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The heat produced by underground cables can cause the soil surrounding the cables to Dry. This drying causes a change in the soil's thermal properties, which may ultimately lead to cable overheating. This phenomenon is known as soil thermal instability. A method was suggested to determine the diameter the soil layer dried by the heat produced by the cables so its effects could be included in cable ampacity calculations. A set of equations was developed to fmd this diameter which may be simplified.

Soil moisture content when qNHR was measured. The values of qNHR and CJmeasured are measured in the field. The value CJdry is the driest moisture content that is expected at the site. While ideally this value would be determined by extensive field measurements done over long periods of time that included both wet and dry conditions, this is seldom practical. As an alternative, the driest expected soil moisture contents near many sites in the United States are available from the National Water and Climate Center.

To determine cable ampacity, the soil ambient thermal resistivity must be measured and corrected to the driest expected ambient soil conditions at the site to fmd the thermal resistivity Pamb. Also, the resistivity of the dry soil must be measured to get Pdry' Once these two values are known, the thermal properties of the soil outside the cable or conduits in question will be modeled as an area of dried soil immediately surrounding the cable with a resistivity of Pdry, surrounded by the ambient soil having thermal resistivity Pamb.

Since no laboratory test can replicate the natural flow of moisture through soil to test Soil Thermal Instability, the non-drying heat rate, qNHR, must be measured in the field using the same equipment used for fmding the soil in situ thermal resistivity. A byproduct of measuring qNHR is a measurement of thermal resistivity of the in situ soil at the soil moisture content that existed at the time of the measurement. However, the two resistivities needed to model the soil thermal properties around the cable are the resistivity at the driest ambient condition expected-Pamb, and the resistivity when the soil is completely drY-Pdry.

How to Include Soil Thermal Instability in Underground Cable Ampacity Calculations-IAS Copyright