A conservative rating procedure for cyclically loaded, buried cables

A conservative and simple current-rating algorithm for buried power cables subjected to a cyclically varying load is presented. The algorithm is based on the assumption that the moisture distribution in the soil is stationary during load variations and that it is determined by the spatial distribution of the maximum heat flux. This assumption is reasonable in view of the hysteretic nature of moisture retention by soils. Only one Fourier component of the heat-loss variation is considered in the algorithm. Applications where adapted equations of Philip and De Vries are used to describe heat and moisture transfer are shown. The critical temperatures found for drying-out of the soil hardly differ for stationary and cyclic loads. The maximum allowed average load decreases with increasing load variation. At maximum load, the maximum current occurring during a cycle is higher than the maximum stationary load allowed.