James C. Cawley

Occupational electrical injuries in the United States, 1992–1998, and recommendations for safety research

PROBLEM CFOI and SOII data show that 2,287 U.S. workers died and 32,807 workers sustained days away from work due to electrical shock or electrical burn injuries between 1992 and 1998. METHOD The narrative, work activity, job title, source of injury, location, and industry for each fatal electrical accident were examined. A primary causal factor was identified for each fatality. RESULTS Electrical fatalities were categorized into five major groups. Overall, 44% of electrical fatalities occurred in the construction industry. Contact with overhead power lines caused 41% of all electrical fatalities. DISCUSSION Electrical shock caused 99% of fatal and 62% of nonfatal electrical accidents. Comprising about 7% of the U.S. workforce, construction workers sustain 44% of electrical fatalities. Power line contact by mobile equipment occurs in many industries and should be the subject of focused research. Other problem areas are identified and opportunities for research are proposed. IMPACT ON INDUSTRY Improvements in electrical safety in one industry often have application in other industries.

An alarm to warn of overhead power line contact by mobile equipment

Overhead electric powerlines are a serious hazard at mining and mining-related operations. Over one fourth of electrical fatalities in the mining industry are due to accidental overhead line contacts, and for each fatality nearly two serious nonfatal injuries occur due to such contacts. In incidents involving high-reaching mobile equipment, many of the victims touched the equipment after the fact, unaware that the machine frame had become energized by the line contact. MSHA data for accidents involving overhead power line contacts in the mining industry between 1980 and 1997 reveal that in 57% of the cases personnel were unaware of the accidental line contact until after one or more workers touched the equipment or a hoisted load and were injured. This suggests that a device that alerts workers when a power line has been contacted could help prevent many of these injuries. Such a device would not prevent power line contacts, yet if widely employed could yield a significant reduction in the number of resulting injuries. Researchers at the NIOSH Pittsburgh Research Laboratory are attempting to develop such a device. The approach being investigated is based on measuring electric current flow to ground through a machine during a line contact. The specific technique being tested involves the diversion of some part of this current through a shunt cable mounted on board the machine, to provide a point at which to install a current sensor. Experiments indicate that this approach is feasible. Research is better defining electric current flow through mobile equipment and refining techniques for measuring this current.

Development of an overhead power line contact alarm for mobile equipment

This paper describes research by the US National Institute for Occupational Safety and Healths Pittsburgh Research Laboratory to develop an overhead power line contact alarm system for mobile equipment. Analysis of accident reports revealed that many workers were unaware of a power line contact until after an injury occurred, suggesting that many injuries could be prevented by an alarm system that alerts operators and other nearby workers when a line has been contacted. Sensing electric current flow through mobile equipment chasses and measuring electric field strength between equipment chasses and ground were studied as possible techniques for detecting power line contact. Experiments involved using these techniques to monitor energised cranes and dump-bed trucks, operating on commonly encountered types of road and work area surfaces. Sensing current flow proved inadequate when operating on a high-resistivity surface such as asphalt, but electric field measurement was more reliable, performing well on several different surface types. Additionally, electrical characteristics of the cranes and trucks were examined, and this confirmed that, in a power line contact accident, the primary hazard to personnel is simultaneously contacting the equipment and ground. A prototype power line contact alarm system was constructed and tested.

Through phase-angle measurements [induction motor starting safety]

The US National Institute for Occupational Safety and Health investigated how the starting of induction motors may cause nuisance tripping of short-circuit protection on coal mine power systems. This research had a dual purpose: (1) to identify how motor-start waveforms differ from those for short circuits; and (2) to devise a method to provide short-circuit protection without intentional time delays to account for motor starts. This technology will help ensure that surface temperatures of energized electrical apparatus will not exceed gas or dust ignition thresholds when short circuits occur.