Conductor to surface attainment factor

This is the attainment factor for the temperature rise of the conductor above the outside surface of the cable according to the IEC 60853-2 standard. This factor takes the thermal capacitance of the cable into account and approaches a value of 1.0 if the transient time periods are sufficiently long compared to the thermal time constant of the cable.

The applied formula is equivalent to the following formula: $\Delta\theta_{c_t}/(n_{c}W_{c}(T_A+T_B))$

$\frac{T_{a0} \left(1-e^{-a_0 \tau}\right)+T_{b0} \left(1-e^{-b_0 \tau}\right)}{T_A+T_B}$