Extended Thermal Probe Method for Measuring Thermal Stability of Soils Surrounding Buried Power Cables

The single most influential factor in determining the rating of underground cables is the thermal resistivity of the soil that surrounds the cables. An accurate measurement of soil thermal resistivity is therefore critical to the determination of cable ratings. Ratings are also strongly influenced by the thermal stability of the soil, because heat generated by the cables can often drive soil moisture away from the cables resulting in an unexpected steep rise in the soil resistivity and a corresponding sharp reduction in cable ratings. Soil thermal stability can be determined by a simple, short-duration test that involves a thermal probe that has traditionally been used to measure thermal resistivity if the duration of the test is extended. Therefore a single test with a thermal probe can be employed to provide a measure of the soil's thermal stability as well as its thermal resistivity. Armed with knowledge of both resistivity and stability of the soil, a utility engineer can confidently calculate cable ratings that will result in safe cable operating temperatures even under the most adverse unstable soil conditions. Measurements of soil stability are shown to be influenced by the soil's properties, the heat generated in the cables, the diameter of the cable/soil interface and the duration that the soil experiences heating by the cables. Experimental measurements of soil stability using the extended probe method are demonstrated for four different soils, and measurements of stability are shown to influence the rating of several different cable installations.