Thickness of insulation to sheath

Insulation thickness between conductors and sheath used to calculate $T_1$ with following considerations:

Type of screen/sheath of multi-core cables

Symbol
$t_1$
Unit
mm
Formulae
$t_i+t_{scb}+t_{scs}+\frac{\Delta d_{sh}}{2}$Single-core cables
$t_i+t_{scb}+t_{scs}+\frac{\Delta d_{sh}}{2}$Multi-core cables type SS
$t_i+t_{scb}+t_{scs}+t_f+\frac{\Delta d_{sh}}{2}$Multi-core cables type SC
$t_i+t_f+t_{scb}+t_{scs}+\frac{\Delta d_{sh}}{2}$Multi-core cables type CC
$t_i+t_{scb}$Pipe-type cables