On the use of the Law of Times in calculating soil thermal stability and underground cable ampacity

The heat generated by underground cables has been known to cause the soil around the cables to dry, increasing its thermal resistivity and potentially causing the cables to overheat. The ability of soil to maintain a constant resistivity while being subjected to a heat source is known as its “thermal stability”. A method using the Law of Times has often been recommended to determine soil stability. To test whether this method can accurately predict soil thermal stability an experiment was performed that tested the hypothesis inherent in the Law of Times that the diameter of the heat source affects the drying time of the soil surrounding it. This paper reports the results of that experiment and includes the statistical analysis of the data. The experimental evidence resulted in rejecting the Law of Times as an accurate predictor of the drying time of soil around a buried cable.