In wire and cable products, shielding structures are divided into two distinct concepts: electromagnetic shielding and electric field shielding. Electromagnetic shielding is primarily used to prevent high-frequency signal cables (such as RF cables and electronic cables) from causing interference to the external environment or to block external electromagnetic waves from interfering with cables transmitting weak currents (such as signal and measurement cables), as well as to reduce mutual interference between cables. Electric field shielding, on the other hand, is designed to balance the strong electric fields on the conductor surface or insulation surface of medium- and high-voltage power cables.
1. Structure and Requirements of Electric Field Shielding Layers
The shielding of power cables is divided into conductor shielding, insulation shielding, and metal shielding. According to relevant standards, cables with a rated voltage greater than 0.6/1 kV should have a metal shielding layer, which can be applied to individual insulated cores or the overall cable core. For cables with a rated voltage of at least 3.6/6 kV using XLPE (cross-linked polyethylene) insulation, or cables with a rated voltage of at least 3.6/6 kV using thin EPR (ethylene propylene rubber) insulation (or thick insulation with a rated voltage of at least 6/10 kV), an inner and outer semi-conductive shielding structure is also required.
(1) Conductor Shielding and Insulation Shielding
Conductor Shielding (Inner Semi-Conductive Shielding): This should be non-metallic, consisting of extruded semi-conductive material or a combination of semi-conductive tape wrapped around the conductor followed by extruded semi-conductive material.
Insulation Shielding (Outer Semi-Conductive Shielding): This is directly extruded onto the outer surface of each insulated core and is tightly bonded to or peelable from the insulation layer.
The extruded inner and outer semi-conductive layers should be tightly bonded to the insulation, with a smooth interface free of noticeable conductor stranding marks, sharp edges, particles, scorching, or scratches. The resistivity before and after aging should be no more than 1000 Ω·m for the conductor shielding layer and no more than 500 Ω·m for the insulation shielding layer.
The inner and outer semi-conductive shielding materials are made by mixing corresponding insulating materials (such as cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR)) with additives like carbon black, anti-aging agents, and ethylene-vinyl acetate copolymer. The carbon black particles should be evenly distributed in the polymer, with no agglomeration or poor dispersion.
The thickness of the inner and outer semi-conductive shielding layers increases with the voltage rating. Since the electric field strength on the insulation layer is higher on the inside and lower on the outside, the thickness of the semi-conductive shielding layers should also be thicker on the inside and thinner on the outside. For cables rated at 6~10~35 kV, the inner layer thickness typically ranges from 0.5~0.6~0.8 mm.
(2) Metal Shielding
Cables with a rated voltage greater than 0.6/1 kV should have a metal shielding layer. The metal shielding layer should cover the outside of each insulated core or the cable core. Metal shielding can consist of one or more metal tapes, metal braids, concentric layers of metal wires, or a combination of metal wires and tapes.
In Europe and developed countries, where resistance-grounded dual-circuit systems are used and short-circuit currents are higher, copper wire shielding is often employed. In China, arc suppression coil-grounded single-circuit power supply systems are more common, so copper tape shielding is typically used. Cable manufacturers process purchased hard copper tapes by slitting and annealing to soften them before use. The soft copper tapes must comply with the GB/T11091-2005 “Copper Tapes for Cables” standard.
Copper tape shielding should consist of one layer of overlapped soft copper tape or two layers of gap-wrapped soft copper tape. The average overlap rate should be 15% of the tape width, with a minimum overlap rate of no less than 5%. The nominal thickness of the copper tape should be no less than 0.12 mm for single-core cables and no less than 0.10 mm for multi-core cables. The minimum thickness should be no less than 90% of the nominal value.
Copper wire shielding consists of loosely wound soft copper wires, with the surface secured by reverse-wrapped copper wires or tapes. Its resistance should comply with the GB/T3956-2008 “Conductors of Cables” standard, and its nominal cross-sectional area should be determined based on fault current capacity.
2. Functions of Shielding Layers and Their Relationship with Voltage Ratings
(1) Functions of Inner and Outer Semi-Conductive Shielding
Cable conductors are typically made up of multiple stranded and compacted wires. During insulation extrusion, local gaps, burrs, or surface irregularities between the conductor surface and the insulation layer can cause electric field concentration, leading to partial discharge and treeing discharge, which degrade electrical performance. By extruding a layer of semi-conductive material (conductor shielding) between the conductor surface and the insulation layer, it can tightly bond with the insulation. Since the semi-conductive layer is at the same potential as the conductor, any gaps between them will not experience electric field effects, thus preventing partial discharge.
Similarly, gaps between the outer insulation surface and the metal sheath (or metal shielding) can also lead to partial discharge, especially at higher voltage ratings. By extruding a layer of semi-conductive material (insulation shielding) on the outer insulation surface, it forms an equipotential surface with the metal sheath, eliminating electric field effects within the gaps and preventing partial discharge.
(2) Functions of Metal Shielding
The functions of metal shielding include: conducting capacitive currents under normal conditions, serving as a path for short-circuit (fault) currents, confining the electric field within the insulation (reducing electromagnetic interference to the external environment), and ensuring uniform electric fields (radial electric fields). In three-phase four-wire systems, it also acts as the neutral line, carrying unbalanced currents, and provides radial waterproofing.
3. About OW Cable
As a leading supplier of raw materials for wire and cable, OW Cable provides high-quality cross-linked polyethylene (XLPE), copper tapes, copper wires, and other shielding materials widely used in the manufacturing of power cables, communication cables, and special cables. Our products comply with international standards, and we are committed to delivering reliable cable shielding solutions to our customers.
Post time: Mar-24-2025