As part of an overall effort relating cable life and current load, the US Bureau of Mines conducted research to determine the effects of elevated temperatures on the mechanical performance of coal mine trailing cable insulation. Insulation samples from six low-voltage unshielded portable power cables were thermally aged in air ovens. The specimens were exposed to constant temperatures ranging from 230 to 290 degrees F for periods up to 10 months. At regular intervals over the test terms, samples were extracted and subjected to mechanical tests in accordance with Insulated Cable Engineers Association Standard S-68-516. These tests included tensile strength, elongation at rupture, hot creep elongation, and set. Characteristic variations over time were plotted for each aging temperature. Arrhenius (useful life) graphs showed that 80% retention of tensile strength and 50% retention of elongation were equally critical determinants of thermal degradation. Extrapolation to 90 degrees C yielded useful lives ranging from 4.4 to 7.5 years for the cables evaluated. The findings support the tentative conclusion that, in service underground, cables fail mechanically long before they deteriorate thermally.
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