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In the case of a cable installed underground, one mechanism by which heat is removed from the cable is the conduction of heat through surrounding soil drying. The amount of heat generated in the cable is determined by the current and varies with the square of the current. In the case of a cable installed underground there are three mechanisms by which heat is removed from the cable: conduction of heat through surrounding soil, removal of heat by vaporizing soil moisture which then migrates away from the cable, and absorption of heat by the surrounding soil thereby increasing the soil temperature. The first two methods are the most important.
Since the cable has a maximum temperature limit, its ampacity will be determined by the rate at which heat is carried away. The conductive heat rate is dependent upon the thermal resistivity of the soil; the lower the resistivity the faster the heat will be conducted away. The resistivity, in turn, is principally controlled by the amount of moisture in the soil.
Higher moisture content results in lower thermal resistivity because water fills in the air voids between soil particles aiding the conduction of heat between particles. In addition to heat conduction, part of the heat is carried away by vaporizing the soil moisture in contact with the cable. This vapor migrates away from the cable thereby drying the soil in contact with the cable. Unless this moisture can be replenished from the surrounding soil quickly enough, the soil in contact with the cable will dry and increase in thermal resistivity. This reduces the conduction of heat away from the cable causing it to overheat.
Some sources have found that rather than changing in resistivity gradually as the soil is heated, it may change suddenly. In some cases a soil may exhibit a wet resistivity value which is constant until a point called the “effective drying time”. At that point the soil resistivity quickly increases in value and rapidly attains a dry resistivity value, which may be many times larger than the wet resistivity. The same sources also suggest that the time it takes soil to effectively dry after the application of heat is based on the square of the cable diameter.
|Effect of Cable Diameter on Soil Drying|