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.”