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.

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.”