# 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).

Symbol
$t_{1t}$
Unit
mm
Formulae
 $t_1$ 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
Related
$D_{it}$
$\Delta d_{sh}$
$t_1$
$t_f$
$t_{scb}$
$t_{scs}$