Development and application of mineral insulated mine cables
Mineral insulated mine cable also known as magnesium oxide cable, the main components are electrical copper, magnesium oxide insulation materials and metal sheath. A highly conductive copper conductor embedded in a seamless steel tube with a tightly compacted magnesia material constitutes a highly secure mineral insulated cable. Special needs can also be squeezed on the metal sheath wrapped with a layer of polyethylene outer sheath. Mineral insulated cable is developed earlier in foreign countries, and has been produced for half a century, and is used in various fields. It is still a new topic at home, and although it has been used by some departments, enterprises and imported equipment in joint ventures with foreign countries in recent years, it is only the beginning. ArnoldFrancoisBorel, a Swiss engineer, first proposed the idea of mineral insulated cables at the end of the 19th century and obtained the patent right in 1936. Subsequently, France and Britain were officially put into production one after another, and developed rapidly after that. Britain, France, Italy, Canada, Australia, the United States, the Soviet Union, Japan and other major countries have large quantities of production. The main equipment has been imported from Austria and Italy. Harbin has developed the cable product and its accessory production line with imported equipment and domestic supporting equipment, with a registered capital of 39 million yuan, making it the first large-scale professional mineral cable manufacturer with an annual production capacity of 1500 kilometers, filling the domestic gap.
The classification of the mine cable structure according to the foreign usually, mineral insulated cable roughly divided into: distribution cables (WiringCable), heating cables and components (HeatingCableandHeatingElement), thermocouple cables and compensation cables (ThermocoupleCableandCompensatingCable) and special cable (SpecialCable), etc. Mineral insulated cables can be divided into heavy duty cables (1000 v class) and light duty cables (600 V class) according to the voltage class. Cable core number is 1, 2, 3, 4, 7, 12, 19 core, 1-4 core is mainly used for power and lighting, more than 7 core is mainly used for control and communication. The maximum manufacturing cross-sectional area of a conductor for a single core cable can be up to 400mm2. According to different requirements, the cable can also be made into square, flat or hollow type and other different shapes. Cable terminals are made of metal or thermoplastic materials. The long-term service temperature of bare wire is 250oC, and the melting point of copper is close to 1083oC in short time or extraordinary period. When used in chemical occasions with erosion of copper sheath, a layer of PVC outer sheath (non-burning) can be extruded outside the copper sheath.
Mine cable structure 1) cable sheath 2) conductive wire core 3) insulation material 4)PVC sheath
Main specifications of mine cables: cable model conductor core number and nominal section Mm2 bare cable outer diameter MM extrusion PVC sheath nominal diameter mm conductive wire nominal diameter mm conductive wire core resistivity 20oC copper sheath resistance 20oC finished cable delivery length ± 5% light load cable
Characteristics of mine cables The cables are all made of inorganic materials, thus giving them some characteristics superior to any other type of cables
1) Fire prevention: Copper and magnesium oxide are both inorganic substances. Cables do not burn or assist combustion, and they do not produce toxic gas. It has passed the test of INTERNATIONAL Electrotechnical Commission IEC331.
2) Explosion-proof: The highly compressed insulation material and cable terminal installed with special sealing sleeve can prevent steam, gas and flame from entering the electrical equipment connected with the cable, so it can be used in places with explosion risk and the connection of various explosion-proof equipment and equipment;
3) Waterproof and corrosion resistance: The seamless metal sheath of the cable has high corrosion resistance, impervious to water, oil and gas. In almost all environments, it does not need to take additional protective measures. PVC sheath is only needed in the case of individual chemical corrosion;
4) Radiation resistance: Inorganic materials with effective neutron capture cross section and high insulation resistance are used, so they have good stability and are suitable for use in nuclear power plants and other nuclear facilities;
5) Large load current: Under the same load flow, the diameter of the cable is smaller than that of the traditional cable section, and it bears more overload (current) energy;
6) High mechanical strength: can withstand severe mechanical damage, and can still work normally under the condition that the cable diameter deformations by one third;
7) Large current carrying capacity: The actual continuous service temperature of copper core and copper sheath cable can reach up to 250oC. Under the same load flow condition, the diameter of the cable is smaller than that of the traditional cable section, and it can bear large overload (current) energy;
8) Safety and reliability: Independent grounding wires are not required. The outer sheath of the cable is installed in conjunction with the phase wire to play the role of grounding, providing excellent low grounding resistance. Due to the strength and toughness of the metal sheath, magnesium oxide is highly compressed during the cable processing, so the cable will not be broken when subjected to bending, flattening, torsion and other deformation between the core wire and the relative position of the core wire and the sheath remain unchanged. Ensure the stability of the cable, no maintenance in the work;
9) Long service life: All cables are made of inorganic materials and do not age. The service life is calculated according to the oxidation corrosion rate of copper sheath, which is 0.25mm, 257 years at 250oC and 583 years at 400oC. The thickness of mineral insulated cable sheath is 0.34mm-1.03mm in general, so the cable has a long service life. As a heating cable, the heating range is wide, the temperature is uniform and the control is convenient. As a thermocouple cable, the reflection is sensitive and inert. Its temperature resistance and electrical performance are very good, its melting point below 2800oC hardly any change. Each cable is fitted with a suitable terminal, and a complete terminal consists of a gland that can reach into the junction box to act as a connection, a terminal that prevents invasion of damp kydl_syey gas and ensures constant insulation resistance, and a corresponding PVC sleeve. Three different forms of terminal materials can be used according to the installation mode and operating temperature of the cable, as shown in the following table: maximum operating temperature sealing material gasket insulating bushing Brass pot 105oCCS Mupren BT185oCHT TEF thermal shrinkable end 125oC
There are special accessories for the installation and laying of mine cables: 1) Brass earthing plate for cable sheath earthing; 2) copper strip (12 x1mm); 3) Adjustable plastic mounting sheet; 4) Brass locking nut; 5)"0" gasket; 6) Brass tightening joint cap; 7) Lubricating oil; 8) PVC sheathed gland for special occasions; 9) Resin for straight joints and installation; 10)PVC outer sheath; 11) gland.
conclusion
Because of mineral insulated cable has the characteristics above, it has been the international electrotechnical commission, the national institute of electrical engineers, such as the institute of electrical engineers, Germany Lloyd's register many international authoritative organizations, some national standards, and preferable in key areas of electrical engineering, the dangerous area such as: the airport, oil depot and gas station, chemical industry, coal mine, (nuclear) power plant, steel smelting and shipbuilding etc. Advanced and important public facilities: satellite ground stations, railway wiring, alarm, fire fighting, etc. Places to ensure personal and property safety: historic buildings, hotels, high-rise buildings, museums, libraries, Banks, hospitals, insurance, shopping malls, scientific research institutions, etc. Once laid, the cable can be used forever, and it is economical to use from the point of view of one-time investment. According to statistics, fires caused by electrical reasons account for about 20-30% of all fires in the world every year. Our loss will not be less than this. It can be seen that mineral insulated cable is an essential part of modern engineering cable system. It will enable China's oil platforms, nuclear power plants, petrochemical and high-rise buildings to meet international standards.