What is shielded cable? What does the shield do?

The shielded cable has the word "shielded". As the name suggests, it is a cable with the ability to resist external electromagnetic interference formed by adding a shielding layer to the transmission cable. The so-called "shielding" on the cable structure is also a measure to improve the distribution of the electric field. The cable conductor is made of twisted multi-strand wires. It is easy to form an air gap between it and the insulating layer. The surface of the conductor is not smooth, which will cause electric field concentration. Let me tell you about the knowledge of shielded cables.

1. Cable shielding layer

1. Add a shielding layer of semiconducting material on the surface of the conductor, which has the same potential as the shielded conductor and is in good contact with the insulating layer, so as to avoid partial discharge between the conductor and the insulating layer. This layer of shielding is also known as the inner shielding layer. There may also be gaps at the contact between the insulating surface and the sheath. When the cable is bent, the insulating surface of the oil-paper cable is prone to cracks, which are all factors that cause partial discharge.

2. Add a shielding layer of semi-conductive material on the surface of the insulating layer, which has good contact with the shielded insulating layer and is equipotential with the metal sheath, so as to avoid partial discharge between the insulating layer and the sheath. The shielding layer of the cable In order to evenly conduct the core and insulating the electric field, the medium and high voltage power cables of 6kV and above generally have a conductor shielding layer and an insulating shielding layer, and some low-voltage cables do not have a shielding layer. There are two types of shielding layers: semi-conductive shielding and metal shielding.

2. Shielded cable

The shielding layer of this kind of cable is mostly made of metal wire braided into a mesh or metal film, and there are many different ways of single shielding and multi-shielding. Single shielding refers to a single shielding mesh or shielding film in which one or more conductors can be wrapped. The multi-shielding method is that there are multiple shielding nets, and the shielding films are co-located in one cable. Some are used to isolate electromagnetic interference between wires, and some are double-layer shielding to strengthen the shielding effect. The mechanism of shielding is to ground the shielding layer to isolate the induced interference voltage of the external pair of wires.

1. Semi-conductive shield

The semiconducting shielding layer is usually disposed on the outer surface of the conductive wire core and the outer surface of the insulating layer, which are respectively called the inner semiconducting shielding layer and the outer semiconducting shielding layer. The semiconducting shielding layer is composed of semiconducting material with low resistivity and thin thickness. The inner semi-conductive shielding layer is to uniform the electric field on the outer surface of the wire core and avoid partial discharge of the conductor and the insulation caused by the unsmooth surface of the conductor and the air gap caused by the twisting of the wire core. The outer semi-conductive shielding layer is in good contact with the outer surface of the insulating layer, and has an equal potential with the metal sheath 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 medium and low voltage power cables without metal sheaths, in addition to the semi-conductive shielding layer, a metal shielding layer should be added. The metal shielding layer is usually made of copper tape or copper wire, which mainly plays the role of shielding the electric field.

Because the current passing through the power cable is relatively large, a magnetic field will be generated around the current. In order not to affect other components, a shielding layer can shield this electromagnetic field in the cable. Moreover, the shielding layer of the cable can play a certain role in grounding protection. If the core wire of the cable is damaged, the leaked current can flow into the grounding network along the shielding layer to play a role in safety protection. It can be seen that the role of the cable shielding layer is still very large.