Thermal resistance between one conductor and sheath, transient

For calculation of the cyclic and emergency current rating of cables acc. IEC 60853, the multi-core cable is replaced by an equivalent single-core construction dissipating the same total conductor losses. The space between the equivalent single-core conductor and the sheath is considered to be completely occuied by insulation (for oil-filled cables, this space is filled partly by the total volume of oil in the ducts and the remainder is oil-impregnated paper).

$\frac{\rho_{i} \ln{\left(1 + \frac{2 t_{1t}}{d_{ct}} \right)}}{2 \pi}$