Correction of thermal resistance for backfill

When the cables or ducts are embedded in concrete, the calculation of the thermal resistance $T_{4iii}$ is first of all made assuming a uniform medium having a thermal resistivity equal to the concrete $\rho_b$. A correction $T_{4db}$ is then added algebraically to take account of the difference between the thermal resistivity of concrete and soil for that part of the thermal circuit exterior to the backfill. The backfill is transformed into an equivalent circle with the help of the geometric factor $G_b$.

When the cables or ducts are not embedded in concrete, $T_{4db}$ is zero.

Reference: IEC 60287-2-1 Ed.2.0

$\frac{N_b \left(\rho_4-\rho_b\right)}{2\pi} G_b$