Fig. 9. Results of performance tests on concentric trailing cables.. 10. Good temporary splice of twin trailing cable...__ 11. Taped splices of concentric and parallel duplex trailing cables found in coal mines____ 12. Splices of concentric and parallel duplex trailing cables found in coal mines; tape removed.... 13. Good and poor permanent splices and good vulcanizing in twin trailing cables.. 14. Typical underground test setup for determining the effectiveness of frame grounding- 15. Outside view of portable fireproof structure housing portable motorgenerator set__. 16. Portable motor-generator set inside portable fireproof structure_ 17. Master circuit breaker in underground fireproof enclosure 18. Sectional automatic reclosing circuit-breaker installation_ 19. Feeder sectionalizing switches... 20. Some types of trolley-guard hangers. 21. Permissible universal-type mining machine.. 22. Universal-type mining machine mounted on rubber tires.. 23. Dangerous condition of mining-machine frame caused by dislodgment of rail hook.. 24. Defect in drill insulation shown at A causes the mining-machine current to divide at B.. 25. A fault occurring at point A causes current to follow direction of dotted line.... 26. Track-mounted loading machine entering a working place__. 27. Controls of mobile coal loader_ _ _ Page 30 28. Fuse-protected trolley-wire tap and rail-return clamp used with trailing cable of mobile coal loader_ 29. Hand-held permissible drill, showing approval plate____ 30. Driller splicing "hot" powerline in a coal mine-a dangerous practice. 31. Permissible post-type coal drill.... 32. Permissible rubber-tire-mounted drilling machine. 33. Cable-reel shuttle car.. 34. Continuous mining machine_ 35. Permissible two-outlet distribution box. 36 and 37. Permissible-type room hoists_ 38. Permissible portable air compressor.. 39. Permissible reciprocating pump, showing motor and control. 40 and 41. Permissible rock-dust distributors... 72, 73 42 and 43. Types of cable installation for borehole. 74, 75 44. Diagram of typical test setup for determining resistance of drill-hole casing to ground... 45. High-potential cable leading from borehole to substation_ 46. Method of suspending high-potential line; messenger wire and sheath are grounded at regular intervals... 47. Typical splice on nonmetallic armored cable.. 48. A, Cable crossing guard; B, cable hanger__ 49. Buried power cable in trench dug with mining machine_ 50. Cable junction for buried power cable.. 51. Substantial telephone installation.. ELECTRICAL ACCIDENTS IN Coal-Mine Accident-Prevention Course- Revised by C. L. Brown 2 and E. J. Gleim 3 PURPOSE AND SCOPE This publication is the sixth in a series of seven miners' circulars covering various phases of accident prevention at bituminous-coal mines. These circulars constitute a textbook for courses of instruction on accident prevention in bituminous-coal mining, serving as a general reference to which may be added other material applicable to the district in which the course is presented. Other circulars in the series are: Accident Statistics as an Aid to Prevention of Accidents in Bituminous-Coal Mines (Miners' Circular 47). Accidents From Falls of Roof and Coal in Bituminous-Coal Mines (Miners' Circular 48). Accidents From Hoisting and Haulage in Bituminous-Coal Mines (Miners' Circular 49). Explosions and Fires in Bituminous-Coal Mines (Miners' Circular 50). Explosives Accidents in Bituminous-Coal Mines (Miners' Circular 58). Miscellaneous Accidents in Bituminous-Coal Mines (Miners' Circular 60). The use of electric power for haulage purposes in the bituminouscoal mines of the United States has increased from year to year since its introduction during the latter part of the nineteenth century (about 1887), a few years after trolley cars were first used for surface transportation. Later, electric power was utilized in cutting coal, pumping, drilling, ventilation, mechanical loading, illumination, heating, and other auxiliary operations. During 1891 only 5 percent of the underground (coal) production was cut by machines. This percentage had increased to 94.9 by 1951. Electric drills were first used about 1911, but the one-man portable drill did not appear until 1917. Hand augers, however, predominated until mechanical loading made it necessary to balance operations, thereby bringing about the greater use of power drills. Records of coal mechanically loaded go back to the year 1923 when three-tenths of 1 percent of total underground output was so loaded. By 1942 that proportion had grown to 45.2 percent, and 10 years later it had risen to a new high of 74.7 percent.* 1 Work on manuscript completed May 1954. 2 Mining engineer (electrical), Accident Prevention and Health Division, Region VIII, Bureau of Mines, Pittsburgh, Pa. 3 Chief, Electrical-Mechanical Branch, Accident Prevention and Health Division, Region VIII, Bureau of Mines, Pittsburgh, Pa. 4 Mechanization, Decade of Progress, 1942-52: Vol. 17, No. 11, November 1953, p. 85. In 1951 a new high was reached in the volume of bituminous coal loaded into shuttle cars by mobile loaders and continuous mining machines; in other words, shuttle cars handled 45.6 percent of the output of mechanically loaded coal. These statistics show the growth of electrically powered devices applied to bituminous-coal mining. The trend likely will continue upward for some time to keep pace with the increase in mechanical loading. Along with this trend has come the development of machines that combine the operations of cutting and loading and at the same time eliminate drilling and blasting. The widespread application of electric power in and about the bituminous-coal mines of the United States has been accompanied by numerous hazards. Unfortunately, in many instances safety has not kept pace with the increase in the use of electrically operated devices; however, in overall perspective the record has shown favorable improvement. The trend of the fatality rate has been downward, but the nonfatal-injury rate, after reaching an all-time low in 1938, increased to slightly under the 1930 high in 1947. The trend since 1947 has been downward. This miners' circular discusses the dangers coincident with the transmission of electric power and the installation and operation of electrical machinery at bituminous-coal mines; it also includes recommendations for eliminating the dangers or minimizing the hazards. Numerous sources of information, predominantly publications of the Bureau of Mines, were utilized in the preparation of this circular. One of the foremost activities of the Bureau is to promote safety in mining through the activities of the Health and Safety Division. The Electrical-Mechanical Branch, through its permissibility tests and other services, has done much to promote safety, particularly in reducing electrical accidents. The Coal-Mine Inspection Branch examines mines, including their equipment, and issues reports with recommendations to assist mine operators, officials, and mechanics to install, operate, and maintain electrical equipment and accessories so that the hazards in connection therewith will be minimized. ELECTRICAL-ACCIDENT STATISTICS Fatal and nonfatal injuries from electricity occur from several sources. Electrical-contact injuries sustained underground in the bituminous-coal mines of the United States have been classified by causes in Bureau of Mines Bulletin 509 5 as follows: (a) Trolley wire or pole. (b) Other permanent or semipermanent lighting circuits. (e) Transformers, generators, etc. (f) Locomotives or shuttle cars. (g) Mining or loading machines. (h) All other. On a million-man-hour basis both fatal and nonfatal injury-frequency rates from electricity show a decrease since 1942. The statistics on electrical accidents do not include gas or dust explosions of electrical origin. The 1942 accident-frequency rate, including fatal and nonfatal injuries from electricity sustained underground in bituminous-coal mines, was 1.58 per million man-hours of exposure, compared with 0.817 for 1951. During the 20-year period 1911 to 1930, 4.6 percent (average) of all underground fatalities were caused by electricity. By 1931 mechanical loading underground had become an important factor in bituminous-coal production; in this year the underground production so loaded exceeded 10 percent of the total production. Since then it has increased almost continuously; in 1952 it was 74.7 percent of the total production. During the last 9 years 85 explosions, probably originating from electric areas, caused 326 deaths. Locomotives and mining machines were charged with originating over half of the explosions of electrical origin from 1944 to 1952. Most of the equipment was of nonpermissible type, but in a few of the explosions permissible equipment improperly maintained was held to be the source. *** Although electrical ignitions have not increased in number, they have not been reduced in frequency, and much more remains to be done.* For the fiscal years 1930 to 1943, inclusive (14 years), the greatest single source of coal-mine fires was electricity, and the greatest number of mine-fire fatalities resulted from fires of electrical origin. Table 1 gives the number of fatal and nonfatal injuries from electricity, including those from mine explosions of electrical origin, for the years 1930 to 1951, inclusive. Surface injuries include those that occurred in bituminous-coal stripping operations. Total fatal and nonfatal injuries from all causes are shown for comparative purposes. Figure 1 illustrates graphically the number of fatal and nonfatal 5 Moyer, Forrest T., Jones, G. D., and Wrenn, V. E., Injury Experience in Coal Mining, 1948 Bureau of Mines Bull. 509, 1952, 109 pp. Fene, W. J., and Humphrey, H. B., Ccal-Mine Explosions and Coal- and Metal-Mine Fires in the United States in 1950, 1951, and 1952: Bureau of Mines Inf. Circ. 7661, 1953, 13 pp. NUMBER OF ACCIDENTS 1930 1932 1934 1936 1938 1940 1942 1944 1946 1948 1950 1952 FIGURE 1.-Fatal and Nonfatal Injuries From Electrical Accidents at Bituminous-Coal Mines in the United States, 1930-51. (Surface accidents include strip mining.) injuries from electricity at the bituminous-coal mines of the United States for the years 1930 to 1951, inclusive. Both surface and underground injuries are shown; surface injuries include strip mining. Nonfatal injuries at bituminous-coal mines underground increased during the period 1942-47. Fatalities, however, decreased during this period, indicating a reduction in the severity rate. For the 39-year period prior to 1949 a total of 2,325 fatalities occurred underground from all electrical causes, or an average number per year of 59.6. The first 20 years of this period accounted for 67.7 percent of the total.' The average number of fatalities per year for the 5-year period 1949-53 was 12.2. See footnote 5. |