In addition to the semiconducting screens around the insulation, the majority of modern 3-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.
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.
$K_{\mathrm{02}} + Z_{\mathrm{K}}^{2} \left(2 K_{\mathrm{02}} - 4 K_{\mathrm{06}} + 2 K_{\mathrm{10}}\right) + Z_{\mathrm{K}} \left(- 3 K_{\mathrm{02}} + 4 K_{\mathrm{06}} - K_{\mathrm{10}}\right)$ | round conductors, and sector-shaped conductors with $3 < X_{\mathrm{K}} < 25$ |
$K_{\mathrm{02}} + \left(- K_{\mathrm{02}} + K_{\mathrm{06}}\right) \left(2.5 Y_{\mathrm{K}} - 0.5\right)$ | sector-shaped conductors with $0 < X_{\mathrm{K}} <= 3$ and $0.2 < Y_{\mathrm{K}} <= 0.6$ |
$0$ | screened cables with round conductors |