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The Heat and Buried Cable Conundrum
When it comes to heat and buried cable, one of the most important factors affecting underground cable ampacity is the thermal resistivity of the soil. A soil’s thermal resistivity varies with moisture content, and the heat generated by cables can cause the soil to dry, increasing its resistivity. the ability of the soil to maintain its thermal resistivity in the presence of a heat source is known as thermal stability. since no soil has perfect stability, accounting for the fluctuating thermal resistivity of soil makes the cable ampacity calculations difficult. this article will examine the information available from standard soil tests and the information these tests may provide relating to the migration of moisture in the soil and the resulting changes in soil resistivity. Furthermore, we suggest a method for including this information in underground cable ampacity calculations.
When a cable is buried in the soil, whether directly buried or in an underground pipe, the heat generated by the resistive losses in the cable must be carried away through the soil surrounding the cable. the rate at which this heat can be carried away determines the temperature the cable will reach during any loading condition. if this temperature becomes too high, the cable can be damaged.
The thermal resistivity of the soil surrounding the cable is the main factor in determining the rate at which heat can be conducted away from the cable and, therefore, the ultimate amount of current the cable can carry. soil thermal resistivity is one of the most important values an engineer must know to calculate the cable ampacity. once the thermal resistivity of the surrounding soil is known, the neher–mcGrath method is commonly used to determine the amount of current a cable can carry without exceeding its allowable temperature.
Measurement of Soil Properties thermal resistivity is a measure of the ability of a material to resist the flow of heat. in the case of soil, this property is commonly measured using either laboratory or field tests. Several soil tests are commonly performed to characterize a soil’s properties. one common test performed determines the soil density and moisture content. to perform this test, a soil sample is taken from the field, and the in-place soil unit weight test gives the overall soil unit weight in pounds per cubic foot. The water content of the soil sample is also tested, and the result of this test gives the weight of water contained in the soil sample divided by the weight of the dried soil and is given in a percentage.
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