This is the thermal resistance of a single isolated cable or duct. It is valid for a single isolated cable only and does not contain the additonal thermal resistance $ΔT_4$ caused by heating from other cables in a group.
The thermal resistance $T_4 + ΔT_4$ is equal to the thermal resistance to ambient $T_{4iii}$ for one buried cable.

This parameter $T_4$ is used for transient calculations acc. IEC 60853 where the thermal resistance of medium in the duct $T_{4i}$ and of the duct wall $T_{4ii}$ are included already in the element B of two-part thermal circuit $Q_B$.

Symbol
$T_{\mathrm{4}}$
Unit
K.m/W
Formulae
$\frac{\rho_{\mathrm{4}}}{2 \pi} \ln{\left (\frac{4 L_{\mathrm{c}}}{Do_{\mathrm{d}}} \right )}$
Related
$Do_{\mathrm{d}}$
$L_{\mathrm{c}}$
$\rho_{\mathrm{4}}$
Used in
$k_{\mathrm{t}}$