The basic knowledge of the cable you have to know
The basic structure of the cable generally consists of the following three parts: the conductive core wire for transmitting electrical energy and signals; the insulating layer, which could ensure the transmission of electrical energy or signal along the conductive core wire and the isolation of conductive core wire from the outside, withstanding the voltage of the power grid; The coating layer, which could protect and seal the insulating layer and keep the insulating property long lasting.
In addition, a slightly higher voltage cable, in order to keep the electric field, magnetic field in the cable and shield the cable from the external magnetic field. Outside the conductor and insulating layer, a shielding layer is generally made from the semiconductor or mental material; In order to make the cable core round, filling the insulation core gap is needed.
About the selection of conductive core wire material
Silver has the best electrical properties, followed by copper and aluminum. Due to the high price of silver, all kinds of cables commonly use copper core wire or aluminum core wire as conductor, except for special occasions and special use. Copper is a good conductor of electricity, its conductive performance is only next to silver, and it has high mechanical strength, easy to carry out calendering, wire drawing and welding with excellent physical and mechanical properties as well as excellent process performance. Due to the low strength of aluminum, it’d more difficult to draw thin lines, and currently process of soft conductor and soft conductor is also not realistic, but the cable wire with more than 4 mm ² cross-sectional area can really safely use the aluminum conductor. Cable products made of aluminum are commonly found in the market as follows: 0.6/1kV aluminum conductor cables, 0.6/ KV aluminum cables and 500kV ultra-high pressure heat-resistant aluminum alloy conductors.
About the selection of insulating material
Because the cable core wire is in the high potential through which a large current gets, so the cable insulation layer mainly withstand voltage to ensure the interval between the multi-core conductor and the sheath. Its dielectric loss is low, and requires a certain degree of heat resistance and stable insulation quality. It determines the reliability, safety and service life of the wire cable.
The insulation thickness of the cable is related to the working voltage and the sectional area of the cable, the higher the voltage is and the thicker the insulating layer is, and the insulating layer can be thinner when the sectional area is large. Insulating layer not only should not be penetrated under the power frequency voltage and impact voltage, but also ensure that the the insulating layer will not be mechanically damaged when in the normal construction.
At present, various kinds of power cables are classified according to insulating material: oil-impregnated paper insulated cable, plastic insulated cable, rubber insulated cable and mineral insulated cable.
About the selection of common protective layer material
In order to protect the the cable insulation from damage and make it adaptable to the various environments, the protective cover layer added outside the insulating layer of the cable is called a cable protective layer. The cable sheath includes an inner layer and an outer sheath.
The inner sheath is a protective layer covering the insulation of the cable, which is used to prevent moisture, mechanical damage, light and chemical erosion, and also allow the short-circuit currents to flow through it to ensure that the insulating properties are unchanged. The inner protective layer has metal sheath (lead sheath, aluminum cover, wrinkle aluminum sheath, steel sheath, comprehensive sheath, etc.) and non-metallic sheath (plastic sheath, rubber sheath).
The outer layer is the protective layer outside the cable sheath (inner sheath). It can increase the mechanical strength of tensile and compressive force of cable, and protect the insulation layer from mechanical damage and various damages from environmental factors during laying and operation, so as to ensure long time and stable electrical performance.
Commonly used cables are in two kinds: outer layer with metal sheath as its inner layer and outer layer with non-metallic sheath (plastic sheath) as its inner layer.
The protective layer of the cable is divided into 3 categories: the metal sheath (including the outer layer), the plastic protective layer and the combination layer. However, in order to meet certain special requirements, cable protection layer against biological contamination is also called special protective layer.