Electrical Trolley systems are used for both mineral haulage and movement of personnel and supples in nearly 50 U.S. mines, mostly in southwestern Pennsylvania and West Virginia. These systems use a trolley wire, energized at 300 or 600 V dc, to power vehicles constrained to run on a network of permanently installed steel track. The bare copper trolley wire along with a feeder wire are suspended about every 6 m fro insulators that are anchored into the roof by bolts. The insulators prevent current from flowing to the grounded return rack via the overlying strata. Coal and rock dust accumulations, as well as acidic drainage and condensation, may jeopardize insulator integrity. Leakage currents from the trolley wire into the roof strata can heat the insulator, as well as the immediate area in which the suspended bolt in anchored. If not detected and corrected, this may result in ignition of roof coal and a catastrophic mine fire. An insulator and surrounding strata that are subject to leakage currents maybe discolored, having an odor, or exhibit no physical evidence of deterioration. Leakage can be confirmed through voltage measurements across the insulator. However, this can be a time-consuming tedious task considering the thousands of insulators distributed over miles of haulage way. A portable infrared detector can be used to scan for heat on the insulators and in the roof form a low-moving vehicle. This can be effective when done regularly, but this method cannot detect impending failures between examinations. An insulator, integrally designed to give some indication of the presence of leakage currents, could be detected and replaced promptly.
NIOSH, Pittsburgh Research Laboratory, P.O. Box 18070, Pittsburgh, PA 15236