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P1254 - Guide for Soil Thermal Stability

The successful operation of all underground cable systems require that the system dissipate heat so that the maximum temperature limit of the cable conductor is not exceeded. In order for the cable system to successfully dissipate this heat, the thermal properties of the surrounding soil or backfill material must be known and remain thermally stable during the operation of the cable. This guide has been prepared to assist planners and engineers in achieving that goal.This guide covers two approaches for evaluating the impact of soil moisture migration under cable heating. With knowledge of conditions that lead to thermal instability, operating conditions of the cable can be adjusted or a thermally stable backfill can be used to prevent the occurrence of thermal instability. The first approach uses the 'extended thermal needle test'. The thermal stability test is essentially an extended time thermal resistivity test using a thermal probe and instrumentation identical to that used to measure the thermal resistivity of a soil. The test procedures which can be performed in the field on an in-situ sample or in the laboratory on a small-scale compacted sample are explained in detail. The test results can be used to predict the time to onset of thermal instability of an operating underground power cable at a given heat generation rate. A second approach uses a 'two-zone model'. This concept considers a dry envelope surrounding the cable and soils at ambient moisture content beyond the critical boundary. Test procedures for determining the 'critical temperature rise' are generally outlined in this guide. A migration test cell is utilized to define the regions of the two zone method.
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