Chapter 4 in NFPA 921 (Guide for Fire and Explosion Investigations) is Methodology. It may be a short chapter when compared to the page count of other chapters, but it is the key to all investigations – effectively the Scientific Method. The proper methodology for carrying out a cause and origin investigation is to

  • determine or establish the origin of the fire, and
  • investigate the cause

Investigating the cause has to consider the situation(s) that brought the fuel, oxidant (often air), and ignition source together at the origin location. This sounds rather straight forward, but fires can be very complex and the investigation of the origin and the determination of the cause can be involved. So it is recommended that fire investigations be carried out systematically and follow the scientific method.

Graphical representation of the scientific method.

The figure shown to the right graphically represents the process used for the scientific method. For a fire cause and origin investigation, the identification of the problem has already been taken care of for us – a fire or explosion has occurred. Since we have a problem that exists – the fire/explosion – we need to determine how we are going to solve that problem. In the case of a fire/explosion investigation we are going to carry out a cause and origin investigation. We need to define or review our process for each investigation we carry out because differences may exist and it may require us to conduct different steps. These steps might include the review of a fire department report, the investigation of the scene, the review of photos from others, interviews, chemical analysis of samples, etc.

So what we are doing during that process is the

  • Collection of data,
  • Analysis of the data,
  • Creating a hypothesis (our theory of the origin and cause), and
  • Then the testing of our hypotheses.

The testing of our hypothesis is an important step because that is where we take a hard look at the data and make sure it supports our hypothesis. If  the data does not support our hypothesis, then we need to discard that hypothesis and develop a new one. Keep in mind, that the “data” we use does not just have to be from the scene, it may also include experiments we conduct later or the scientific literature.

Before I wrap up this discussion, we always need to makes sure we don’t make our mind up too soon and jump to the wrong conclusion. For example, we may show up at the scene and hear someone say “… yeah it was the regulator on the propane system that caused the explosion.” With that suggestion in mind, we go about our work only collecting data that supports that hypothesis. That may certainly may be the cause and origin of the explosion, but we need to remember to consider the scene as a whole rather than only focusing on this one aspect. There are times when the first hypothesis is not correct and we need to make sure we have collected all the data we need for other potential hypothesis.  NFPA 921 does a nice job and covers this topic area through sections on Avoid Presumption, Expectation Bias, and Confirmation Bias.


Fire investigations – The basic approach to a fire investigation

Randy Clarksean

Randy is a Ph.D., P.E., CFEI, CFII Mechanical Engineer with over 30 years of experience in failure analysis, fires, and forensic engineering. In addition he has expertise in areas of technical due diligence consulting, heat transfer, thermal systems, management, and general technical consulting services.

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