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

Type of screen/sheath of multi-core cables

Symbol
$t_{\mathrm{1}}$
Unit
mm
Formulae
$\frac{t}{2} + t_{\mathrm{scb}}$single-core with tape screen
$\frac{t}{2} + t_{\mathrm{f}} + t_{\mathrm{scb}}$multi-core cables type CC with tape screen and no sheath
$\frac{\Delta d_{\mathrm{sh}}}{2} + \frac{t}{2} + t_{\mathrm{scb}} + t_{\mathrm{scs}}$other single-core cables / three-core cables type SS with sheath
$\frac{\Delta d_{\mathrm{sh}}}{2} + \frac{t}{2} + t_{\mathrm{f}} + t_{\mathrm{scb}} + t_{\mathrm{scs}}$other multi-core cables
Related
$\Delta d_{\mathrm{sh}}$
$t_{\mathrm{scb}}$
$t_{\mathrm{scs}}$
Used in
$G_{\mathrm{1}}$
$X_{\mathrm{G}}$
$Y_{\mathrm{G}}$
$Y_{\mathrm{K}}$