This is the transient temperature rise of conductor above outer surface caused by dielectric losses.

For emergency rating calculations acc. IEC 60853, the formula is equal as $Δθ_{d}$ for cables in air.

Symbol
$\Delta \theta_{\mathrm{d_{\mathrm{t}}}}$
Unit
K
Formulae
 $\frac{W_{\mathrm{d}} n_{\mathrm{c}}}{2} \left(T_{\mathrm{a0}} \left(1 - e^{- a_{\mathrm{0}} \tau}\right) + T_{\mathrm{b0}} \left(1 - e^{- b_{\mathrm{0}} \tau}\right)\right)$ transient $W_{\mathrm{d}} \left(\frac{T_{\mathrm{1}}}{2} + n_{\mathrm{c}} \left(T_{\mathrm{2}} + T_{\mathrm{3}}\right) + n_{\mathrm{cc}} \left(T_{\mathrm{4i}} + T_{\mathrm{4ii}} + T_{\mathrm{4iii}}\right)\right)$ emergency rating
Related
$a_{\mathrm{0}}$
$b_{\mathrm{0}}$
$n_{\mathrm{c}}$
$n_{\mathrm{cc}}$
$T_{\mathrm{1}}$
$T_{\mathrm{2}}$
$T_{\mathrm{3}}$
$T_{\mathrm{4i}}$
$T_{\mathrm{4ii}}$
$T_{\mathrm{4iii}}$
$T_{\mathrm{a0}}$
$T_{\mathrm{b0}}$
$\tau$
$W_{\mathrm{d}}$