With the rapid and healthy development of China's social economy and modernization construction, the electricity load is increasing. In recent years, replacing cables with bus ducts has become a common phenomenon in developed countries and has formed a trend of directional development in China. However, because some designers, users, and quality inspectors do not have a deep understanding and recognition of the most important safety technical parameters of bus ducts? Limiting temperature rise, causing safety hazards, and wasting engineering investment. Let's talk about the limitations of bus ducts. Several issues related to temperature rise.

More than 60% of fire safety accidents in Chinese enterprises are caused by electricity. The perpetrators of social fire risk accidents caused by electricity include cables, wires, high and low voltage complete technical equipment, transformers, bus ducts, electrical control components, etc. Most of them are caused by long-term temperature rise and heating in China, which leads to aging and short circuit of insulation structural materials, resulting in fire accidents. The standard term for thermal testing is extreme temperature rise.

Therefore, in order to ensure the safe production and operation of the power supply management system, as well as energy conservation and emission reduction, the maximum temperature rise of the bus duct is the main technical parameter for performance evaluation of CESKO bus duct product information, which is sufficient to attract the attention of the design and supervision, the construction unit of Party A, and the acceptance unit.

1、 Why does temperature rise determine the current carrying capacity of bus ducts

The low-voltage system transmission main line mainly includes cables, busbars, bare conductive bars, and through cables through wires, cables, and branches. Due to the different heat dissipation of various products, the current carrying capacity per square millimeter also varies: the same product, the same conductor size, and the same current passing through it result in different temperature rises; The temperature rise of the same conductor varies due to different design structures. Of course, as the ambient temperature continues to rise, the resistance value can increase, the voltage drop also increases, and the power quality loss also increases. For example, a 35mm2 wire can have a low temperature rise when passing through a current of 80A, and meets national standards when passing through a current of 100A. If a current of 120A or 150A can be passed, the temperature rise will exceed the standard, and the insulation structure material will quickly age, ultimately leading to various short circuit accidents. If a 35mm2 wire passes 100A current, the current flowing through a 6mm2 wire is 2.8 A/mm2. If a 6mm2 wire passes 38A current, the current flowing through a 6mm2 wire is 6.3 A/mm2. If the current passing through a 6mm2 wire is less than 2.8 A/mm2, the current flowing through a 6mm2 wire is 18A. Due to the decrease in conductor temperature rise and electrical loss, the voltage drop and electrical loss are significantly less than 35mm2. The same goes for bus ducts, so calculating the conductivity of bus duct conductors based on the conductivity per square millimeter (current density) is incorrect. However, for different designs, factors such as structure and heat dissipation, skin effect, impedance, inductance, etc. are closely related to the development of current carrying capacity. Therefore, the national standard GB7251-2006 (equivalent to the electrical standard IEC60439.2-2000) stipulates that the current carrying capacity is determined by the rated current that rises through the limit temperature.

2、 Standard temperature rise requirements for busbar trunking:

Electrical Technical Standard IEC60439.2? 2000 is the same as the national standard GB7251.2-2006: the temperature rise of the bus duct is determined by the heat resistance level of the insulation material, and the allowable temperature rise is determined. If the insulation structure material of the bus duct used is Class F, its heat resistance is ≥ 155 ℃. Under the conditions of the surrounding living environment, its allowable temperature rise is 115K (155 ℃ minus the enterprise environment temperature of 40 ℃). Therefore, the current carrying capacity of the bus duct can only be finally determined after conducting full load production tests, and the ultimate temperature rise is the most critical technical parameter of the bus duct. The national mandatory 3C certification system testing technical standard, with a maximum temperature rise of ≤ 70K for bus ducts, is a reasonable standard for safety management.

3、 Temperature rise involves busbar trunking issues:

Bus ducts are like wires and cables, and are therefore also used as mainline equipment for power transmission. The same 35mm2 wire can carry a rated current of 80A or a rated current of 12. The difference is that the rated working current of 80A and the temperature rise of 12 are completely different. The bus duct is the same. When the maximum temperature rise is 70K and 90K, the current carrying capacity of the same bus duct differs by more than 15%. At present, there are bus duct temperature rises of 55K, 70K, 90K, 100K, and even higher in the market, but high temperature rise involves the following issues. It is recommended that users choose bus ducts with a maximum temperature rise of ≤ 70K or ≤ 55K.

The increase in temperature directly reflects the increase in power loss.

3.2 The higher the temperature rise, the faster the aging rate of insulation structural materials, and the service life of the busbar trunking network is rapidly shortened.

3.3 Temperature rise can accelerate the aging of surrounding insulation materials and equipment (such as wires and cables adjacent to or transferred to bus ducts; or electrical insulation supports, etc.), and may even lead to fire accidents.

3.4 The internal temperature of the bus duct increases, leading to an increase in voltage drop.

3.5 An increase in temperature will reduce the mechanical strength of the bus duct. After being heated, the stress of metal conductors begins to relax, and the mechanical design strength can be reduced;

3.6 The safety factor is reduced, and the shell is prone to scalding and injuring people at high temperatures.

3.7 The increase in temperature has a significant impact on the ambient temperature.

4、 The origin of temperature rise:

4.1 Copper bars have low copper content and high electrical resistivity.

People often need to mention the copper content and resistivity of copper bars, which are indeed related to the current carrying capacity of the busbar. Copper bars with a copper content of 99.95% or ≥ 99.93% and a resistivity of ρ ≤ 0.01777 (ohms · square up to millimeters/meter) belong to busbar copper bars. Copper bars. If the copper content is low, the resistivity will be high, and the current carrying capacity and temperature rise can only be ensured by increasing the conductor size. Otherwise, the temperature rise will be too high.

4.2 Structural design of insulation materials and shells for heat dissipation differences.

Bus duct conductors with better structure, insulation performance, and heat dissipation performance can meet the current carrying requirements of design manuals or electrician manuals after discounts. But for some products, insulation materials such as cast resin or other insulation materials with poor heat dissipation, air type bus bar structures, and dense bus bar structures with poor heat dissipation will have more discounts. Some product structures and insulation materials have poor heat dissipation. The conductor was selected based on the 30 ℃ ambient temperature in the electrical manual, which misled the user. It is understood that some of these products can only achieve 60% to 70% of the current interception capability, causing serious problems in China. Power supply. Its potential safety hazards and huge power losses deserve attention.

4.3 Overload operation.

Some engineering technologies increase their load with the increase of network equipment, or the originally designed busbar cannot meet the needs of the construction site. Some projects use variable capacity during ordering and construction without taking effective protective measures. During overload operation, the ambient temperature rises., Moreover, the switch at the beginning of the variable capacity cannot guarantee the overload of small current after the variable capacity, so there are potential information security risks for enterprises.

4.4 Unstable connector connection leads to increased electrical resistivity of the connector.

An unstable connector connection, poor contact with society, and increased electrical resistivity can all affect the temperature rise of the bus duct.

4.5 Temperature rise is related to skin effects.

Inside the conductor, the heat generated by resistance is difficult to dissipate, resulting in high temperature, high cost and speed of electrons, and uneven circuit, leading to relatively narrow electronic circuits. Path and high resistance. On the surface of a conductor, heat dissipation is fast, temperature is low, the price and speed of electrons are low, and the circuit is flat, resulting in a relatively wide electronic path. Therefore, the surface resistance of conductors is small, and electrons run faster, which is also the reason for the current skin. One.

For example, the cross-sectional area of the 6 × 100 and 10 × 60 bus conductors is also 600mm2, but the former is 19% larger than the latter. This is the effect caused by the skin effect of current. By the same current, the former is larger, while the latter has lower operating temperature rise, lower electrical loss, and smaller voltage drop. That is to say, at the same temperature rise, the current carrying capacity of the latter is 19% less than that of the former. It can be seen that unilaterally determining the current carrying capacity and electrical energy loss of a conductor based on its cross-sectional area is completely incorrect.

4.6 Misleading conductor calculations:

Some technicians, regardless of the structure of the bus duct, calculate the specifications of the conductors used in the "Electrical Manual" (or "Electrical Design Manual") list and infer the service life of the bus duct based on the current carrying capacity per square millimeter This is incorrect. The conductor is made of copper or aluminum, and the service life of the bus duct depends on its operating temperature. The working environment temperature is increasing, and its aging development rate is faster (including copper, aluminum rods, and insulation materials. Electrical copper and electrical aluminum used for passing conductors have a close relationship with temperature in terms of creep strength, tensile strength, and oxidation reaction rate development). Due to differences in the design and structural analysis of busbars, as well as variations in heat dissipation, the internal temperature control within the enterprise also varies. According to the conductor table in the design manual, the ambient temperature is selected as 35 ℃. A dense bus duct with better heat dissipation will reduce the current carrying capacity by 5% to 15% to meet the temperature rise of ≤ 70K. Dense busbars with poor heat dissipation should be reduced by 20%, and the current carrying capacity of air type busbars will be reduced even more. In summary, the current carrying capacity of the busbar does not differentiate between product structure and skin effect. It is determined based on the cross-sectional area of the conductor and the current carrying capacity per square millimeter. The ability to determine the size selection of conductors is incorrect. A dense bus duct with better heat dissipation will reduce the current carrying capacity by 5% to 15% to meet the temperature rise of ≤ 70K. Dense busbars with poor heat dissipation should be reduced by 20%, and the current carrying capacity of air type busbars will be reduced even more. In summary, the current carrying capacity of the busbar does not differentiate between product structure and skin effect. It is determined based on the cross-sectional area of the conductor and the current carrying capacity per square millimeter. The ability to determine the size selection of conductors is incorrect. A dense bus duct with better heat dissipation will reduce the current carrying capacity by 5% to 15% to meet the temperature rise of ≤ 70K. Dense busbars with poor heat dissipation should be reduced by 20%, and the current carrying capacity of air type busbars will be reduced even more. In summary, the current carrying capacity of the busbar does not differentiate between product structure and skin effect. It is determined based on the cross-sectional area of the conductor and the current carrying capacity per square millimeter. The ability to determine the size selection of conductors is incorrect. The current carrying capacity of the busbar does not differentiate between product structure and skin effect. It is determined based on the cross-sectional area of the conductor and the current carrying capacity per square millimeter. The ability to determine the size selection of conductors is incorrect. The current carrying capacity of the busbar does not differentiate between product structure and skin effect. It is determined based on the cross-sectional area of the conductor and the current carrying capacity per square millimeter. The ability to determine the size selection of conductors is incorrect.

5、 Requirements for temperature rise of conductors by relevant units

5.1 Design Institute Design and Temperature Rise

At present, most design institutes do not have temperature rise protocols during design, only rated current, three-phase four wire or three-phase five wire. This is a relatively universal design. In the 115K project using F-grade insulation materials, if a bus duct with a current specification of 1000A and a temperature rise value ≤ 55K is used, a rated current mark can be added above 1600A, so it is important to agree on the temperature rise value. Bus duct during the design phase. of It is recommended that the maximum temperature rise of the bus duct be ≤ 70K or ≤ 55K, which is the basis of engineering quality. If the temperature rise of all power equipment in our country is controlled below 55K (1000V), not only will the line loss of electrical energy be greatly reduced, but also the fire accidents caused by electricity consumption will be reduced.

5.2 Temperature rise requirements for engineering construction supervision, quality inspection stations, and power system testing units.

Currently, most projects are unable to directly verify the current carrying capacity of bus ducts. According to our national safety standards GB7251.2 and IEC60439.2, the maximum temperature rise of the busbar trunking is determined by the heat resistance rating system of the insulation structure material, as long as environmental conditions permit, to determine whether temperature rise is allowed. The design drawings of both the design institute and the party A do not clearly specify the temperature rise value, making it impossible to determine the current carrying capacity of the bus duct. Moreover, there is no agreed maximum temperature rise between the user and the enterprise, so it is impossible to determine how much K the temperature rise of the bus duct is required to be considered a qualified product for engineering use. It is recommended that construction supervision and quality inspection stations, as well as power system acceptance personnel, check the 3C certificate to ensure that the maximum temperature rise of various current conductor specifications in the 3C testing analysis report is consistent with the product. At the same time, use a wire tester to measure the conductivity of the wire, calculate the copper content and resistivity, and check whether the 3C certificate and technical parameters are consistent with the test report at the certification center or laboratory. Ensure the current carrying capacity and low-temperature system operation of the production bus duct. At the same time, use a wire tester to measure the conductivity of the wire, calculate the copper content and resistivity, and check whether the 3C certificate and technical parameters are consistent with the test report at the certification center or laboratory. Ensure the current carrying capacity and low-temperature system operation of the production bus duct. At the same time, use a wire tester to measure the conductivity of the wire, calculate the copper content and resistivity, and check whether the 3C certificate and technical parameters are consistent with the test report at the certification center or laboratory. Ensure the current carrying capacity and low-temperature system operation of the production bus duct.

5.3 The country conducts mandatory 3C certification to verify the use of busbar trunking with maximum temperature rise.

The mandatory 3C certification implemented in our country has verified the maximum temperature rise of the busbar. Except for special fire-resistant busbar products, all other busbars are uniformly tested according to the ≤ 70K temperature rise test standard, but currently there are many busbar specifications. Each product testing and certification fee for 3C certification requires tens of thousands of yuan. Therefore, in order to reduce the burden on enterprises, the certification service center divides each unit based on the short-circuit withstand strength. We can cover several rated values and current specifications for each unit. The maximum current specification for each unit is now standardized for testing, while other specifications are calculated by the enterprise and reviewed by the laboratory. The conductor specifications for calculating current within the current range covered by certification should be determined based on the current carrying standard per square millimeter of the test sample. If it is smaller than the test sample, it must undergo temperature rise commissioning testing, or if there are conductors of this specification in other units, it can be allowed, otherwise it is not allowed to pass, ensuring certification risk. But now some testing institutions have approved testing reports containing conductor specifications that are not proportional to the sample current and its current carrying capacity, which were filled out by the enterprise itself. For example, in the CCC type test report, the current covered by the 2500A unit is 2000A, 1600A, and 1250A, and the manufacturer uses a 2500A busbar to test the unit. The conductor size of sample 2500a is 6 × 205, with a maximum temperature rise of ≤ 70k. The conductor specification should have a current carrying capacity of 22.032 per millimeter. The recommended conductor size for 2000A is 6 × 165, but the product description is written by the manufacturer as 6 × 125. Obviously, it can only increase the temperature rise. The current carrying capacity reaches 2000A, making it difficult for the certification center to control the specifications of the covered current conductors. When the current carrying capacity per square millimeter covered by certain certifications differs significantly from the current carrying capacity per square millimeter of the certification test sample, it is worth considering. The conductor size of sample 2500a is 6 × 205, with a maximum temperature rise of ≤ 70k. The conductor specification should have a current carrying capacity of 22.032 per millimeter. The recommended conductor size for 2000A is 6 × 165, but the product description is written by the manufacturer as 6 × 125. Obviously, it can only increase the temperature rise. The current carrying capacity reaches 2000A, making it difficult for the certification center to control the specifications of the covered current conductors. When the current carrying capacity per square millimeter covered by certain certifications differs significantly from the current carrying capacity per square millimeter of the certification test sample, it is worth considering. The conductor size of sample 2500a is 6 × 205, with a maximum temperature rise of ≤ 70k. The conductor specification should have a current carrying capacity of 22.032 per mm. The recommended conductor size for 2000A is 6 × 165, but the product description is written by the manufacturer as 6 × 125. Obviously, it can only increase the temperature rise. The current carrying capacity reaches 2000A, making it difficult for the certification center to control the specifications of the covered current conductors. When the current carrying capacity per square millimeter covered by certain certifications differs significantly from the current carrying capacity per square millimeter of the certification test sample, it is worth considering. The conductor specification should have a current carrying capacity of 22.032 per millimeter, and the recommended conductor for 2000A should be 6 × 165, but the manufacturer states it as 6 × 125 in the product description. Obviously, it can only increase the temperature rise. The current carrying capacity reaches 2000A, making it difficult for the certification center to control the specifications of the covered current conductors. When the current carrying capacity per square millimeter covered by certain certifications differs significantly from the current carrying capacity per square millimeter of the certification test sample, it is worth considering. The conductor specification should have a current carrying capacity of 22.032 per millimeter, and the recommended conductor for 2000A should be 6 × 165, but the manufacturer states it as 6 × 125 in the product description. Obviously, it can only increase the temperature rise. The current carrying capacity reaches 2000A, making it difficult for the certification center to control the specifications of the covered current conductors. When the current carrying capacity per square millimeter covered by certain certifications differs significantly from the current carrying capacity per square millimeter of the certification test sample, it is worth considering. The current carrying capacity reaches 2000A, making it difficult for the certification center to control the specifications of the covered current conductors. When the current carrying capacity per square millimeter covered by certain certifications differs significantly from the current carrying capacity per square millimeter of the certification test sample, it is worth considering. The current carrying capacity reaches 2000A, making it difficult for the certification center to control the specifications of the covered current conductors. When the current carrying capacity per square millimeter covered by certain certifications differs significantly from the current carrying capacity per square millimeter of the certification test sample, it is worth considering.

6、 How to ensure the current carrying capacity and safe use of bus ducts.

The China Quality Certification Center and the 3C testing unit suggest that the certification test should be strictly controlled, and the safe, reliable and accurate data, such as the extreme temperature rise, specifications, conductor materials of the certified products, should be published on the Internet. It is convenient for users to inquire and ensure the life of Renmin University of China. Starting from the safety of property and the personal interests of users, mobilize the entire society to ensure the quality of information engineering construction.

6.1 designs. When designing bus ducts, the maximum temperature rise should be indicated on the drawings, and bus protection devices (or temperature controllers) should be installed for each current level according to the technical requirements or drawings. Design diagram for monitoring working temperature. Suggest setting it at each current level. At the current connector.

Note: The protection device has two signal output points, an over temperature alarm and a temperature limiting cutoff current, to ensure internal temperature rise during bus operation.

6.2 The Party A and the supervisor may inquire about the conductor specifications and temperature rise values in the 3C testing work report, or log in to the China Development Quality Management Certification Center to inquire about relevant information and technical parameters from other countries. The 3C certificate and some information technology parameters and announcements have been made, such as IP protection capability level, ICW=___ KA (short-circuit withstand current signal strength), rated working current specifications, etc., to ensure that the bus duct purchased by the enterprise is certified consistent with the bus duct.

6.3 Extreme temperature rise test.

6.3.1 To ensure that the purchased bus duct meets the full load conditions of low temperature, safety, and low loss operation, one method is to conduct an extreme temperature rise test.

6.3.2 The location for detecting extreme temperature rise is also crucial. Detect the temperature rise of incoming sections, conductors, plug-in interfaces through conductors, connectors, enclosures, etc. using bus ducts. After running at full load current, subtracting a stable maximum temperature from different ambient temperatures did not result in a temperature rise value, expressed in K. Other current specifications are calculated based on the current carrying capacity per mm2 that the sample passes through They are calculated from maximum current to low current. The current per square millimeter can be analyzed by density to pass through high currents. The same product structure has the same thickness as the copper bar. The low current bus conductor is very good.