The thermal stability of soil has been recognized as an important factor governing the rating of underground power cables. Several studies have suggested critical property values that can be used to predict soil thermal stability. The present study predicts a two-stage drying process that occurs in soil. The first stage is characterized by a decrease in moisture content until a critical value is reached. In the second stage, the drying occurs more rapidly, eventually leading to a high thermal resistivity layer surrounding the cable. The second stage produces a "thermal runaway" condition which has been classified as a thermally unstable soil. The paper describes the results of a computer program that solves the transient form of the conservation equations in radial coordinates. The program is applied to the soil surrounding the cable in order to predict the local instantaneous temperature and moisture content. The results are then used to predict those soil properties and cable operating conditions which will produce stable and unstable behavior. The program is capable of predicting drying rates and the time at which thermal instability will occur. The results of the program are compared with two separate experimental scale laboratory tests. The experimental results show that the prediction of thermal instability can be accurately achieved with small diameter heat sources. Therefore, it is not necessary to perform simulations on full size cable systems.
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