Branch cable is a new type of prefabricated building distribution cable, which is widely used in the distribution of middle and high-rise buildings, large factories and stadiums. According to the structural characteristics and distribution requirements of specific buildings, this product integrates main cables, branch cables and branch connectors, and has excellent technical and economic indicators. It has outstanding advantages over traditional cables and buses in engineering economy, advanced technology and convenient installation. Based on the brief introduction of the structure and performance of branch cable, the technical progress of this product in meeting the relevant building electrical codes is emphatically introduced.
I. Structure and Performance of Branch Cables
1, the first generation and technical standard branch cables are developed on the basis of ordinary plastic cables. Due to the development of modern civilization, urban high-rise buildings are becoming more and more popular. In the electrical design of high-rise building distribution system, power supply availability, engineering economy and construction convenience become more and more important. When ordinary power cables supply power, the contradictions between them are always difficult to be completely unified, and they can only be emphasized according to different projects. According to the traditional method, there are three common methods in floor distribution design:
(1) radial type, the underground power distribution room directly supplies power to the guide cables of each floor, but it needs a large number of cables, bridges and larger cable shafts, which is of high cost and the worst economy.
(2) Linking method, which leads the cable from the distribution room to the bottom distribution box, and then links the power supply from the bottom layer to the bottom layer, is the most economical, but the more layers, the lower the safety factor (the safety factor is multiplied step by step).
(3) Partition trunk line type, a high-rise building is divided into n unit areas, each unit is powered by the power distribution room through cables, and then distributed to each floor in the unit area. The economy is good and it is often adopted.
(4) Trunk cable branching method: one (or more) trunk cable is led out from the distribution room, and each layer is branched on the trunk cable. This method is the most economical, but the construction is the most troublesome. What's more troublesome is that when making floor branch heads on the main cable, the quality of the joints is uneven due to the influence of cable structure, site construction conditions and personnel quality, but this method has prompted people to think of making the joints together with cables, thus giving birth to a new generation.
Branch cable is a specially treated single-core power cable, which is used as the main cable of the building. According to the structural characteristics and dimensions of each specific building, branch connectors are designed and manufactured in advance together with branch lines and trunk cables. The site construction and management in the above method (4) are completed by professional manufacturers, and the consistency of process also brings the consistency of quality.
Branch cables appeared earlier in Britain and Japan. In terms of technical standards, Japan Electric Wire Industry Association issued the first industrial standard JCS 376 (1980) in1980. With the development and progress of technology, 1992 revised the standard, relaxed the requirements for structural materials of products, and improved the technical indicators of finished products. At present,
2. Branch cables can be divided into single-core type and multi-core stranded type in structure, and each single-core branch cable can be divided into three parts:
(1) backbone cable; (2) Branch cables; (3) Branch connector.
At present, single-core branch cable is widely used because of its simple structure and convenient production and construction. According to Japanese standards, multi-core branch cable is essentially a twisted body of multiple single-core cables, rather than the structure of traditional multi-core cables. Each conductor of multi-core branch cable has a separate insulation and sheath, and each core has a separate branch connector. Multi-core branch cable has the running performance of general multi-core cable, and only a few large-scale integrated cable factories in China have the production capacity, which has been popularized and applied at present.
3. Performance branch cable is a new type of distribution cable, and its key performance has two aspects: First, the branch cable with good quality must be a power cable with excellent performance. For domestic products, the conductor performance, insulation performance and mechanical and physical properties of materials should meet GB 12706-9 1 standard-cable performance is the basic index of branch cable products.
Secondly, the performance of branch connector is very important, which is the key performance of branch cable. The branch connector connects the conductors of the trunk cable and the branch cable into a whole, and makes insulation and moisture-proof treatment. You can't know the quality of internal joints from the appearance. There are two important tests that can detect the performance of joints, namely mechanical tensile test and electric heating cycle test. For mechanical tensile test, the breaking force of branch connectors (including trunk lines and branch lines) should be kept above 80% before connection. During the electrothermal cycle test, after the rated load of 125 and the no-load cycle at a certain time interval, the temperature of the branch joint should not be higher than 8℃ of the cable surface temperature. The key to determine the mechanical and electrical properties of the branch connector lies in the material and technology of the branch connector. For the vast number of users, we should pay full attention to the cable quality of branch cables, the material selection of joints, production technology and tooling.
We say that branch cables are more suitable for the distribution system of modern buildings. Why? To analyze this problem, we must first understand the requirements of distribution lines in relevant electrical design codes.
Second, the design requirements of relevant specifications for distribution lines of building electrical system
1, design specifications related to building electricity At present, the specifications related to the design of low-voltage distribution system of building electricity mainly include:
(1)GB50052- 1995 Code for Design of Power Supply and Distribution
(2)GB50054- 1995 Code for Design of Low Voltage Distribution
(3) JJ/T16-92 Code for Electrical Design of Civil Buildings
(4) Code for Fire Protection Design of Buildings GBJ16-87 (version1997)
(5)GB50045- 1995 Code for Fire Protection Design of High-rise Civil Buildings, in which Code for Design of Power Supply and Distribution System and Code for Design of Low-voltage Distribution are two basic codes, and the main contents refer to IEC standards. The power supply system and low-voltage distribution in Code for Electrical Design of Civil Buildings are basically consistent with their provisions. However, as this is a professional standard in the construction industry, some provisions related to buildings are more specific, such as the simple load grade of power supply system, which not only stipulates the classification principle, but also stipulates the load grades of various specific building names.
Because there is no precedent for the application of branch cable products in China when the above specifications are promulgated and implemented, branch cables are not mentioned in the specifications, but the design direction is reflected in many clauses. Generally speaking, there are three views:
1, about distribution series-the less the better;
2. Regarding the power distribution mode, the order from high to low is radial >: trunk line > district trunk line > link type;
3. Installation and laying methods should adapt to the environment, architectural features, electromechanical stress and other factors.
(1) As for the distribution series, clause 3.07 of GB50052-95 stipulates that the power supply system should be simple *, and the distribution series of the same voltage power supply system should not exceed two levels. Article 8 of JJ/T16-92 Code for Electrical Design of Civil Buildings stipulates: "From primary side of transformer to service. The above specification embodies an essential point, that is, the fewer distribution series, the better, the lower the reliability and the more advanced the technology.
(2) Regarding the power distribution mode, Articles 6.02, 6.03, 6.04 and 6.05 of GB50052-95 put forward: "In a factory or building with normal environment, when most electrical equipment is of medium or small capacity, but there is no special requirement, trunk power distribution should be adopted", "When the capacity of electrical equipment is large, or the load nature is important, or there are special requirements." When some electrical equipment is far away from the power supply point, but close to each other, the electrical equipment with small capacity can adopt interlocking distribution, but each loop of interlocking equipment should not exceed 5 sets, and its total capacity should not exceed 10kW ":"In a high-rise building, when supplying power to the distribution points in the floor, it is appropriate to adopt the district trunk line distribution, but some concentrated loads or important loads with large capacity should be distributed radially from the low-voltage distribution room.
(3) The Code for Electrical Design of Civil Buildings /T 16-92 has more detailed provisions on power distribution methods, such as: "8.2. 15 High-rise buildings in residential quarters should adopt radial power distribution" and "Other civil buildings in 8.2.2.4, except multi-storey houses, should adopt radial power distribution from the power distribution room for larger concentrated loads or more important loads. For the distribution of multi-storey distribution rooms or distribution boxes, trunk and sub-district trunk methods should be adopted. " From the interlayer distribution room or the distribution box to the distribution box, the combination of radial and trunk distribution should be adopted. "For concentrated loads or important loads with large capacity, 8.2.3.2 should adopt one of the following methods to distribute power to distribution rooms at all levels through radial distribution:
(1) The working power supply adopts the partition trunk type, and the standby power supply also adopts the partition trunk type or the vertical trunk type from the first floor to the top floor.
(2) The working power supply and multi-purpose power supply adopt the vertical trunk line mode from the first floor to the top floor.
(3) The working power supply is divided into main lines, and the multi-purpose power supply is taken from main lines such as emergency lighting.
The above provisions are limited to the formulation of specifications. Time-division branch cables have not been popularized and applied in China, and power supply lines mainly rely on ordinary power cables and buses. The author believes that the above provisions should be simplified after the application of branch cable wiring. Radial view is higher than trunk view and link view.
(4) Installation and laying methods of cables and buses.
5.5. 1 in GB50054-94 and 9. 12. 1 in JJ/T 16-92 all stipulate that "the enclosed bus should be used in dry and corrosion-free indoor places".
5.7.2 in GB50054-94 and 4. 13.5 in JGJ/T/kloc-0-92 stipulate that "the following factors should be considered in vertical wiring of shafts: .... Connection method of vertical trunk line and branch line. "
In 5.7.3 of GB50054-94, when a large-capacity single-core cable is used as a trunk line for vertical wiring in a shaft, the following conditions shall be met:
1, bearing capacity should have a certain margin; 2. The installation and maintenance are convenient and economical.
GBJ 16-87 Code for Fire Protection of Building Design (10.10.4) stipulates: "The distribution line of fire-fighting electrical equipment shall be protected by penetrating pipes. When concealed, it should be laid in a non-combustible structure, and the thickness of its protective layer should not be less than 3cm. When exposed, you must wear a metal tube and take fire prevention measures. When using cables with discontinuous combustible insulation and sheath, it is not necessary to wear metal pipes for protection, but should be laid in cable trench.
In GB50045-95 Code for Fire Protection Design of High-rise Civil Buildings, 9. 1.4 also stipulates the same content for fire power supply and its distribution.
The above specifications show that cable wiring has better environmental adaptability and more convenient installation and laying than bus.
After being familiar with the relevant provisions of the electrical code, let's analyze the compliance and technical advancement of the branch cable wiring mode and code.
Third, the technical progress of branch cable wiring
1. Distribution method of branch cables The branch cable distribution system is generally shown in the figure. In an N-storey building, the power supply of the main shaft and each floor is completed by prefabricated branch cables. PG is the main distribution cabinet, PX is the floor distribution box, and ZJX is the transfer box. When the distance between PG and ZJX is not far, it is generally not selected (to meet the requirements of current carrying capacity and start-up pressure drop), which can reduce one connection point and save money.
2. Technical progress of branch cable wiring From the analysis of the above wiring system, we can know that branch cable can simplify floor wiring into secondary wiring, and each floor can realize the simplest secondary wiring, which is in line with the principle of minimum wiring series in the specification, which is one of the progress.
The essence of branch cable distribution system is radial distribution system, which is suitable for all kinds of important occasions, even especially important occasions. This is the second advanced.
Branch cable is a kind of prefabricated power cable, its shape and structural characteristics still have the characteristics of cable, its joints have been sealed and insulated, and the withstand voltage and insulation resistance tests have been carried out in water before leaving the factory. Therefore, the requirements for the environment are low, and it can be applied to humid, salt spray and acid-base environments, but the bus in the specification is clearly not applicable to these environments, and its application scope is wider than that of the bus. Moreover, its installation mode is simple, the construction period is short, and the labor cost is low, which conforms to the viewpoint that the design should pay attention to economy in the specification and is the third technological progress.
Four. Matters needing attention in branch cable wiring design
We have analyzed that the branch cable distribution system is technologically advanced. It can be said that branch cable is a professional product tailored for modern architecture, which has the best applicability and technical economy. However, in engineering design, we need to pay attention to one thing-that is, the protection of branch lines. Yes, because the cross section of the branch line is generally smaller than that of the main line, when the branch line is overloaded or short-circuited, the main line protection system will generally not act on it. Therefore, a protector must be set in the branch distribution box, and the problem between the protector and the branch connector shall not exceed 3m. If it is exceeded, the branch line must be laid in a nonflammable pipeline or groove, and the main line protection should be able to be disconnected when a single-phase or two-phase short circuit occurs in this section. Pay attention to this.
Branch cable, as a new type of building distribution cable introduced from abroad, has been widely used in many domestic projects and has been recognized and used by designers. This paper aims to explain the consistency between branch cable wiring and current building electrical code, and better reflect the guiding ideology of the code. This is the most advanced and economical distribution mode, which can meet the existing specifications. Due to the limited level, please correct me if there are any mistakes in the article.
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd