We all have had the experience of dealing with an "infernal machine," one seemingly possessed by the devil. This case was one of those times.
Late on a chilly February night, a police officer was working his beat, checking the perimeter of a suburban shopping center. After verifying that all doors were solidly locked and closed, he noticed through the window of a carpeting store the flickering light of a fire.
He dashed to his patrol car and radioed in a call for help. Firefighters quickly arrived, forced open the door, and proceeded to strike out the fire. Fire officials and the insurance carrier's expert examined the site, noting that a battery-powered lift truck had struck some racking with enough force to spew rolls of carpet onto the floor. Given the extensive fire, water, and smoke damage, losses were estimated to exceed $1M. The insurance carrier filed suit against the truck manufacturer and maintenance contractor that had serviced the truck just a week before the accident.
Although experts agreed that the fire originated with the truck, they were mystified as to how the unoccupied vehicle came to ram into the racks. The owner's son testified he was the last person to drive the truck, parking it near, but not connected to, a battery charger. He left the battery connected to the truck, turned off the key switch, and placed the forward-reverse control in neutral. Security logs confirmed no forced entry except for the fire department's efforts.
The attorney representing the insurance carrier retained my services to inspect the 4,000-lb, sit-down-style truck and find the "devil"—a rational explanation for the accident.
It was impossible to tell whether the melted key switch was in the "on" or "off" position, but I was able to confirm that the direction switch was in the "neutral" position. With the toasted seat raised, I was able to rotate the parking-brake drum by hand, proof that the brake was not functioning. In the rear control compartment, there was no damage to the contactors, cables, or wires, and no other visible physical damage that might have caused motor operation and truck motion.
But low on the truck frame in the speed control compartment, I found burned insulation of the cables, wires, and all of the non-metallic components of the motor control. The stranded copper wires in some cables had gouged-out burn marks from electrical arcing. Other burned cables had globules of melted copper on each strand. Some cables had totally melted and resolidified, others were welded together or to the truck frame. The frame edge and cable clamp had marks of electrical arcing. I found spilled hydraulic fluid on the motor and frame. A cable opening in the frame created a pathway for the arcing cables to ignite the fluid.
It was possible to trace and identify the cables connected to the motor control panel by their termination points on the traction motor and control panel itself. I marked a spot on the manufacturer's installation drawings for each burned or melted cable. By connecting the dots, a picture emerged of an electrical path that could power up the drive motor without the controller being energized. The non-functioning parking brake allowed uncontrolled travel and the subsequent collision.
My next task was to determine a cause for the insulation failure and the resulting electrical fault paths. Service records confirmed that the truck received monthly maintenance during which the hydraulic oil tank was topped off. The technician also regularly opened the hinged control compartment door to blow out any foreign material. In testimony, he said the cables were a tight fit in the compartment. It was obvious that under repeated opening and closing, the sharp edges of the cable clamp and the frame edges gradually cut through the cable insulation, permitting an electrical fault path and arcing between cables of opposite polarity. These fault paths caused drive motor operation and truck travel.
At the trial, I presented my conclusions that the fire was due to both the faulty cable restraint means and poor cable layout, as well as the technician's lack of diligence in dressing the cables when closing the control door, his failure to adjust the parking brake, and failure to prevent spillage of the hydraulic oil.
The jury found the service technician at fault and liable for damages. It also found the owner's son partially at fault, as regulations clearly specify that the battery be disconnected when a truck is not in use. Although the jury may not have agreed with all of my opinions regarding the cause of the fire, they certainly agreed that "the devil didn't make me do it."
Myron J. Boyajian, P.E., (ntesla@ieee.org) is president of Engineering Consultants, a consulting service for forensic and design activities. Cases here are drawn from his files.
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