Shielded cable, as the name implies, is a cable with a shielding layer applied to the transmission cable to enhance its resistance to external electromagnetic interference. The so-called "shielding" in cable construction is also a measure to improve electric field distribution. Cable conductors are made of multiple strands of wire, which can easily form air gaps between them and the insulation layer. The uneven surface of the conductor can also cause electric field concentration. Below, I will explain in detail about shielded cables.
1. Cable Shielding
1. A shielding layer made of semi-conductive material is applied to the surface of the conductor. It maintains the same potential as the shielded conductor and maintains good contact with the insulation layer, thus preventing partial discharge between the conductor and the insulation layer. This shielding layer is also called the inner shield. Gaps may also exist where the insulation surface meets the sheath. When the cable is bent, cracks may form on the insulation surface of oil-paper cables, which can cause partial discharge.
2. A shielding layer made of semi-conductive material is applied to the surface of the insulation layer. It maintains good contact with the shielded insulation layer and maintains the same potential as the metal sheath, thus preventing partial discharge between the insulation layer and the sheath.
To ensure uniform electric fields between the conductive core and the insulation, cable shielding generally includes both a conductor shield and an insulation shield. Some low-voltage cables (6kV and above) do not have a shield. Shielding can be either semi-conductive or metallic.
2. Shielded Cables
The shielding of these cables is typically made of a woven mesh of metal wire or a thin metal film. There are various shielding methods, including single and multiple shielding. Single shielding refers to a single screen or film that surrounds one or more conductors. Multiple shielding involves multiple screens and films within a single cable. Some shielding methods are used to isolate electromagnetic interference between conductors, while others employ double shielding to enhance shielding effectiveness. The shielding mechanism is to ground the shield layer to isolate it from external induced interference voltages.
1. Semi-conductive Shielding
Semi-conductive shielding layers are typically applied to the outer surface of the conductive core and the outer surface of the insulation layer, referred to as the inner and outer semi-conductive shielding layers, respectively. The semi-conductive shielding layers are composed of a thin, semi-conductive material with very low resistivity. The inner semi-conductive shielding layer is designed to even out the electric field on the outer surface of the core, preventing partial discharges between the conductor and insulation caused by uneven conductor surfaces and air gaps created by twisting the cores. The outer semi-conductive shielding layer maintains good contact with the outer surface of the insulation layer and is at the same potential as the metal sheath, preventing partial discharges between the conductor and the metal sheath caused by defects such as cracks in the cable insulation.
2. Metal Shielding
For medium and low voltage power cables without metal sheaths, in addition to the semi-conductive shielding layer, a metal shielding layer is also required. The metal shielding layer is typically made of copper tape or copper wire wrapped around it and primarily serves to shield against electric fields.
Because power cables carry relatively high currents, magnetic fields are generated around the current. To prevent interference with other components, the shielding layer is added to confine this electromagnetic field within the cable. Furthermore, the cable shielding layer provides a certain degree of grounding protection. If a cable core is damaged, the leaked current can flow through the shielding layer to the grounding grid, providing a safety barrier. This demonstrates the significant role of cable shielding.
