Several Technical Recommendations for Busway Safety Assurance
With the rapid development of our country's economy and modernization, people's awareness of the safety of electricity is getting stronger and stronger, and the busway instead of wires and cables has been more popular in developed countries, and the high-current trunk line from the transformer to the distribution cabinet and the distribution cabinet outlet in our country is also commonly used busway, because the busway is in terms of safety, environmental protection, service life and practicality in the cable, so at the time of design, the busway has been a product of the power trunk transmission equipment. However, if the safety and technical issues of the busway are not paid attention to when designing and selecting the busway, it will cause accidents such as short circuits in the busway. I have been engaged in busway installation for many years, and now I will briefly talk about some safety technologies of busway for your reference:
1. Busway protection level and use environment table:
The protection level of the busway shell mainly refers to the processing accuracy of the busbar, and the sealing measures are an important indicator to prevent people or animals from directly touching the live equipment, and to prevent foreign objects and water from entering the rebusway and affecting the safety of the equipment.
In general, most of the electrical accidents caused by busways are caused by the installation of connectors and the protection level of protection that do not meet the requirements. The protection level of the busway refers not only to the protection level of the busbar body, but also to the protection level of the installation connector. At present, many busways on the market are marked with IP54, IP65, IP66 protection levels, and the protection level of the bus body can meet their requirements, but the protection level of the connector can only reach IP30 or IP40. Therefore, the design and processing of the connector is a very critical factor, if the connector is not sealed well, no matter how high the protection level of the busbar body is, it cannot truly effectively protect the safe operation of the busway. The reason for the negligence is that the electrical IEC60439.2 and the national standard GB7251.2 busway protection level test standard only requires that the body for inspection does not need to send the connector for inspection, which is the safe and stable operation of the busway.
When choosing a busway, the manufacturer must be required to send samples, and the sample must be a busway with a plug-in interface body and two or more units connected to the connector. Through the investigation of the protection performance of the busbar body, plug-in interface, and connector, it is possible to determine whether the protection level of the busway meets the design requirements. A single busbar sample or a busbar sample with a connector cannot determine whether the waterproof and dustproof performance of the connector meets the requirements of the protection level, and the importance of the protection level of the connector is worthy of attention when choosing a busway.
Many projects do not indicate the protection level of the busway shell during the engineering design, and the owner does not put forward specific requirements, resulting in many projects choosing IP30 or IP40 busway due to too much consideration of the project cost. In the process of use, due to the accumulation of dust, coupled with the condensation generated on the surface by the change of humid climate and the temperature difference between day and night, or the electrical short circuit caused by water ingress in the equipment, this kind of accident occurs particularly in Shanghai. When selecting busways, the method is implemented to ensure the safety of the busway. Be sure to choose the appropriate protection level according to the requirements of the use environment.
Selection of busway protection level (see Table 1) Suggestion: In the distribution room, the protection measures are better, and the protection level of the busway from the compressor to the distribution cabinet can be IP30 or IP40; The outlet busway of the distribution cabinet should be IP54; In the basement or horizontally installed busway, due to fire spraying and vehicle entry and exit, the environment is humid, there are more dust particles, coupled with fire water or regular fire testing, the busway has more opportunities to be damp or watered, which seriously affects the safe operation and service life of the busway, so horizontal installation and outdoor installation should use a busway with a protection level ≥IP65; IP68 should be used for buried installation or installation in cable trenches; The busway installed in the vertical hoistway is closed and has good protection performance, and the busway with a protection level of ≥ IP40 can meet the requirements.
2. Insulating medium and insulation resistance:
Insulation performance is another important indicator to ensure the safe operation of the busway. The factors affecting the insulation performance are: the insulation material that wraps the conductor, the insulating parts used for installation, the insulation material of the connecting parts and the insulation material of the plug-in switch box, and the insulation material can ensure the high insulation performance of the busway.
There are a variety of busway insulation materials available on the market, because the busway standard only requires insulation resistance, which must be greater than 20MQ, but there is no clear requirement for the material or grade of the insulation material. Even some busway bodies have good insulation performance, poor insulation materials of connectors and plug connectors, and the insulation resistance is lower than the national standard after installation, and short circuit accidents are prone to occur after a period of operation. It is recommended that the insulation medium of the busway be flame-retardant, with a temperature resistance of more than 130 °C and an insulation resistance of ≥ 20MQ after installation. Electrical fires caused by poor busway connection contact or arcing caused by overload or short circuit, and then igniting insulation materials caused by arcing, such accidents are not uncommon around us.
3. Copper-aluminum transition technology of aluminum conductor busway:
The price of copper materials has increased several times since 2005, and the price of aluminum conductor busways is only less than half of that of copper conductor busways, and the cost is more than half less than that of copper cables.
In developed countries, aluminum instead of copper as a busbar conductor has accounted for a large proportion, in our country has begun to use a large number of aluminum instead of copper as conductors, in actual use of its stability but the selection of materials is larger than copper, but important safety technology should be worthy of everyone's attention, the busbar inlet is copper bar-type copper cable, the outlet line and the plug-in box, the switch box is connected to the copper prongs, the copper and aluminum transition technology in these two places must be properly solved, if not properly handled may occur within 3-5 years of oxidation. Therefore, when choosing aluminum conductor busways, it is necessary to clarify the manufacturer's copper-aluminum transition technology treatment, so that the selected aluminum conductor busway is safe and reliable in the future use process.
4. Protection system after busway displacement:
There are multiple branch plug-in boxes for power supply output in a busway, and the trunk current of the bus of each branch plug-in box is reduced, so people often adopt mutation and displacement to reduce the cost of the bus. If an automatic air switch is used after the transformation, it is often inconvenient to operate during use, and some places have small space and cannot be installed, and their cost is also high. If the switch is not installed, then the bus overload after the displacement is not protected, it is recommended that the load of the busway branch is reduced to reduce the cost, and the connector after the displacement is set with a busbar protector MCPP-2, which detects the operating temperature of the busway. If the over-temperature alarm and over-limit temperature exceed the limit, the main power switch will be cut off to ensure the safe operation of the busbar, or you can also set up 2 thermostats with a control box to ensure the safety of the busbar.
5. The plug-in box should be equipped with an interlocking device
The plug-in switch box should be equipped with a mechanical interlock device, and when the switch is in the closed position, the plug-in box cannot be plugged and unplugged, so as to prevent safety accidents caused by arcing when plugging and unplugging with load.
Convenient tap is a major advantage of busway, and the plug-in switch box can be plugged and unplugged in the case of uninterrupted power supply of the system, making maintenance, branch road modification, and capacity increase more convenient. However, if the plug-in box is plugged and unplugged with a load, an arc will be generated at the moment of disconnection or contact, and the larger the rated current of the switch, the larger the arc will be, the more difficult it will be to extinguish the arc. Strong arcing can cause electrical fire or even serious personal injury or death when severe, so the use of mechanical interlocking to prevent loaded plugging and unplugging is an important measure related to safety.
The plug-in switch box is equipped with a mechanical interlock device, and the circuit breaker of the plug-in box can only be inserted or pulled out when it is opened, and the plug-in box can only operate the circuit breaker by closing the box door after inserting into the busway plug-in interface, so that the circuit breaker can be closed. When the circuit breaker is in the closed position, the plug-in box door has been automatically locked and cannot be opened, thus eliminating electrical accidents caused by arcing when plugging and unplugging with load.
6. Calculation of conductor and ampacity:
At present, most users still have the wrong theorem that the ampacity is calculated according to the cross-sectional area of 2A per square millimeter, that is, "the cross-sectional area S (nun2) X2 (A/ nun2) = ampacity (A). The same cross-sectional area has different ampacity if the product design structure, conductor row specifications, and conductor emissions are different, such as the same cross-sectional area: 6X 100=600 nur, 10X 60=600 nunf, the former is about 19% larger than the latter, and there is also a difference between flat and vertical placement, a difference of 5% to 10%, different structural designs, and the quality of heat dissipation also directly affects the ampacity of the conductor, and the temperature of the busbar part is 4%-6% for every 5"C difference. The resistivity is also related to the ampacity, so it is unreliable to confirm only the cross-sectional area when selecting the busway.
It is recommended that the current reference material for busbar conductors is only the conductor selection table in the electrician's manual or design manual, which is only suitable for places without enclosed space, and for busway because of its insulation wrapping and shell closure, the ampacity should be discounted by 10% to 30%, which is related to the structural design and the resistivity of the conductor.
