This is the apparent thermal resistance $T_{a0}$ used to calculate cable partial transient temperature rise acc. to IEC 60853-2.

Symbol
$T_{\mathrm{a0}}$
Unit
1/s
Formulae
$\frac{1}{a_{\mathrm{0}} - b_{\mathrm{0}}} \left(- b_{\mathrm{0}} \left(T_{\mathrm{A}} + T_{\mathrm{B}}\right) + \frac{1}{Q_{\mathrm{A}}}\right)$
Used in
$\alpha_{\mathrm{t}}$
$\Delta \theta_{\mathrm{c_{\mathrm{t}}}}$
$\Delta \theta_{\mathrm{d_{\mathrm{t}}}}$
$T_{\mathrm{b0}}$