Michael R. Yenchek

NIOSH-Sponsored Research in Through-the-Earth Communications for Mines: A Status Report

This paper presents the results of recent contractual research sponsored by the National Institute for Occupational Safety and Health that aimed at demonstrating the feasibility of through-the-earth (TTE) wireless communication in mining. TTE systems, developed by five different contractors, are discussed with a focus on technical approach, prototype hardware, and field test results. System features include both magnetic and electric field sensing, loop and line antennas, digital and analog processing, noise filtering and cancelation, and direction finding. The systems were demonstrated at commercial mine sites. The results of these tests are characterized by transmission range and power levels. This paper concludes with a discussion of issues that remain to be resolved as TTE communications are implemented. These include text versus voice format, acceptable time delays, portability, ease of deployment, an interface with existing communications systems, permissibility, and the effect of geological variations.

Distinguishing motor starts from short circuits through phase-angle measurements

The Pittsburgh Research Center conducted an investigation to improve electrical safety in coal mines through research into short-circuit protection of mine power systems. Distribution voltage, transformer impedance, power center location, and motor size were found to critically influence the magnitude of short circuits and motor starts on high-voltage longwalls. The need for accurate and rapid fault detection was confirmed through computer models of power systems. Voltage and current waveforms of mine induction motors were recorded during field tests, and their salient characteristics were evaluated.

NIOSH-sponsored research in through-the-earth communications for mines-a status report-

This paper presents the results of recent contractual research sponsored by the National Institute for Occupational Safety and Health that aimed at demonstrating the feasibility of through-the-earth (TTE) wireless communication in mining. TTE systems, developed by five different contractors, are discussed with a focus on technical approach, prototype hardware, and field test results. System features include both magnetic and electric field sensing, loop and line antennas, digital and analog processing, noise filtering and cancelation, and direction finding. The systems were demonstrated at commercial mine sites. The results of these tests are characterized by transmission range and power levels. This paper concludes with a discussion of issues that remain to be resolved as TTE communications are implemented. These include text versus voice format, acceptable time delays, portability, ease of deployment, an interface with existing communications systems, permissibility, and the effect of geological variations.

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.

Remote Vision System for Dozers on Coal Stockpiles

The National Institute for Occupational Safety and Health (NIOSH) Pittsburgh Research Laboratory (PRL) participated in an experiment to facilitate the remote control of dozers used on coal stockpiles. The experiment consisted of supplementing the normal sensory cues an operator requires to manipulate the dozer from a remote location. NIOSH provided and tested a vision systems intended to give the operator the ability to operate the dozer safely and efficiently from a remote position. This paper highlights the genesis of the experiment, identifies the contributions of Consol Energy and Caterpillar, and details the NIOSH efforts and successful test results.

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.

Operational characteristics of trailing cable splices

The US Bureau of Mines investigated the operational characteristics of spliced portable power cables. This research had a dual purpose: (1) to, determine the thermal and mechanical performance of repaired trailing cables and compare them with undamaged cables, and (2) to gauge the impact of long-term localized heating on the insulating and jacketing materials contained in cable splice kits accepted or approved by the Mine Safety and Health Administration. The ranges of splice joint resistance and tensile breaking strength were determined from laboratory measurements. The choice of crimping tools affected the strength of the splice under tension. Thermal profiles of energized spliced cables were constructed, which showed that spliced conductor joints operated 5 to 20/spl deg/C hotter than the intact cable at rated currents. Accelerated life tests of thermally-aged samples of splice kit insulation and jacket materials confirmed a deficiency in the thermal rating of the insulating tape. The recommendations in this paper may be utilized to revise splice kit design, splice kit acceptance criteria, and trailing cable loading guidelines. Characterizing the thermal operating limits of spliced trailing cables may help to minimize associated risks from explosions, fires, personnel burns, and shock.