of origin.

Summary (3 of 5) In most fires, the oxygen in the air will serve as the oxidizing agent, or oxidant. Fire intensity can be enhanced by introduction of other oxidants into a fire. You should identify the ignition sequence or the events that brought the ignition source and the fuel together, thus establishing the fire cause and responsibility.

Summary (4 of 5) The appropriate way to apply the scientific method to testing the cause hypothesis is to attempt to disprove the hypothesis. Essential data to be collected to determine cause include identification of fuels, potential ignition sources, and unusual oxidants in the area of origin.

Summary (5 of 5) Once all of the potential initial fuels, potential ignition sources, oxidants, and relevant circumstances/activities have been identified, analysis of the whole of the data can be conducted. You should establish a level of certainty to opinions formed.