Thermal Stability and its Prediction in Cable Backfill Soils

Thermal instability, in the context of heat dissipation from buried cables, refers to the condition tion in which the thermal resistivity of the soil continuously increases with time to a value close to that of its dry state. This "thermal runaway" condition leads to a large increase in cable temperature. The thermal instability in cable backfills is a direct result of net moisture migration away from the heat source below a critical moisture level. Moisture migration in cable backfills can be caused by either a natural drying process resulting from evapotranspiration or by the thermal gradient from the heat source. This paper deals with the latter case, for which a theoretical prediction and experimental verification are provided. A transient heat probe method of predicting thermal instability in cable backfills is discussed.