Ratio of cable surface temperature rise above ambient to conductor temperature rise above ambient under steady-state conditions, and with all cables of the group having equal losses.

For transient calculations acc. IEC 60853, the thermal resistance of medium in the duct $T_{4i}$ and of the duct wall $T_{4ii}$ are included in the element B of two-part thermal circuit $Q_B$, therefore they must not be added again for the thermal resistance to ambient.

Note: The thermal resistance to ambient $T_{4iii} = T_4 + ΔT_4$.

Symbol
$k_{\mathrm{t}}$
Unit
p.u.
Formulae
 $\frac{T_{\mathrm{4}} W_{\mathrm{I}}}{T_{\mathrm{4}} W_{\mathrm{tot}} + W_{\mathrm{c}} \left(T_{\mathrm{A}} + T_{\mathrm{B}}\right)}$ N = 1 $\frac{W_{\mathrm{I}} \left(\Delta T_{\mathrm{4}} + T_{\mathrm{4}}\right)}{W_{\mathrm{I}} \left(\Delta T_{\mathrm{4}} + T_{\mathrm{4}}\right) + W_{\mathrm{c}} \left(T_{\mathrm{A}} + T_{\mathrm{B}}\right)}$ N > 1
Related
$\Delta T_{\mathrm{4}}$
$T_{\mathrm{4}}$
$T_{\mathrm{A}}$
$T_{\mathrm{B}}$
$W_{\mathrm{c}}$
$W_{\mathrm{I}}$
$W_{\mathrm{tot}}$
Used in
$M_{\mathrm{1}}$
$\theta_{\mathrm{R}}$