Thickness of insulation to sheath, transient

For calculation of the cyclic and emergency current rating of cables acc. IEC 60853, the multi-core cable is replaced by an equivalent single-core construction dissipating the same total conductor losses. The space between the equivalent single-core conductor and the sheath is considered to be completely occuied by insulation (for oil-filled cables, this space is filled partly by the total volume of oil in the ducts and the remainder is oil-impregnated paper).

$t_1$Single-core cables
$\frac{D_{it}-d_{ct}}{2}+t_{scb}+t_{scs}+\frac{\Delta d_{sh}}{2}$Multi-core cables type SS
$\frac{D_{it}-d_{ct}}{2}+t_{scb}+t_{scs}+t_f+\frac{\Delta d_{sh}}{2}$Multi-core cables type SC
$\frac{D_{it}-d_{ct}}{2}+t_f+t_{scb}+t_{scs}+\frac{\Delta d_{sh}}{2}$Multi-core cables type CC
$\frac{D_{it}-d_{ct}}{2}+t_{scb}$Pipe-type cables