Screening factor

In addition to the semi-conducting screens around the insulation, the majority of modern three-core cable constructions also have a metallic screen made of copper tapes or wires added around each core. The main purpose of this screen is to provide a uniform electric field inside the cable. Screening provides additional heat paths along the screening material of high thermal conductivity. In order to take account of the thermal conductivity of the metallic screens, the thermal resistance $T_1$ shall be multiplied by a factor $K$, called the screening factor, which is given in IEC 60287-2-1, Figure 4 for circular conductiors and Figure 5 for sector-shaped conductors with different values of $\delta_1$ and different cable specifications.

For flat wires, screening is considered if there are sufficient wires to be considered touching. We consider a coverage of 90% of the circumference sufficient. For round wires, screening is not considered.

Symbol
$K$
Formulae
 $K_{02}+Z_K \left(-3K_{02}+4K_{06}-K_{10}\right)+{Z_K}^2 \left(2K_{02}-4K_{06}+2K_{10}\right)$ Round conductors, and sector-shaped conductors, $3 < X_K < 25$ $K_{02}+2.5\left(Y_K-0.2\right) \left(K_{06}-K_{02}\right)$ Sector-shaped conductors, $0 < X_K <= 3$, $0.2 < Y_K <= 0.6$ $0$ Screened cables with round conductors
Used in
$T_1$
Image IEC standard 60287-2-1 Ed. 2.0 (2015) Figure 4 (left) and Figure 5 (right)