205 Electrical Fires

A majority of electrical fires in a vehicle are caused by the following.

• Wires that have become cut by abrasion and have grounded out.

• Wires that have become displaced and have contacted the manifold or hot portions of the engine block and grounded out.

• Wires that have simply overheated due to overload where the fuses have been bypassed (usually done in home-installed aftermarket equipment).

• Appliances that have failed and have inadequate fusing to catch the fault (also popular in home-installed aftermarket equipment like CBs, radios, and tape decks).

Often, the damages associated with an electrical fire are located on either side of the bulkhead or fire wall. This is, of course, because this is where most of the wiring harnesses are located. Because of the way equipment in the engine compartment is packed together, it is possible for an electrically caused fire to turn into a fuel system fed fire. If the fuel system fed fire is severe enough, it may hide or obscure the electrical origins of the fire.

An electrical fire under the dashboard will usually create inside-out damages, that is, damages that are most severe on the interior, and whose severity diminishes toward the exterior or perimeter of the area. Because the wiring is usually run just under the dashboard, the area will have a hollowed out appearance.

Most of the plastic materials used in modern automobile dashboards do not readily support a flame or fire. However, they will melt, char, and emit copious dense smoke, and may support flames and fire as long as the electrical fault can supply heat energy.

An examination of the fuses will often provide information as to which grouping of equipment was involved in the fault. Then, the point of origin has to be tracked down like any other electrical fault. However, in an automobile, a ground fault can occur any time the bare wiring can make contact with the metal frame or parts of a car.

The metal frame is the return wire for most circuits. The negative side of all the circuits in the vehicle are run through the metal frame as a common ground return. Thus, once heat reaches a group of wires in a harness that is positioned on a metal fender or the bulkhead, and the protective insulation melts away, it is possible for nearly all the wires in the harness to ground fault at more or less the same time at the same place.

The home installation of aftermarket equipment often causes electrical fires in vehicles. The handyman is often unfamiliar with the circuits, and will simply tap into the most convenient wire he can find. Usually, the fuse block is bypassed, so the new item does not have the benefit of overcurrent protection. Further, the wires to the new item will be loose, and often simply tucked loosely under the carpet (where they can be stepped on repeatedly), or laid over metal pieces that may eventually abrade and cut into the insulation.

Because a car is constantly vibrated when rolling along, loose wiring has a much higher probability of abrading itself on nearby edges or rough surfaces. This is one reason why the manufacturer groups the wiring in harnesses. Sometimes, when repair work is done on an electrical circuit, it is considered too much trouble for the mechanic to put the particular wire back into the bundle or harness as before. He may simply tuck it in at a convenient spot, or tape it to something nearby to "hold it down."

Of course, loose wiring in the engine compartment may also eventually come to rest on either the manifold or the engine block. Since much of the wiring is simply sheathed in relatively low temperature plastic insulation, proximity to the hot items may be enough to damage the insulation and allow the circuit to ground fault.

Forensic Engineering Investigation

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