Machine Design

Truck Fire Started in Wiring Bundle

A mechanic opened his shop one morning to discover a smoldering lump of metal where he had towed a truck the day before.

The mechanic had previously worked on the truck’s two transmissions — one that drove the vehicle and one that drove its utilities — in the field. When the initial repairs failed to solve the problem, the mechanic towed the truck into his shop for a more-thorough work over. The truck was towed and stored without being started. Investigators were called in to see if the transmission work caused the fire.

The edges of the fiberglass hood had burned away, but its center had simply melted. The front tires and some other combustible material in the engine compartment were intact, leading investigators to rule out the engine compartment as the point of ignition.

The cab of the truck sustained worse damage: Anything combustible, from seat covers and foam to steering-wheel and dashboard covers, had burned away. Because combustion was so complete, it appeared the fire had burned hot which is typical of electrical fires.

Wires near the steering wheel that fed the driver’s instrument cluster were mostly intact. However, wires that led to the CB radio and other accessory instruments mounted on the “doghouse” between the passenger and driver seats were broken and bare of insulation. Investigators determined that the accessory wire bundle was the point of ignition.

As Lanny Berke pointed out in his column, many vehicle electrical fires are caused by third-party electronics that are improperly installed. If wiring passes through a vehicle’s frame without a grommet, normal vibration can wear away the wiring insulation and set up conditions for a fire.

In this case, the wires serving the truck’s CB radio and supplementary instrument cluster ignited and set the truck on fire. Those who installed the CB radio and other electronics in the cab should have been careful to protect wires from vibration and wear with grommets.

This month’s safety violation comes from the files of Lanny Berke, a registered professional engineer and Certified Safety Professional involved in forensic engineering since 1972. Got a safety violation to share? Send your images and explanations to [email protected].

Edited by Jessica Shapiro

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