What is the function of the metal tape shield in power cables?
The metal tape shield in power cables is typically made of copper tape wrapped around the outer semi-conductive layer. Its functions are as follows:
1) It strengthens the electric field confinement within the insulation layer, aligning the electric field direction with the insulation radius (i.e., radially). The metal tape is grounded, and the electric field terminates at the tape, with no electric field outside the tape.
2) In a three-phase, four-wire system, it serves as the center conductor to carry unbalanced current.
3) It prevents axial surface discharge. In an environment without a good ground connection, the semi-conductive layer has a certain resistivity, which can cause uneven potential distribution along the cable's axis, leading to surface discharge along the cable.
4) Power station protection systems require an outer conductor shield. Wrapping copper tape provides excellent lightning protection.
5) Under normal circumstances, capacitive current flows; in the event of a short circuit, the metal strip serves as a short-circuit fault current path. 11. What are the commonly used sheath structures for power cables?
The sheath of a power cable generally refers to the covering layer that surrounds the insulating core and protects the cable from mechanical damage and corrosion. It is typically composed of a uniform, continuous metal or non-metallic material. Common sheath structures include unarmored and armored (liner or insulation + armor + outer sheath; low-voltage interlocking armored cable may not have an outer sheath).
The cable sheath structure depends on the cable installation location and environmental conditions. Unarmored cable can be selected when there is no mechanical force. Steel tape armored cable should be used for installations subject to radial pressure. Steel wire armored cable should be used for installations subject to longitudinal tension or vertical installation.
