Analysis of the Manufacturing Limitations of Magnesium Oxide Insulated Fire-Resistant Cables

I. Introduction

With the rapid development of the scale and height of urban complexes, high-rise and super high-rise buildings in urban construction, the fire protection requirements for buildings are getting higher and higher, and more strict requirements are put forward for the fire performance of power supply wires inside buildings. Many places require the use of fire-resistant cables. At present, there are many types of fire-resistant cables on the market that can meet the current national relevant standards and requirements. Through the analysis of the manufacturing limitations of magnesium oxide insulated fire-resistant cables, this article puts forward the problems that should be noted in the design and construction processes of building engineering projects, and proposes corresponding solutions.

Ii. Magnesium Oxide Insulated Fire-resistant Cable (MI Cable)

At present, there are many varieties of mineral insulated cables on the market. This article mainly analyzes and discusses the problems that occur in the design and construction of building engineering with magnesium oxide insulated fire-resistant cables.

(1) Magnesium oxide insulated fire-resistant cable

What exactly is magnesium oxide insulated fire-resistant cable? It is made by embedding high-conductivity copper conductors in seamless copper tubes filled with tightly compacted magnesium oxide insulating material, thus forming magnesium oxide Insulated fire-resistant cables, which are called MI cables (Mineral Insulated Cable) abroad. As the composition of MI cables is entirely made of inorganic materials, they possess some characteristics that other types of organic insulated cables do not have.

(II) Main Characteristics of MI Cables

MI cables mainly feature fire resistance, high-temperature resistance, large current carrying capacity, overload resistance, mechanical damage resistance, water and corrosion resistance, radiation resistance, long service life, halogen-free and non-toxic, small bending radius, termite resistance, rodent bite resistance, and the copper sheath can be used as a protective grounding wire.

Here is a brief introduction to several features of MI cables, such as fire resistance and high-temperature resistance:

a) Fire resistance performance

Since all the cables are composed of inorganic substances (copper and magnesium oxide powder), they themselves will not cause fires and cannot burn or fuel combustion. The melting point of copper is 1083℃ and that of magnesium oxide is 2800℃. Therefore, MI cables can continue to supply power in the event of a fire that does not exceed the melting point of copper and are a type of fire-resistant cable with excellent fire resistance.

b) High-temperature resistance

The cable can operate continuously and normally at a high temperature of 250℃. When not exposed to touch and not in contact with flammable materials, it can be used at a temperature of 105℃ or higher. When working for a short time at a temperature close to the melting point of copper, 1083℃, the magnesium oxide insulating material will not undergo any changes at this time. Therefore, it is particularly suitable for use in metallurgy, cement and other high-temperature environments.

c) Good bending performance

The MI cable is a compact whole with a minimum bending radius of no more than 6D (as shown in Table 1), so it is easy to install and reduces the space occupied by the line laying. It is suitable to replace bus ducts and be laid in places with limited space.

Table 1 Bending Radius Table of MI Cable

(3) Limitations in MI cable manufacturing

To clarify the limitations of MI cable manufacturing, it is necessary for us to take a look at its manufacturing process.

There are three production and manufacturing processes for MI cables: the prefabricated magnesium oxide porcelain column method, the automatic magnesium oxide powder filling method, and the copper strip longitudinal wrapping argon arc welding method.

The first prefabricated magnesium oxide ceramic column method is the earliest and most commonly used method in China. Currently, most of the manufacturers of MI cables in China are using this process. This process involves sintering to form magnesium oxide porcelain columns of specific shapes, which are then manually assembled between copper tubes and copper rods. Through a series of drawing, annealing and other procedures, they are finally formed into MI cables. The biggest drawback of this process is that it involves high labor intensity and relatively low production efficiency.

The second automatic magnesium oxide powder filling method involves filling magnesium oxide powder between a vertically fixed copper rod and a copper tube. This method can make the concentricity between the copper rod and the copper tube better, but this kind of equipment is rarely used in China.

The third type of copper strip longitudinal wrapping argon arc welding method is currently one of the more advanced methods in the world. The principle is that the copper strip and the copper rod can be welded infinitely (theoretically), and the copper strip is longitudinally wrapped and welded to form a shape to wrap around the copper rod. Magnesium oxide powder is injected between the copper rod and the welded copper strip. This process can produce finished products of any length while manufacturing, and the production cycle is short.

From the above three production and manufacturing processes, it can be known that in the first and second methods, the production and delivery length of MI cables is greatly affected by the production site and raw materials, especially the production and delivery length of large cross-section cables is relatively short. According to the data in the current national architectural standard design drawing set 09D101-6, the delivery length of small-sized cables is longer, while that of large-sized cables is relatively shorter. For details, please refer to Table 2 (only the data of some single-core cables are selected in the table for illustration). The delivery length of cables produced by the third method is longer. According to the information provided by a certain manufacturer, for cables produced by the copper strip longitudinal wrapping argon arc welding method, the delivery length is 2000m when the cross-sectional area is 35mm² or less, and 500m when the cross-sectional area is greater than 35mm². Therefore, for cables produced by the first and second methods, when larger-sized cables are selected in engineering applications, intermediate connections need to be added. Cables produced by the third method basically do not require intermediate connections in practical engineering applications.

Iii. Analysis of MI Cable Demand in Engineering Projects

(1) Analysis of MI Cable Sales in a Certain Company

In response to the limitations of MI cable manufacturing, the author of this article specially consulted the MI cable manufacturer. With the manufacturer's consent, the sales situation of MI cables and MI cable intermediate connectors over the past three years as shown in Table 3 is presented.

Through analysis, it can be found that in actual construction projects, when MI cables are selected, the proportion of the intermediate connection of the MI cables. In Table 3, the number of intermediate connections used accounts for 0.43% to 0.48% of the total length. This means that only when a cable length of 234 to 210 meters is sold will one MI cable intermediate connector be used.

In addition, the author of the article learned from the MI manufacturer that the use of some MI cable intermediate connections is due to the fact that the production enterprises supply cables in coils (i.e., the maximum length of a single cable). After each coil of cable is used separately, there will be extra cables. The installation units, in order not to waste these extra cables, Intermediate connectors will also be used to connect this part of the cable for continued use.

(II) Analysis of the Use of Fire-resistant Cables in Construction Projects

The author conducted a statistical analysis on the use of fire-resistant cables in a certain engineering project, covering the specifications of fire-resistant cables used in the project, the length of individual power supply circuits, and the load conditions provided.

Example of construction project: Plot C of the first phase of a certain international trade city, which is commercial land, has a total construction area of 175,960 square meters. Among them, the above-ground buildings are a modern trade city integrating department stores, clothing, children's playgrounds, catering and parking lots, while the underground buildings mainly consist of equipment rooms, garages and civil air defense projects. The above-ground building has five floors, with a roof height of 23.10 meters and one floor underground.

Iv. Analysis of the Usage of MI Cable Intermediate Connections

(1) Under what circumstances is an intermediate connection required for MI cables

Based on the above analysis, in actual construction projects, the intermediate connection of MI cables may be needed in the following several situations. Firstly, when a larger cross-sectional area is used and the distance of the power supply circuit exceeds the delivery length of the cable; Secondly, when the location of the substation is restricted by certain conditions, resulting in a long power supply distance for the circuit (exceeding the delivery length of the cable); Thirdly, during the construction of building projects, when some redundant cables are generated and have certain utilization value; Fourth, other special circumstances other than the above three situations.

(2) Reduce the use of MI cable intermediate connections

In construction projects, when choosing MI cables, only through reasonable design, meticulous planning and other methods can the intermediate connections of MI cables be reduced.

First of all, reasonable design is very important. Here, it is necessary to fully understand the load distribution of the construction project, and place the transformer deep in the load center, as close as possible to the fire pump room (or power center). In addition, for large-capacity power supply circuits, a double-cable configuration can also be adopted for power supply, such as: The calculated load of a certain power supply circuit is 560A, and 300mm² is selected. The delivery length of a single cable is 79m. According to Table 1, if the length of the power supply circuit is greater than 79m but less than 90m, 185mm² double splicing can be chosen, and the delivery length can reach 95m. This can avoid the intermediate connection of MI cables.

Secondly, a detailed plan is also very important. Here, it mainly refers to when ordering MI cables, planning the required cables as much as possible according to the delivery lengths of each specification of cables in Table 1, ensuring that the cable lengths of each power supply circuit are customized based on the actual on-site requirements. In this way, during the actual construction of the building project, no extra wires will be generated, thus avoiding unnecessary waste.

In actual engineering, when using MI intermediate connections, the following points should be noted: First, refer to the national architectural design standard drawing set and the installation process, regulations and norms recommended by the manufacturer for construction; Second, a dedicated MI cable intermediate connector should be used to connect the MI cable. Thirdly, during construction, if moisture is found to have invaded the ends of the cables that need intermediate connection, corresponding dehumidification measures should be taken. Only after the insulation resistance meets the requirements can the intermediate connection of MI cables be carried out. It should be noted that after the installation of the intermediate connector is completed, the insulation resistance should also be tested.