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Element A of equivalent thermal circuit

A three-core cable is represented as an equivalent single-core cable for long-term transients with duration < $PQ$. However, for very short transients with duration < $TQ$ where $TQ$ refers to a single core, the mutual heating of the cores is neglected, and a three-core cable is treated as a single-core cable with the dimensions corresponding to the one core. For durations between these two limits, the transient is assumed to be given by a straight line interpolation in a diagram with axes of linear temperature rise and logarithmic times.

Symbol
$T_A$
Unit
K.m/W
Formulae
$T_1$long-term transients
$\frac{\frac{T_1}{n_c}}{2}$short-term transients
Related
$n_c$
Number of conductors in object
$T_1$
Thermal resistance between one conductor and sheath [K.m/W]
$T_B$
Element B of equivalent thermal circuit [K.m/W]
Used in
$\alpha_t$
Conductor to surface attainment factor
$\delta \theta_c$
Ohmic steady-state temperature rise [K]
$M_0$
Coefficient M for partial transient temperature rise [s]
$N_0$
Coefficient N for partial transient temperature rise [s$^2$]
$T_{a0}$
Apparent thermal resistance a [K.m/W]
$T_{b0}$
Apparent thermal resistance b [K.m/W]