1 Problems of welding connection
Except the Atlas of Building Electrical Installation Engineering published in 1980s [1]? In the first volume of data, apart from the description of the wire connection technology, there is no clear stipulation on the thin wire connection technology in building electrical engineering in the current domestic construction or acceptance specifications, but the practice of splicing and tin-plated insulation tape coating seems to have become a conventional or self-evident standard practice, and it has been widely adopted by the majority of engineering construction units. However, there are the following problems when conducting wire connection by welding process:
(1) is inconsistent with Article 526 of GB 16895.6-2000 (idt is equivalent to IEC 60364-5-52: 1993 "Building Electrical Equipment Part 5: Selection and Installation of Electrical Equipment Chapter 52: Wiring System") [2] Except for communication lines, welding should be avoided;
(2) Need auxiliary materials and tools (soldering iron, tin cans, soldering tin, solder resist, insulating tape, etc.). );
(3) The site heating conditions (naked flame, electricity) are required;
(4) Welding fume pollutes the working environment;
(5) Affected by the operator's skills and construction conditions, the contact quality consistency is poor, and there are hidden dangers of virtual welding and false welding;
(6) The conductor insulation layer is damaged by welding high temperature;
(7) Operation difficulty and danger caused by contact position (for example, operation of holding molten solder high).
In addition to technical factors, from the perspective of economic benefits, the biggest problems of brazing process are: low working efficiency and high man-hour cost, which directly affect the engineering cost and the profits of construction enterprises. If the conductor is twisted and not welded in the connection link due to time limitation and convenience, there will be potential safety hazards (it is not easy to check after the insulation tape is wrapped, and it is impossible to judge whether welding has been carried out).
Two-wire connector
The history of using wire connectors in European and American countries can be traced back to the 1920s. The use of wire connectors not only realizes highly reliable electrical connection, but also makes the installation process very fast and efficient because it can be completely operated by hand without special tools, and the average time for each electrical connection is only 10s, which is one third of the traditional welding process. By the 1940s, wire connectors had completely replaced the welding tape technology and were widely used in building electrical engineering.
2. 1 Classification of wire connectors
Wire connectors used in electrical engineering of civil buildings belong to low-voltage circuit connecting devices for household and similar purposes. According to GB 1 3140.1-2008/iec 60998-1:2002 "low-voltage circuit connecting devices for household and similar purposes-part1:general requirements" [
(1) threaded connector
It conforms to GB13140.2-2008/IEC 60998-2-1:2002 "Connecting devices for low-voltage circuits for household and similar purposes Part 2: Special requirements for connecting devices with threaded clamps as independent units" [4]. Select the appropriate bushing (core wire) according to the wire diameter (as shown in step 1 in figure 1), while tightening the screw to generate pressure, squeeze a plurality of wires into the bushing, and install the insulating sleeve (also using threaded connection) to form a complete electrical connection (as shown in step 2 in figure 1).
The threaded joint in Figure 1 needs to be specially pointed out: the threaded joint is not suitable for directly connecting the welded multi-strand flexible wires. Due to thermal expansion and cold contraction and the creep characteristics of solder (the thermal expansion coefficient of Sn-Pb alloy is -25, which is17 times that of Cu-1.47 times), there will be a slight gap between the clip of threaded joint and the end of multi-strand wire of solder. The clip of threaded joint can't compensate this gap, which will lead to poor contact. Therefore, the note in article 7. 1. 1 in GB/t1iec 60947-7-1copper conductor terminal block for low-voltage switchgear and control equipment auxiliary equipment [5] stipulates that the threaded clamp is not suitable for flexible wires with soldering ends. Gb 2099. 1/iec 60884-1,E3. 1 plugs and sockets for household and similar purposes part1:general requirements [6] part12.1./kloc-
(2) The non-threaded connector conforms to GB13140.3-2008/IEC 60998-2-2: 2002 "Connecting devices for household and similar low-voltage circuits Part 2: Special requirements for connecting devices with non-threaded clamps as independent units" [7]. Through the unthreaded clamping part (as shown in Figure 2) composed of reed, spring or cam mechanism, contact force is generated on the connected wires to realize electrical connection.
This kind of connector is also called plug-in connector or push-wire connector (as shown in Figure 3). According to the connection ability of connecting wires (the number of connecting wires at the same time), the threadless connector can be divided into various specifications such as 2 holes to 8 holes, and can also be divided into parallel insertion and reverse insertion.
It should be noted that the unthreaded wire connector itself participates in conduction, so it has the limitation of current carrying capacity. Pay attention to matching the connected wires when selecting, otherwise it may be damaged due to overload.
(3) twisted connector
It complies with GB13140.5-2008/IEC 60998-2-4: 2004 "Connecting devices for household and similar low-voltage circuits Part 2: Special requirements for stranded connection13123" [8]. The contact force between conductors comes from the insulating shell and the embedded conical spiral steel wire. During the rotation of the connector, the edge line of the steel wire with square section will form a small gap on the surface of the conductor, which will distort the conductor (as shown in Figure 4), and at the same time, the conical spiral steel wire will expand and exert sufficient pressure on the conductor. This kind of connector is also called wiring cap, because its appearance and usage are similar to hat nuts in mechanical equipment.
The twisted connector is superior to the welding process in electrical continuity, mechanical strength and insulation protection, and it is completely operated by hand (if auxiliary tools such as nuts and sleeves are used, the working efficiency can be further improved), so the construction is convenient and efficient. In its 90-year development process (invention patent registration number 1920), it has gradually formed various forms to meet different needs (as shown in Figure 5).
Unlike threaded connectors and non-threaded connectors, stranded connectors only provide clamping force for wires and do not participate in conduction. Early twisted connectors were even made entirely of ceramics, so the connectors had no current-carrying index. When installing the twisted connector, it is not necessary to pre-twist the connected wires. Just align the insulated wires as required, install the connector and rotate it. When the external wires of the connector are also twisted, the reliable connection is completed.
The experimental results show that when two AWG 12 copper wires (with an equivalent cross-sectional area of 3.3mm2) are connected, the tensile strength of the twisted connector is 79.8kg, which is three times that of brazing (25.8kg). The contact resistance is 3.1mΩ, which is 22.5% lower than that of tin solder joint (4mΩ).
2.2 Comparison of characteristics of different connectors
The following table shows the horizontal comparison of different connectors. Because of its outstanding advantages, twisted and untwisted connectors have become the most widely used and widely used wire connection devices in low-voltage distribution systems and building electrical equipment all over the world.
3 About the safety line pressing cap
The selection data and usage of safety crimping cap (commonly known as nipple, as shown in Figure 6) are introduced in the Atlas of Building Electrical Installation Engineering, which is also used in building electrical engineering at present. However, except GB 13 140, there is no product standard of this kind of wiring device in other Chinese national standards and IEC standards, and only this kind of connector is involved in UL486C[9] of the United States.
In addition to the fact that the products can't be tested according to national or international standards, the gland still has the following problems to pay attention to:
(1) needs to be installed with special tools to ensure the consistency between the compaction force and the finished product, otherwise it is easy to be compacted and fall off;
(2) It is suitable for positioning and assembly of household appliances, lamps and other production lines (it is no longer used in the production of lamps and lanterns in the United States due to changes in electrical specifications);
(3) Once used, once installed, it cannot be disassembled, so it does not meet the maintenance requirements;
(4) the work efficiency is not high. Therefore, the author does not recommend using this connector in domestic building electrical engineering.
4 Typical application
4. 1 Conductor connection, T-connection and branch connection
In the process of conductor laying, conductor joints are widely used in the connection, branch and T-connection of low-voltage distribution branch (terminal) lines of building electricity (as shown in Figure 7). In particular, when the junction box is set in the ceiling and other high places, the use of wire joints instead of welding can effectively reduce the danger caused by the use of high-temperature welding in the construction process, improve work efficiency and reduce labor intensity.
4.2 Socket wiring
When installing the socket, if the PE line is transmitted through the socket terminal, poor contact will occur, and the downstream socket at the fault point will lose the protection of the PE line. In order to avoid such security risks, the usual practice is to adopt claw-shaped (commonly known as chicken claw) wiring technology (as shown in Figure 8a). Using wire connector instead of welding achieves the purpose of safety, high efficiency, space saving and easy maintenance. Considering the same safety, reliability and maintainability, it is suggested that the L line and N line of the socket adopt the same process connection (as shown in Figure 8b).
4.3 Electrical Equipment Installation
Wire connectors are widely used in wiring links of lamps, ceiling fans and other electrical equipment (as shown in Figure 9). In order to facilitate the installation of users, many European and American brands of lamps use wire connectors as standard accessories in lamps, and users do not need to buy them separately.
5 concluding remarks
China's first atomic bomb exploded (196410/0 16) 20 years later than the United States (1945 July 16), and China launched its first manned space flight (/kloc-0). In contrast, China lags behind the United States for at least 70 years in low-tech building wire connection technology.
In order to change this situation as soon as possible, the new version of "Code for Acceptance of Construction Quality of Building Electrical Engineering" GB50303 will put forward requirements for the application of conductor joints; CAS2 12-20 13 "Code for Design, Construction and Acceptance of Electrical and Intelligent Systems in Residential Decoration Engineering" compiled by China Standardization Association puts forward requirements for the use of wire connectors in home decoration engineering. China Engineering Construction Standardization Association is compiling the Technical Specification for the Application of Fine Wire Connectors in Building Electricity, which will face front-line engineers and put forward more operable requirements for the selection, use and quality inspection of wire connectors. It is believed that in the near future, wire connectors will play their due role in building electrical engineering in China.
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