NIOSH Mining Safety and Health Research

During the 1990s, electricity was the fourth leading cause of death in the mining workplace; 1 of every 26 electrical incidents resulted in a fatality. During this same time, there were 31,370 lost workdays from electrically related incidents. The most common result of incidental electrical contacts was burns, which were rarely fatal. In contrast, electrical shock caused 70 of the reported 75 electrical fatalities during this period. The objective of PRL´s electrical safety research program is to improve electrical safety in the workplace. In the past, research has focused on trailing cables, motors, trolley systems, ground-fault protection, explosion-proof enclosures, and intrinsically safe circuits.

In the project "Fundamental Studies in Electrical Hazards," researchers are identifying, analyzing, and classifying electrocutions and injuries from incidents across workplaces to determine appropriate strategies and technologies to reduce their occurrence. Data from the Bureau of Labor Statistics (BLS) Census of Fatal Occupational Injuries (CFOI) show that 2,287 U.S. workers died of occupationally sustained electrical shock or electrical burn injuries during 1992-98. In addition, the BLS Survey of Occupational Injury and Illnesses (SOII) statistically estimates that 33,890 workers had lost workdays due to electrical shocks and burns during the same period. Although electricity caused only 1 in 500 nonfatal lost workday incidents, it caused 1 in 20 occupational deaths. Analysis of 2,287 CFOI narratives classified fatal electrical incidents into major causal categories by work activity or equipment. Lack of a work activity designation and narrative in the SOII prevented a similar analysis of nonfatal incidents. Based on the incident analyses, intervention strategies to reduce the occurrence of burn injuries have been proposed in a mining concept paper for 2003. These include examining alternate methods of detecting arcs in their incipient stages, improving battery jumper cable technology to isolate personnel, and using an energy-limited power source for troubleshooting. These proposed engineering controls would be combined with recommendations for appropriate training and personal protective equipment.

Overhead electric power lines are a serious electrocution hazard to crane, truck, and drill rig operators in the mining industry. In the project "Overhead Power Line Contact Alarm for Mobile Equipment," two methods to sense contact with a power line overhead are being investigated. One uses a current transformer to detect leakage currents through the vehicle into the ground surface; the other is an electric field sensor. Patents have been sought for both concepts. Both have been tested at PRL using a crane and a dump-bed truck on bare earth, grass, gravel, concrete, and asphalt surfaces. The findings indicate that the current sensing approach may not be appropriate for asphalt surfaces due to extremely low current leakage to earth. Consequently, a technique using dual-sensing means may be more applicable to all possible vehicle-to-ground contacts. Prototype devices are being built. Two commercial firms have signed nondisclosure agreements expressing interest in eventual commercialization of the prototype. An additional safety device, the insulated load link, is also being investigated. Insulated load links are commercially available to electrically isolate a hoisted load from the metallic hoist rope. They are effective, but are limited in use due to costs ranging from $3,000 to $10,000. A low-cost link using Kevlar rope has been designed and built and will be tested for tensile and dielectric strength. Research progress has been presented at technical conferences and documented in a peer-reviewed journal article.

Laser technology has been used in underground mine surveys and is being considered for measuring methane concentrations at the face areas of underground coal mines. On the surface, lasers are used to measure coal levels in storage silos. However, lasers can pose ignition hazards in gassy and dusty atmospheres if energy levels are not constrained. Under a NORA Traumatic Injury project called "Laser Safety in Potentially Flammable Environments," the use of lasers in atmospheres containing flammable dusts and gases, including Powder River Basin coal dust, Pittsburgh coal dust, and methane, is being investigated. Igniting optical power thresholds for each flammable material in a 20-liter chamber are being recorded as a function of laser beam diameter and concentration of the flammable material in air. The time to ignite a material after initial laser exposure, or ignition delay time, is also being recorded. The knowledge gained will help develop explosion prevention engineering controls. Another facet of the work focuses on a means to prevent incidental ignitions of hazardous gassy or dusty atmospheres by lasers. Through infrared thermography, the temperatures of surfaces absorbing optical energy from a laser are monitored. When the temperatures exceed predefined safe levels, the laser power source is deenergized to prevent incidental ignitions. Preliminary tests with a laser in a methane-air atmosphere have proven that the concept is feasible. Recommendations resulting from this project are expected to impact consensus and statutory safety standards, such as the American National Standards Institute (ANSI) Z136 series laser standards, ANSI Instrumentation, Systems and Automation (ISA) hazardous location standards, and International Electrotechnical Commission (IEC) standards.