A fire has severely damaged a two-story single family home and the room of origin has been determined to be the living room. A fire marshal has determined that the area of the fire’s origin is between a window and the front door, a space of about 36 inches. The basis for that determination is an area of clean burning in that 36-inch space between the door and the window. In that space is a small electrical appliance plugged into a wall outlet. Apparently unknown to the fire marshal is the fact that the housing of the small electrical appliance is made of V0 plastic and the electrical circuit board within the product is non-flammable. The plastic housing of the appliance melted away during the fire leaving behind two tiny power conductors that have evidence of melting on their ends. There are virtually no fuels in the immediate area available for fire spread and development from the fire marshal’s point of origin on the wall to the rest of the room.
It developed that at the time of this fire investigation, the fire marshal was relatively new to the job and had investigated only 34 fires. A fire investigator hired by the subrogation interest quickly agreed with the fire marshal that the fire started at the outlet and that the electrical product had to be the cause. Fire investigators hired by the product manufacturer concluded that the area of clean burn, relied upon by the fire marshal, was caused by the effects of ventilation and that the total destruction of the living room prevented the clear identification of the fire’s origin. They concluded that the correct determination was that the cause of the fire was “undetermined.”
As this is an active claim, I am unable to more clearly identify the product and present pictures of the burn patterns in question, however, I will do so in part two of this editorial once the matter has been resolved.
In defending this litigation, I became more familiar with the somewhat confusing effects of a ventilation-controlled fire as it became evident that the fire marshal was confusing evidence of ventilation-controlled fire with fire patterns indicating the incipient stages of a fire. Consequently, this topic is a focus of this edition of Perspectives. Those of us in the business of fire investigation or fire litigation, understand that a correct determination of the origin of a fire is absolutely critical for the correct determination of the fire’s cause.
Burn Pattern Analysis
Burn pattern analysis is heavily relied upon as a tool in the determination of a fire’s origin. Areas of “greatest burning,” “clean burn” and “V” patterns are often the basis upon which the area of origin is determined. Certainly, in the initial stages of a fire, where fire growth, spread and development are based upon the location of fuel, the reliance upon these and other burn patterns can be quite helpful in determining the area of the fire’s origin. However, after a fire transitions into full room involvement, it is likely to become ventilation-controlled. To understand the concept of a ventilation-controlled fire, it is important to remember that the materials which make-up the various fuels in a room, such as a couch or a chair, do not themselves burn. The fuel must first be heated to a point where flammable vapors are given off by the object and it is the flammable vapor which actually “burns” as visible fire. This is called flaming ignition and it is what we normally think of when we think of fire. Yellow, red, or orange flames are observed during the burning of the flammable vapors when they reach their ignition temperature in the presence of sufficient oxygen. When a room reaches full room involvement, usually after transitioning through flash-over, nearly all fuels in the room are heated to the point where they emit flammable vapors. It is at this stage, that a fire could become ventilation-controlled and flaming ignition will only take place in those areas of the room where there is sufficient ventilation, usually as the result of a broken window or an open door.
Picture a room with several windows and a door in which a fire has been started and where the fire has spread to the point where all oxygen has been consumed. At that point in the fire, it will either self-extinguish, smolder, or continue to burn in the room as oxygen is allowed to enter. If a door is opened, oxygen will rush in at floor level, pushing the hot gases out the upper level of the door. The super-heated gases will now self-ignite as they flow out the doorway and in those areas of the room where there is oxygen present. You will see fire being emitted out the top of the open window or doorway as the hot gases are pushed out into the open air. Fire (flaming ignition) will occur in those areas of the room where the oxygen is flowing into the room from the outside. The burning of the super-heated gases will not necessarily be at the same location where the initial “fuels” are located. The flaming ignition will occur only where the super-heated vapors, having been emitted from the original fuel, come into contact with the newly arrived oxygen supply. As an example, flammable vapors being emitted from a couch into a room with depleted oxygen will flow up to the ceiling and move throughout the room. When a door is opened or a window breaks and oxygen is allowed to enter, those super-heated vapors from the couch will self-ignite when mixed with the proper amount of oxygen. Quite often this occurs at a great distance from the couch and perhaps at a location where no fuel was initially available for combustion. The fire is controlled, not by the original fuels in the room, but by the available ventilation. The room now burns wildly wherever oxygen is available. These burn patterns left behind by a ventilation-controlled fire can be quite confusing to the fire origin investigator.
Burn patterns which were present during the initial stages of the fire and which could be correctly relied upon to determine the area of origin, may be obliterated and could certainly be confused by other patterns which developed during the period of time when the fire was ventilation-controlled. Classic “V” patterns, clean areas of burning and areas of greatest burning may now be correctly attributed to the effects of ventilation at later stages of the fire, rather than the fire’s incipient stage.
Conclusion
I have witnessed many fires where areas of “clean burning” and “V” patterns were thought to point to a fire’s origin, when in fact, it later became apparent that no fuel or very little fuel was in the area where these patterns appeared and therefore could not have been caused by the fire’s incipient or initial stage. Similarly, burn patterns along the floor caused by ventilation entering through an open door have been known to mimic pour patterns often attributable to an arson fire, when in fact this low intense burning resulted from the ignition of super-heated gases when mixed with oxygen flowing through the open doorways.
The effects of ventilation-controlled fires and their impact on fire origin and cause are founded in scientific principles and warrant an analysis beyond that which can be provided in this commentary. Fortunately, it is the subject of the in-depth article (Improving the Understanding of Post-Flashover Fire Behavior) in this edition of Perspectives written by Steven Carman, a highly qualified and respected fire investigator who graciously allowed me to print his article written following burn cell testing which he and others conducted in 2005. (Two additional Carman articles on related subjects appear in full text on our website: Investigation of an Elevated Fire – Perspectives on the ‘Z-Factor’ and Progressive Burn Pattern Development in Post-Flashover Fires). Another valuable resource which clearly explains and dramatizes the effects of a ventilation- controlled fire can be found at www.cfitrainer.net, a free website which has been developed to provide training to fire investigators and anyone interested in learning more about fire and fire investigation. I highly recommend that everyone reading this editorial access that website and open the two modules entitled “A Ventilation-Focused Approach to the Impact of Building Structures and Systems on Fire Development,” and, “Post Flashover Fires.” Steve and others did a remarkable job in creating these modules to explain the analysis that must be undertaken when attempting to determine the area of origin of a fire in a room in which the burning was controlled by ventilation.
Unfortunately, the effects of a ventilation-controlled fire are still not fully understood by many fire investigators. Most fire investigators today understand that burning around a broken window or open door may be caused by the effects of ventilation. However, the cause of burn patterns that occur at low levels or in areas of the room of origin, away from a broken window or open door which can be correctly attributable to the effects of ventilation are still sometimes confused with incipient fire patterns.
— Frederick B. Tedford
