1. Brief introduction of magnesium oxide cable:
Copper core copper sheathed magnesium oxide insulated fireproof cable (hereinafter referred to as magnesium oxide cable) is a new type of seamless copper tube sheathed with magnesium oxide crystal powder filled in the middle as insulating material, and single-strand copper rod as the conductor. cable. It not only can withstand high temperature, fireproof, explosion-proof, and flame retardant, but also has large current carrying capacity, small outer diameter, high mechanical strength, long service life, and generally does not require an independent grounding wire. Therefore, magnesium oxide cables are widely used in dangerous, harsh and high-temperature environments, such as nuclear power plants, metallurgy, chemicals, mines, kilns, etc. In recent years, with the vigorous development of various technical processes, magnesium oxide cables have also been widely used in high-rise buildings, airports, docks, subways and other civil buildings to protect fire pumps, fire elevators, local lighting, etc. in case of fire. Uninterrupted operation of fire-fighting power and important equipment such as emergency evacuation instructions, security monitoring, smoke prevention and exhaust systems, and self-provided power supplies.
　　 Magnesium oxide cable structure and model specifications:
　　 The manufacturing process of magnesium oxide fire-resistant cables is more complicated. There are two main methods. One is to pour magnesium oxide crystal powder between the outer seamless copper tube and the core copper core conductor, and then compact; the other is to press the magnesium oxide powder into a hollow cylinder. It penetrates between the conductor and the copper tube, and then is made into a cable through multiple processes such as drawing-annealing-drawing and so on. It should be noted that during the processing of magnesium oxide cables, it is necessary to continuously test the compactness of magnesium oxide and the air tightness of copper pipes. Through the understanding of the manufacturing process and method of magnesium oxide cable, we can see that the structure of the cable is mainly composed of three parts: the seamless copper tube sheath on the outer layer, the magnesium oxide crystal powder that acts as an insulation in the middle, and the copper core as the conductor. Great.
　　 Magnesium oxide cables can be divided into two types: BTTQ light cables (500V class) and BTTZ heavy cables (750V class). The Kempinski project is mainly BTTZ heavy cables. Its specifications mainly include single-core, two-core, three-core, four-core, seven-core, twelve-core, etc. Single-core to four-core magnesium oxide cables are mainly used for power and lighting, and seven-core or more cables are mainly used for communication and control. The maximum cross-section of single-core cables can reach 4002mm square meters, and the maximum cross-section of two-core to four-core cables is 25 mm square meters.
　　 2. Advantages of Magnesium Oxide Cable
　　1. Completely fireproof
　　 Magnesium oxide cable does not burn at all. It can run continuously for a long time at 250°C, and can maintain normal operation for 30 minutes at the limit state of 1000°C without causing fire. Even in the case of flame grilling, as long as the flame temperature is lower than the melting point of copper, the cable can continue to be used without replacing the flame after the flame is eliminated. In the case of being grilled by flames, no toxic smoke and gas will be produced. ｜
　　2, strong overload protection ability
　　 When the line is overloaded, the cable will not be damaged as long as the heat does not reach the melting point of copper. Even if the breakdown occurs instantaneously, the magnesium oxide crystal at the breakdown site will not form carbides. After the overload is eliminated, the cable performance will not change, and normal use can still be continued.
　　3, high working temperature
　　4, good anti-corrosion and explosion-proof performance
　　 Because the seamless copper tube is used as the sheath, the magnesium oxide cable has the properties of waterproof, moisture, oil and some chemical substances. The copper tube has considerable mechanical strength and has better explosion-proof performance.
　　5, long service life
　　 Magnesium oxide cables are all made of inorganic materials, so there is no insulation aging, and the service life can reach more than 3 times that of ordinary cables.
　　6, the laying flexibility is greater
　　 Magnesium oxide cable can be laid on the bridge with ordinary cables, and it can also be laid with special brackets, which is more flexible than ordinary cable laying. It also saves the investment in cable bridges and can reduce the overall cost of the project.
　　 3. Disadvantages of Magnesium Oxide Cable
　　Everything has two sides, and magnesium oxide cable is no exception. While having many advantages, there are also some shortcomings, which should be paid attention to when selecting and constructing.
　　 1. High investment cost
Because the outer sheath of magnesium oxide cable is made of seamless copper tube, the overall copper content is much more than that of ordinary cables. At the same time, the construction process of magnesium oxide cables requires higher equipment than ordinary cables, resulting in the price of magnesium oxide cables compared with ordinary cables. About 30% higher.
　　2, the joints are easy to get damp
　　The insulating layer is composed of mineral magnesium oxide, which can easily chemically react with moisture in the air to produce conductive magnesium hydroxide. In the construction of the cable head, when the bare conductor is stripped at the cable end, the insulation resistance of the cable is generally above 10MΩ. However, if the cable head is not completed within 1 hour, the insulation resistance can drop below 10MΩ, or even drop to 0.5. Under the condition of MΩ, if the outer layer is not scratched carefully, and the sealing and moisture-proof treatment are not found in time, the insulation value will drop quickly and gradually drop to 0, which will cause the cable to be unusable.
　　3, the construction is difficult
The hardness of magnesium oxide cable is higher than that of ordinary cables, and the weight is about twice that of ordinary cables. It is not easy to achieve parallel and clean appearance when laying. Also, it is difficult to find fault points because of long lines and many joints. Therefore, the construction is more difficult. It is difficult to bend and form at the wiring box and inside the bridge.
　　4, the construction workload is large
Magnesium oxide cables whose specifications exceed 35mm are single-core cables, such as a 70mm cable, ordinary cables only need 3×70+2×35 five conductors in the same outer sheath, while magnesium oxide must be equivalent The specification must be composed of 3 single-core cables of 70mm plus 1 35mm. The delivery length of single-core cables is relatively short. For example, the delivery length of 240mm cables is 69 meters. If the laying distance is long, it will increase the production of a large number of intermediate joints, which will double the construction workload.