See circuit diagram.
An unfortunate forklift driver becomes part of a lethal circuit
Improper installation of equipment can result in serious injury or worse, but other evidence may hide an accident’s actual cause.
At a tanning plant, cow hides were dunked into a caustic solution to remove hair. A driver lifted a metal pallet of hides over the tank. As he dunked the hides, he called out in pain, fell to the ground and expired. An autopsy confirmed he died from an electric-current-induced cardiac arrest. The driver’s family’s attorney suspected an electrical fault in the forklift and retained me to determine the accident’s cause.
I reviewed the forklift service records, operator and maintenance/repair manuals, and witness statements. I was puzzled because the truck met UL Standard 583. With its isolated 36V dc electrical system, the truck frame was not part of any control or power circuit. Barring catastrophic component or wiring failure, or misuse or tampering, it seemed unlikely the truck hurt the driver. I considered the possibility that the conductive caustic solution in some way splashed onto or into the forklift control system, resulting in an electrically “live” truck. This didn’t add up as the driver’s body would still have to become part of an electrical circuit. Cardiac arrest can occur with only a few milliamps of current, so low voltage could cause injury with a sufficiently low resistance path into the driver’s body, say, if the conductive caustic solution splashed on the driver. The forklift was tested immediately after the accident and found to be in good order, with no electrical fault paths to the truck frame.
Housekeeping and maintenance was a high priority. Spills were cleaned up regularly. In fact, a maintenance crew replaced a broken fluorescent fixture over the caustic tank only a week before the accident. That time, another driver raised the forks, hit and bent the fixture, breaking the lamps. The crew quickly replaced the fixture and its 8-ft-long, 220V ac lamps.
Plant personnel admitted the fixture had also been broken during the fatal accident. The electrician who replaced the fixture prior to this accident admitted, “I might have missed” making a secure ground connection. Whoa! Suddenly, in a perverse way, the broken light fixture lit up the dark corners of this puzzle. The accident sequence seemed clear. The raised mast smashed the ungrounded light fixture and energized the truck with 220V. Without lowering the forks, the driver began dunking the hides into the tank. The driver must have touched both the metal pallet and tank, completing the fault circuit with fatal results. Had the fixture been properly grounded, a circuit breaker would have tripped and removed power. Despite my conclusions, the attorney insisted the truck was at fault. Monetary damages could not be recovered from the employer, hence the need to find fault within the forklift.
My insistence that the truck did not have any electrical faults put an end to my participation in this case.
Myron J. Boyajian, P.E., (firstname.lastname@example.org) is president of Engineering Consultants, a consulting service for forensic and design activities. Cases presented here are from his actual files.