This is the temperature rise of the boundary between the dry and moist zones above the ambient temperature of the soil.
The method is based on a simple two-zone approximate physical model of the soil where the zone adjacent to the cable is dried out whilst the other zone retains the site's thermal resistivity, the zone boundary being on isotherm. This method is considered to be appropriate for those applications in which soil behaviour is considered in simple terms only.
The method should be applied to a single isolated cable or circuit only, laid at conventional depths. Installations of more than one circuit shall consider necessary spacing between circuits (not covered in IEC 60287-1-1 Ed. 2.1).
When the permissible current rating is being calculated under conditions of partial drying out of the soil, it is also necessary to calculate a rating for conditions where drying out of the soil does not occur. The lower of the two ratings shall be used.