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 and multi-core cables type SS
$D_{1} - d_{ct} - t_{sc}$multi-core cables type SC
$D_{1} - d_{ct}$type CC with tape screen and no sheath
$D_{it} - d_{ct}$other multi-core cables type CC