Characteristics of soil affecting cable ratings

The paper gives an account of extensive co-operative field studies of the thermal properties of soils in relation to the performance of buried cables. It describes tests made during the past seven years on cables under controlled loading conditions in various types of natural and imported soils at 16 sites in the south of England. The part played by soil moisture is discussed and the influence of the seasonal changes in the atmospheric and soil ambient conditions on the retention of soil moisture near the cables is examined. The effect of the type of load on the retention of the moisture, and thus on the thermal resistivity, is considered. The use of imported backfill and of other means of improving thermal conditions of the power cables, such as impermeable covers or water cooling, is briefly discussed. The investigations have demonstrated how loaded cables can cause drying out of the surrounding soil and have given a lead as to the measures that can be taken for the avoidance of this condition. Although further work is proceeding, some recommendations are made for the realistic assessment of soil thermal resistivity, and the desirability of using winter and summer cable ratings is indicated.