| Because of the large number of mining fatalities in the 1950's and 1960's, federal legislation was enacted in an attempt to curtail the rising number of work related injuries and fatalities. The Mine Health and Safety Act of 1969 required such things as the use of protective cabs and canopies on all face equipment, methane monitoring on all coal-cutting face equipment, and sprinkler systems on all rubber belts, etc. As a result of this legislation, mining accidents and fatalities did decline. All areas of reported causes of mine fires showed a marked decline, except one--fires caused by trolley system faults.;In many electrical systems, the magnitude of the available fault current is several times that of the normal load current. This is not the case with most coal mine trolley systems, where the fault current may be as low as 125% of the normal load current. Therein lies the problem.;What is needed is a way to detect low level system faults (less than 300 amperes), and trip all necessary circuit breakers, while allowing legitimate 5000 ampere trolley system loading to occur.;This dissertation describes such a system based on rate of current rise and duration of the current pulse. Low level faults that previously went undetected can now be recognized, and quickly and safely de-energized; thereby significantly reducing the risk of a mine fire. It is strongly believed that the system of protection outlined in this dissertation will greatly improve the safety of any coal mine trolley system.;Between the years of 1970 and 1981, 31 out of 134 reportable mine fires were caused directly or indirectly by trolley system faults. Since 1981, the author has personally investigated three fires in Consolidation Coal Company mines. All three fires were caused by trolley system faults, and two of these fires persisted for more than thirty minutes. |
