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Basic knowledge of cable must be known
Time：2019-01-24

The basic structure of cables generally consists of three parts: conductive core wires for transmitting electric energy and signals; insulating layer for ensuring transmission of electric energy or signals along conductive core wires and isolation of conductive core wires from the outside world to withstand grid voltage; and protective layer for protecting and sealing the insulation layer and keeping the insulation performance for a long time.

In addition, in order to shield the electric field and magnetic field in the cable and protect the cable from the influence of external electromagnetic field, the shielding layer is usually made of semiconductor or metal materials outside the conductor and insulation layer. In order to make the cable core rounded, it is necessary to fill the gap in the insulating core.

Selection of Conductive Core Material

The best electrical properties of metals are silver, followed by copper and aluminium. Because of the high price of silver, in addition to the use of silver in special occasions and special uses, the conductors commonly used in various cables are copper or aluminium core wires. Copper is a good conductor of electricity, and its conductivity is second only to silver. It has high mechanical strength and is easy to be processed by calendering, drawing and welding. It has good physical and mechanical properties and excellent technological properties. Because of the low strength of aluminium, it is difficult to draw thin wires. At present, it is not realistic to process soft conductors and Extra-soft conductors. However, the wire and cable with a cross-section of more than 4 mm can safely use aluminium conductors. Cable products based on aluminium are commonly used in the market, such as 0.6/1 kV aluminium conductor cable, 0.6/1 kV aluminium alloy cable and ultra-high voltage 500 kV heat-resistant aluminium alloy conductor.

Selection of Insulation Layer Material

Because the core wire of the cable is in high potential and large current passes through, the insulation layer of the cable mainly bears the effect of voltage to ensure the isolation between multi-core wires and sheaths. Its dielectric loss is low, and it requires certain heat resistance and stable insulation quality. It determines the reliability, safety and service life of wire and cable.

The insulation thickness of cable is related to working voltage and cable cross-sectional area. The higher the voltage, the thicker the insulation layer is. When the cross-sectional area of conductor is large, the thickness of the insulation layer can be thinner. Insulation layer should not only ensure that it will not be broken down under power frequency voltage and impulse voltage, but also ensure that the insulation layer will not be damaged by machinery during normal construction.

At present, different kinds of power cables are divided into oil-immersed paper insulated cables, plastic insulated cables, rubber insulated cables and mineral insulated cables according to their insulation materials.

Selection of Material for Common Protective Layer

In order to protect cable insulation from damage and adapt to the requirements of various use environments, the protective covering layer added outside the cable insulation layer is called cable protection layer. The cable sheath includes inner sheath and outer sheath.

The inner shield is a protective covering covering covering covering on cable insulation. It is used to prevent damp, mechanical damage and optical and chemical erosion of the insulating layer. It can also flow through short-circuit current to ensure the insulation performance remains unchanged. There are metal sheaths (lead sheath, aluminium sheath, wrinkle aluminium sheath, steel sheath, comprehensive sheath, etc.) and non-metal sheaths (plastic sheath, rubber sheath).

The outer shield is a protective covering covering covering covering on the outside of the cable sheath (inner shield). It can increase the mechanical strength of the cable under tension and compression, and protect the insulating layer of the cable from mechanical damage and various environmental factors during laying and operation, so as to ensure long-term and stable electrical performance.

Commonly used cables have inner sheath as outer sheath of metal sheath and inner sheath as outer sheath of non-metal sheath (plastic sheath).

Cable protective layer is mainly divided into three categories: metal protective layer (including outer protective layer), plastic protective layer and composite household layer. However, in order to meet some special requirements, such as anti-biological cable sheath is also known as special sheath.