【Summary description】 Copper core copper sheathed magnesia insulated fireproof cable (hereinafter referred to as magnesia cable) is a new type of cable with seamless copper tube sheathed on the outer layer and filled with magnesia crystal powder as insulation material and single copper rod as conductor.

I. Introduction of magnesium oxide cable:
Copper core copper sheathed magnesia insulated fireproof cable (hereinafter referred to as magnesia cable) is a new type of cable with seamless copper tube sheathed on the outer layer and filled with magnesia crystal powder as insulation material and single copper rod as conductor. It is not only resistant to high temperature, fire prevention, explosion proof, flame retardant, and large load capacity, small diameter, high mechanical strength, long service life, generally do not need independent grounding wire. Therefore, magnesium oxide cable is widely used in dangerous, harsh, high temperature environment, such as nuclear power plant, metallurgy, chemical industry, mine, kiln and so on. In recent years, with the vigorous development of various technologies, magnesium oxide cable is also more used in high-rise buildings, airports, docks, subways and other civil buildings, to ensure the fire in the case of fire pump, fire elevator, local lighting, emergency evacuation instructions, security monitoring, smoke control smoke exhaust system, self-provided power supply and other fire power and important equipment uninterrupted operation.
Construction and model specifications of magnesia cable:
The manufacturing process of magnesia fireproof cable 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 compacted; The second is the first magnesium oxide powder pressed into a hollow cylinder, it is inserted between the conductor and the copper pipe, and then through the drawing - annealing - drawing and other processes cycle repeatedly and then made of cable. It should be noted that in the processing of magnesium oxide cable, it is necessary to constantly test the compactness of magnesium oxide and the air tightness of copper pipe. Through the understanding of the production process and method of magnesium oxide cable, we can see that the structure of the cable is mainly composed of three parts: the outer seamless copper tube jacket, the middle plays the role of insulation magnesium oxide crystal powder, the core as a conductor of copper rod.
The types of magnesia cable can be divided into BTTQ light cable (500V class) and BTTZ heavy cable (750V class). The Kempinski project mainly uses BTTZ heavy cable. Its main specifications are single core, two core, three core, four core, seven core, twelve core and so on. Single-core to four-core magnesia cable is mainly used for power, lighting, more than seven core for communication, control. The maximum section of single-core cable can reach 4002m㎡, and the maximum section of two-core to four-core cable core is 25m㎡.
Two, the advantages of magnesium oxide cable
1. Complete fire prevention
The magnesia cable itself does not burn at all. It can run continuously for a long time at 250℃, and can maintain normal operation for 30 minutes at 1000℃ limit state. At the same time, it will not cause fire source. Even in the case of flame barbecue, as long as the flame temperature is below the melting point temperature of copper, the cable can continue to be used after flame elimination without replacement. No toxic fumes and gases are produced when barbecued by flame. |
2. Strong overload protection ability
When the line is overloaded, as long as the heat does not reach the melting point temperature of copper, the cable will not be damaged. Even if the breakdown is instantaneous, the magnesium oxide crystal at the breakdown will not form carbide. After the overload is eliminated, the performance of the cable will not change and it can continue to be used normally.
3. High working temperature
4, anti-corrosion, explosion-proof performance is good
Due to the use of seamless copper tube as sheathed, so the magnesium oxide cable has waterproof, moisture, oil and some chemical substances damage performance, copper tube has considerable mechanical strength so it has better explosion-proof performance.
5, long service life
Magnesia cable is composed of inorganic materials, so there is no insulation aging, the service life can reach more than 3 times of the ordinary cable.
6. Greater flexibility in laying
Magnesium oxide cable can be laid on the bridge with ordinary cable, but also can use special support, more flexible than ordinary cable laying, but also save the investment of cable bridge, can reduce the overall cost of the project.
Three, the shortcomings of magnesium oxide cable
Everything has two sides, magnesium oxide cable is no exception, has many advantages at the same time, there are also some shortcomings, should be paid attention to in the selection and construction.
1. High investment cost
Because the outer sheath of magnesium oxide cable is composed of seamless copper tube, the overall copper content is much more than ordinary cable, and the construction technology of magnesium oxide cable has higher requirements for equipment than ordinary cable, resulting in the price of magnesium oxide cable is about 30% higher than that of ordinary cable.
2, the joint is susceptible to moisture
The insulating layer consists of the mineral magnesium oxide, which readily reacts with moisture in the air to form magnesium hydroxide, which conducts electricity. During the construction of cable end, when the bare conductor is peeled off at the cable end, the insulation resistance of the cable is generally above 10MΩ. However, if the cable end is not finished within 1 hour, the insulation resistance can be reduced to less than 10 mω or even to less than 0.5 mω. If the outer layer is not scratched and the sealing and moisture-proof treatment is not done timely, the insulation resistance can be reduced to less than 10 mω. The insulation value will drop quickly and gradually drop to 0, which will make the cable unusable.
3. The construction is difficult
Compared with the general cable hardness is higher, the weight is about 2 times that of the general cable, it is not easy to achieve the parallel and clean appearance effect when laying, and the line is long, the joints are more, it is difficult to find the fault point, so the construction is more difficult, in the access to the distribution box and the bending and forming in the bridge.
4, the construction workload is large
Magnesia cables whose specifications exceed 35mm are single-core cables. For example, a 70mm cable, ordinary cables only need 3×70+2×35 five conductors in the same outer sheath, while magnesia cables to achieve the same specifications must be composed of 3 70mm and 1 35mm single-core cable. The delivery length of single-core cable is short, for example, the delivery length of 240mm cable is 69 meters. If the laying distance is longer, the production of a large number of intermediate joints will be increased, which makes the construction workload increase exponentially.