As farfetched as it sounds, this author investigated a Recreational Vehicle (RV) fire where the cause was tire failure. Consider these facts:

• RV being operated on highway when right-front tire blew out.
• Driver pulled over to right side of highway.
• Witnesses photographed active fire at right-front corner of RV.
• Fire damage to RV included near-complete melting of right-front aluminum wheel hub, with zero melting of the other five aluminum hubs.
• Burn patterns on RV exterior indicated a low point of burning at right-front wheel well, and V-like patterns rising upward and spreading outward from there.

While the facts above are fully consistent with a fire origin at or near the right front wheel well, and no other origin location would be equally consistent, the evidence above fails to provide direct evidence of any fire causation mechanism. To discover the cause of the fire, our team found and inspected an exemplar RV of the same make and model and discovered an entirely new set of facts about the RV design – none of which survived the fire.  Consider the inset photo below and the following additional facts:

• Several sources of combustible plastic and rubber were present inside the wheel well.
• A set of four conductors (two of which were 10 AWG solid copper wire) inside a plastic wire loom was run through the wheel well.
• The heavy-gauge wires provided power to the front passenger seat adjustment motor and were energized whenever the ignition key was in run or accessory mode.
• The wire loom and wires were a few inches above the tire’s upper surface and a few inches inward from the tire’s inward edge.
• When a rotating tire fails, elements of steel belting can partially disengage and whip around repeatedly at high speed, impacting softer materials within their reach.

Thus, after inspecting the exemplar, our team was able to supplement the burn pattern information with design information that confirmed a source of ignition (energized conductors with contemporaneously-damaged insulation) with several sources of fuel (plastic, rubber and plywood) in the area of origin.  Our causation scenario was the only hypothesis under consideration that was fully consistent with all of the facts – tire failure, followed by steel-belt whipping and damaging energized conductors, followed by ignition of nearby combustible plastics, followed by fire spread to right-front corner of RV structure.

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations.  We hope you are instructed by this content, and we encourage you to contact us if you seek additional information.

Many people my age will remember the television show Mission Impossible from the late 1960s. At the very beginning of each episode, the lead character, Jim Phelps (played by actor Peter Graves) and his team of secret agents would receive a new mission from headquarters, and they would have 60 minutes (of TV time) to execute it in the face of all sorts of obstacles. The instructions and background information for the Impossible Mission Force team were always provided in a large brown envelope and a reel-to-reel audio tape player. At the end of the tape, the voice would always say, “This tape will self-destruct in five seconds. Good luck, Jim.” The tape player would suddenly start smoking and you would see the tape warping and melting as it succumbed to the mysterious fuming and heating. It almost seemed that the device was undergoing spontaneous combustion.
If you think about it, self-destruction is the essence of spontaneous combustion, but spontaneous combustion is not limited to items that have outlived their usefulness. In fact, when spontaneous combustion occurs, it almost never happens because someone intended for the material to be incinerated.
Regardless, the nagging question about spontaneous combustion not how, but why would the material survive for years or decades without undergoing self-ignition, and then one day, its surroundings change and its own tendency to undergo self-heating chemistry launches its voyage toward self-ignition and self-destruction? To paraphrase James Carville, “…It’s the environment, stupid.”
The classic scenario for enabling spontaneous combustion is filling a waste basket with linseed oil-soaked rags and waiting 3 to 12 hours. Linseed oil, like most vegetable oils, contains oxygenated hydrocarbon and these constituents continuously undergo exothermic reactions. Thankfully, the reaction rate is so slow that under normal conditions, the (neat) oils naturally dissipate such heat through conduction and convection and the liquid never warms up or approaches thermal runaway.
Conversely, when a film of the same oil is adhering to the woven interstices of cloth rags accumulated into a pile, the self-heating reactions occurring at the center of the pile deposit their heat into a space that is well insulated from the surroundings, and the heat loss is minimal. This allows the temperature to rise locally, which causes the reaction rate to grow exponentially, and eventually thermal runaway is reached. At that point, the limitation on how much longer it will take for the material to begin smoldering and to ultimately ignite is governed by the ability of fresh air to diffuse through the openings in the rags and to provide oxygen to the reacting organic material at the center of the pile.
NFPA cites the following statistics about spontaneous combustion or chemical reaction caused fires: average of 14,070 fires per year between 2005 and 2009, including 3,200 structure fires, 1,150 vehicle fires, 5,250 outside non-trash fires, and 4,460 outside trash or rubbish fires. The following are known to be capable of spontaneous combustion: Activated carbon, Oily cotton, Paper, Seed cake, Celluloid scrap, Linen stacks, Log piles, Coal piles, Haystacks, Compost piles.
Agricultural raw materials and food or feed products are very likely to undergo spontaneous combustion – especially if they are being heated in an oven or dryer. NFPA 61 states: “Spontaneous ignition is a primary cause of dryer fires and explosions. The requisites of this phenomenon are a heated surface or a hot airstream, a layer of product exposed to this heat, and time.” The four factors that make spontaneous ignition more likely are: (a) size of pile and effectiveness of self-insulation; (b) temperature of surroundings; (c) ability of oxygen to diffuse to the reaction zone; (d) sufficient time for self-heating reactions to accelerate.
Unfortunately, safe handling of linseed oil-soaked rags is not entirely intuitive, and spontaneously ignited fires continue to occur via many different scenarios. Warning labels on cans of linseed oil state: “USE EXTREME CAUTION. Immediately after use and before disposal or storage, you MUST (1) hand-wash rags thoroughly with water and detergent outside in a bucket and rinse. Repeat washing and rinsing until you have removed all oil from all cloths, rags, paper, … and any other materials contacted during use or because of an accidental spill. (2) spread all rinsed materials outside to dry by flattening them out to their full size in an airy spot for at least 24 hours until completely dry.”
This author has investigated fires where spontaneous combustion was the only likely ignition source and others where allegations of spontaneous combustion were proven incorrect upon further investigation.
The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations. We hope you are instructed by this content, and we encourage you to contact us if you seek additional information.

The author of this blog is pleased to announce that a new Mobile Investigation Workshop has been added to the Martin Thermal Engineering collection of tools.

The workshop is equipped with many types of hand and power tools for disassembling products and extracting evidence from a fire scene, as well as instruments that are vital to the conduct of appliance tests and exemplar examinations, including thermocouples, pressure gauges, and flow meters. Multiple video cameras with tripods can be deployed to monitor and record mechanical meters (e.g. gas volume).

We can also extract samples of automotive fluids and combustion gases for submission to an analytical lab for chemical characterization. Evidence chain of custody paperwork is maintained in a folder on board. Marking and labeling tags and signs can be employed when multiple pieces of evidence require identification.

A battery charging station, complete with inverter is soon to be installed, which will make the mobile workshop self-sustaining for multi-day inspections.

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations. We hope you are instructed by this content, and we encourage you to contact us if you seek additional information.

Over the past 25 years, the National Institute of Standards and Technology (NIST) has developed, augmented and improved an important computational tool for fire investigators – the Fire Dynamics Simulator (FDS) code.

FDS is a flow, heat, and chemistry modeling application that is utilizes Computational Fluid Dynamics (CFD) to model fires and other flows that are important to fire safety engineers and fire investigators. While other commercial CFD programs exist, their cost to license and their computational cost (hours of runtime) tend to make them inaccessible for small enterprises, including many fire investigation firms.

Fortuitously, FDS offered breakthroughs on several fronts that helped bring the power of CFD modeling to smaller practitioners. (Recently, a user-friendly, front-end package was developed by Thunderhead Engineering http://www.thunderheadeng.com/ and can be licensed for a reasonable annual fee.) Because the underlying code was developed by the U.S. Government, it is downloadable and usable by the general public with no license fees. Secondly, the program can be run on Windows-based desktop and laptop computers, so there is no need to purchase expensive hardware. Perhaps most importantly, the program uses a computational simplification called “Large Eddy Simulation” (LES) to enhance the speed at which complex flows can be solved numerically.

While LES does employ a computational shortcut (where only large eddies are directly solved and the dissipative energy generation of the small eddies is modeled as a byproduct of the large eddies) as compared to CFD programs that utilize Direct Numerical Simulation (where all the equations are solved for all sizes of turbulent eddies), the LES technique nevertheless produces fully-validated results for many fire problems.

The video posted below is a FDS simulation of a gasoline leak under a car inside a garage. The model incorporates a source of heptane vapor being released from a square “puddle” under the vehicle’s engine compartment. Contours of heptane concentration versus time are shown in the simulation output – red is flammable, orange is possibly flammable, yellow is probably not flammable, and green/blue/purple are not flammable. It is noteworthy that the red contours do cover the entire floor surface, but because gasoline vapors are heavier than air, the flammable concentrations don’t rise up very high above the floor level.

Also posted below is a plot of hydrocarbon concentration measured by four sensors located in the corners of the garage, at an elevation of approximately 18 inches above the floor. The Consumer Product Safety Commission performed tests of the safety of water heaters in garages approximately 25 years ago and found that flash fires involving gasoline spills ignited by the pilot flame of a water heater were largely preventable if the water heaters were installed on pedestals so that the flames were at least 18 inches up in the air.

The plot below validates the CPSC’s finding. The maximum concentration at the four sensors during the 10 minute duration of the simulation run was 7 parts per million, by volume. This is more than 99.9% smaller (three orders of magnitude smaller) than the Lower Flammable Limit of heptane (1.0 percent by volume). In other words, the mixture is too lean to produce a deflagration at the elevation of the water heater’s burner, if the heater is installed on pedestal, in adherence with the requirements of the National Fuel Gas Code.

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations. We hope you are instructed by this content, and we encourage you to contact us if you seek additional information.

When I was a teenager, I remember my Dad occasionally becoming frustrated with my reluctance to rake leaves and pull weeds.  Thinking myself a modern-day Tom Sawyer, I once suggested to him that my interest in weeding and raking would go up tremendously if I could have a couple of friends over to assist.  In response, he recited an old proverb he had been told in his youth, “As my Dad always said, ‘Two boys is half a boy, and three boys is no boy at all.’”

While I wasn’t exactly thrilled with my Dad’s cold water pronouncement, the adage stuck with me.  Years later, it popped back into my brain in a most unusual context – while I was performing an automobile fire investigation.

According to NFPA 921 “Guide for Fire and Explosion Investigations” there are a wide variety of substances that may serve as the material first ignited in a motor vehicle fire, including gasoline or diesel fuel; lubricating oil; transmission, power steering, brake, or windshield wiper fluids; coolant; battery vapors; plastic wire insulation, casings, trays, or trim; rubber hoses or belts; natural or synthetic fabrics or upholstery; cargo; and other contents.  Sources of ignition are only slightly less plentiful – hot surfaces; friction; mechanical sparks; electric arcs; overloaded wiring; open flames; and smoking materials.

A relatively common cause of vehicle fires is the unsafe installation of aftermarket electrical appliances, and this author has investigated many such incidents.  While many third-party audio/video systems are installed with kits that are engineered to work flawlessly in conjunction with the vehicle’s original wiring, some are not meant for automobile installation at all.  The biggest problems are (a) devices that consume too much power, (b) use of wiring that is improperly sized or inadequately safeguarded against excess current, and (c) installation of wiring in locations that are insufficiently protected and susceptible to insulation damage.

In at least three vehicle fire investigations, I have determined the cause to be improper wire installation that led to (a) chafing, (b) insulation failure, (c) short-circuit current flow from battery to ground, and (d) melting and ignition of the surrounding plastic wire insulation.

Not surprisingly, the other common feature in each of these fire incidents was inadequate overcurrent protection.  In one case, there was no fuse installed anywhere in the circuit.  In the second case, the fuse was installed on the wrong leg (neutral instead of hot).  And in the third case, a fuse was present, but its rating was too high to protect the conductor from overheating.

Which brings us back full circle to the adage my father told me (with minor adaptation) – “A fuse on the wrong leg is like half a fuse, and an overrated fuse is like no fuse at all.”

A fire that originated inside a restaurant in a strip mall became the subject of litigation almost three years after the date of the fire. An adjacent tenant in the mall was pursuing a claim against the restaurant owner seeking recovery for his losses.

Subsequent to the fire, but before we were retained by the adjacent tenant’s law firm, the property had been rebuilt. Consequently, it was impossible for us to perform a site inspection, but we were able to review photographs taken by the responding fire department’s arson investigator.

Every investigator will acknowledge that his level of confidence in his own conclusions is usually higher when he has had the opportunity to actually examine the site and the physical evidence. Nonetheless, good investigators can often develop strong conclusions from photographic evidence alone, as long as other available evidence (e.g., eyewitness testimony, building records, fire personnel observations) corroborates the findings.

One technique that can be especially helpful for analyzing historical photos is photogrammetry. In the subject case, we wanted to determine if an oven’s exhaust duct was properly installed (i.e., did it have sufficient clearance between the duct’s hot walls and the building’s wood beams and joists). We used photogrammetry to assess the dimensions of both items.

Photogrammetry is the art and science of determining geometric properties of objects from photographic images. The two images on this anecdote page give examples of a photogrammetric analysis conducted as part of our strip mall fire investigation. In both images, the superimposed green lines represented known dimensions and the superimposed red lines represented unknown dimensions. By computing the relative lengths of the red and green lines, a multiplier was obtained and the unknown dimensions were determined from the known dimensions.

In the first photo above, the known dimension (green line) was a 4-inch electrical junction box, and the unknown dimension (red line) was the width of the square exhaust duct (determined by the analysis to be 24 inches).

In the second photo, the known dimension (green) was a 16-inch cement block, and the unknown dimension (red) was the distance between joist hangers (also determined to be 24 inches).

The photogrammetric results were ultimately compared to eyewitness statements and burn patterns to form a sound conclusion about the origin and cause of the fire.

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations. We hope you are instructed by this content, and we encourage you to contact us if you seek additional information. Copyright Martin Thermal Engineering, Inc. (2013)

Portable cookstoves are used by caterers and campers to bring the convenience of a “kitchen appliance” to places where such appliances are normally not available. Some cookstoves are fueled with liquefied propane that is supplied in 1-lb canisters (first photo) or 20-lb containers (second photo). Others utilize liquefied butane that is supplied in aerosol-like cans (third photo). While all these systems are equipped with unique safeguards, certain types have failed catastrophically, injuring workers and guests.

Cookstove Safeguards. Butane cookstoves (example in fourth photo) are generally equipped with a multi-function gas safety valve. When the fuel can is installed, a lever must be pressed to latch the can into place so that the gas will begin flowing through the valve to the burner. The valve also has a pressure-safety pin that is designed to trip the latch and disengage the can when the fuel pressure exceeds a safety threshold. However, in some models, the retracting mechanism has been observed to fail intermittently and the can fails to disengage even though the pressure-safety pin performed as it should.

Can Safeguards. Small butane fuel cans are also equipped with certain safety features that are designed to prevent or mitigate catastrophic releases of flammable gas. Because the fuel cans are installed horizontally into the cook-stove chassis, the fuel withdrawal tube is equipped with a right-angle extension to ensure that butane vapor is withdrawn from the headspace, rather than butane liquid from the lower portion of the can. The neck of the can contains a feature that is intended to prevent improper rotation in the stove, but users can unknowingly defeat this safeguard and install the can in the wrong orientation, permitting butane liquid to be withdrawn instead of butane vapor, which is undesirable.

This author has investigated several flash fires involving portable cookstoves and has identified product defects as well as certain ways the products can be misused. In the worst-case scenario, the fuel canister overheats and ruptures, releasing a fireball that causes personal injury and property damage.

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations. We hope you are instructed by this content, and we encourage you to contact us if you seek additional information. Copyright Martin Thermal Engineering, Inc. (2013)