The shield on the shielded cable is the cable formed in the way of the transmission cable plus a shield against external electromagnetic interference. The so-called ‘cable shielding’ on the cable structure is also a measure to improve the distribution of electric fields. The cable conductor is made of multiple strands of wire stranded, it is easy to form an air gap between the insulation layer, and the conductor surface is not smooth, which will cause the electric field concentration. Then the following will give you detailed knowledge about shielded cables.
I. Cable shielding layer
1. In the conductor surface plus a layer of semi-conductive material shield, it is equipotential with the shielded conductor, and in good contact with the insulation layer, to avoid partial discharge between the conductor and the insulation layer. This layer of shielding is also known as the inner shield. In the insulation surface and sheath contact, there may also be gaps, cable bending, oil paper cable insulation surface is prone to cracking, these are the factors that cause partial discharge.
2. Add a layer of semi-conductive material shield layer on the surface of the insulation layer, which has good contact with the shielded insulation layer, and the metal sheath isotropic. Thus avoiding partial discharge between the insulation layer and the sheath.
The shield of the cable in order to uniform conductive core and insulation electric field, 6kV and above of the medium and high voltage power cables generally have conductor shield and insulation shield, some low-voltage cables do not set shield. The shield layer has two kinds of semi-conductive shield and metal shield.
II. Shielded cable
Most of the shielding layer of this cable is woven into a mesh of metal wire or using a metal film, there are a variety of different ways of single shielding and multi-shield. Single shielding refers to a single shielding network or shielding film, which can be wrapped around one or more wires. The multi-shield approach is multiple shielding mesh, shielding film together in a cable. Some are used to isolate the electromagnetic interference between the wires, and some are double shielding to enhance the shielding effect. The shielding mechanism is to ground the shield layer to isolate the external induced interference voltage on the wire.
1. Semi-conductive shielding
The semi-conductive shield is usually set on the outer surface of the conductive core and the outer surface of the insulation layer, respectively, known as the inner and outer semi-conductive shield. The semi-conductive shield is composed of very low resistivity and thin thickness of semi-conductive materials. The inner semi-conductive shield is to uniform the electric field on the outer surface of the core, to avoid the conductor surface is not smooth and the core stranded air gap caused by the conductor and insulation of partial discharge. The outer semi-conductive shield layer and the outer surface of the insulation layer are in good contact with the metal sheath and isotropic, to avoid partial discharge with the metal sheath due to defects such as cracks on the surface of the cable insulation.
2. Metal shield
For low-voltage power cable without metal sheath, in addition to setting a semi-conductive shield, but also increase the metal shield. The metal shielding layer is usually made of copper tape or copper wire wrapped, mainly to play a role in shielding the electric field.
Because the power cable through the current is relatively large, the current around the magnetic field will be generated, in order not to affect other components, so adding a shielding layer can shield this electromagnetic field in the cable. And the cable shield can play a certain role in grounding protection, if the cable core is within the breakage, the leakage of current can flow along with the shield layer such as the grounding network, which plays a role in safety protection. This shows that the role of the cable shield is still very large.
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