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.

$T_{1t}$long-term transients
$\frac{T_{1t}}{2 n_{c}}$short-term transients