What caused a crane operator to become a high-voltage conductor
By Myron J. Boyajian, Contributing Editor
A chilly and wet fall day saw a crew of workers unloading roofing shingles at a luxury home on a wooded job site by using a truck-mounted, articulated-boom (knuckle-boom) crane. The crane was mounted on a large flatbed truck between the cab and the flat cargo deck that held the bundles of shingles. In use, this crane looked like a Transformer. As it moved from its transverse storage position, the articulated arm unfolded and extended while its forks turned and rotated to a horizontal position to facilitate lifting loads.
The hydraulically powered crane had a set of control levers near the truck cab and a remote control connected to solenoid valves that duplicated the manual valve functions.
While unloading, the crane operator stood away from the truck and used the remote control, which hung from his neck by a nylon strap to permit two-handed operation. As the last of the roofing was being moved, the workers heard a loud crackle and saw a brilliant electric flash. The crane operator then fell to the ground, his clothes on fire as he writhed in pain. His fellow workers saw that the crane arm was lowered and ran to smother the flames. His overalls were burned from his body, leaving him now shivering from cold and the onset of physical shock.
First responders administered aid for electric shock and serious burns. They brought the man to a trauma center, and he later recovered and brought suit against a host of defendants, including the utility company, the crane and the flatbed truck manufacturer, and the supplier of the remote control, whose attorney retained me to investigate the incident. My task was to investigate the plaintiff’s claim that the control console was “not properly grounded.”
Several issues made this a murky case. The power company argued its 13,000V power lines were not at fault as the rainy weather resulted in lightning strikes. The utility proposed that lightning hit the crane and the operator did not stand sufficiently far away to prevent being struck. They also argued that the truck chassis was negligently designed by not being grounded and was electrically live during the strike. The plaintiff’s employer, normally isolated by workers’ compensation laws, was brought in as a third party for failure to properly train the operator to avoid power line contact.
At the accident site, power lines running through a tree line that paralleled the access road should have been visible. However, I later learned that the wires were invisible at the time of the accident because the utility company failed to trim the tree line on schedule.
The knuckle-boom/truck inspection was attended by a platoon of lawyers and engineers. Among the findings we could see a burn mark on the crane arm, the result of electrical contact. For my part, I was able to carefully lay out the control cable and open the remote control box. In the cable, several control wires were enclosed by a woven metallic shield that was wrapped by a protective and flexible insulated sleeve. The woven metallic cover shielded the control wires from electromagnetic interference. This shield was charred from end to end, and most of the exterior cable insulation was burned off. The switches and potentiometers in the console were destroyed with only stubs of wire and some metallic components remaining.
In my opinion report and deposition, I stated that the remote control and cable were correctly grounded. The shielding prevented electromagnetic interference, not electrical fault currents. The crane arm hit the concealed power lines, but because the truck’s rubber tires were insulators, the high voltage forced current through the control cable, console, and the operator. My client was dismissed from the 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.