Performance comparison between bus duct and cable
Traditional bus ducts are being challenged by cables, especially branch cables. Years of practical experience has proved that the two have their own strengths in product performance, application fields and economic costs, and cannot be simply concluded. This article makes a multi-faceted comparison of cables and busbars for reference. Cables have always played an important role in power transmission and distribution in low-voltage power distribution systems before the 1990s. However, since the 1990s, the busway system has been gradually introduced and developed, and has quickly been widely used in construction projects. Compared with traditional power transmission equipment, it has its unique performance advantages, such as large current carrying capacity, strong overload capacity, convenient tapping, good heat dissipation, and convenient maintenance.
1 Performance characteristics of ordinary bus duct and cable:
The so-called ordinary busway is to wrap several conductors in a metal shell through a reasonable and safe structure to form a whole power transmission and distribution system with electrical continuity. Ordinary bus ducts are generally divided into two types: dense insulation and air insulation. The air-insulated type has a simple structure and large transmission current; the densely insulated type has a compact structure and good heat dissipation capacity. They all have the characteristics of strong overload capacity, convenient tapping, and small footprint. Cables are divided into single-core cables and multi-core cables. Single-core cables mainly use various insulating materials, and multiple copper wires are wrapped in an insulating layer as a current conductor of a certain phase. A multi-core cable is a complete cable formed by twisting multiple insulated cores together and wrapping the outer sheath. The number of insulated cores of power cables is usually 1, 2, 3, 4, 5, etc., as well as 4+1 and 3+2 cores. The main advantage of the cable is that it is flexible and adaptable to the environment, but it also has its inherent limitations. The following is a performance comparison of bus ducts and cables from several main aspects. a) Current carrying capacity: The maximum cross-sectional area of the low-voltage cable is 1000mm2, and the rated current is 1600A. Because of its large size and weight, cables of this size are rarely used in actual projects.
Generally, cables of 400 mm2 and below are commonly used in engineering, which requires the use of multiple cables to achieve simultaneous power supply. The maximum rated current of the busway can reach 6300A, and its powerful current-carrying capacity is unmatched by cables.
b) Overload capacity: Regardless of the cable or bus duct, the overload capacity depends on the working temperature of the insulating material used. The working temperature of the insulating material of the busway is 105℃, and radiation cross-linked flame-retardant winding tape (PER) and radiation cross-linked polyhydrocarbon heat shrinkable tube with working temperature above 140℃ have been developed. At present, the normal working temperature of insulating materials used in cables is generally 90°C and 105°C, and the maximum working temperature of radiation cross-linked cables is 125°C. Therefore, the overload capacity of the bus duct is much greater than that of the cable.
c) Installation tapping: The busway generally adopts plug-in installation tapping method. The so-called plug-in bus duct is to use the plug-in method to branch the power supply of the main line to the branch line. There is a plug-in box opening every several meters, so the branching is very convenient. However, the cables need to be tapped on site, which has poor reliability, and even prefabricated branch cables have their drawbacks. The obvious shortcomings of branch cables are mainly the need to customize branch connection equipment from the factory, usually using open "C" type hoop. Over time, whether this kind of hoop can guarantee the tightness is also a question. In addition, the branch cable branch head is expensive, so the prefabricated branch cable is still not widely used. When installing branch cables, you need to cut off the power supply on the floor, and there is no need to cut off the power when installing the bus duct. Just remove the plug box of the bus duct under no-load conditions. But to cut off the branch power supply of the prefabricated branch cable, it is very dangerous to operate under the condition of electrification.
d) Fire resistance: The insulating layer and outer sheath of ordinary cables will burn. Even flame-retardant cables will burn under a flame, and only burn after the flame leaves. Fire-resistant cables will not burn, but they are expensive. Fire-resistant cables are only used where power outages are not allowed, such as fire alarm circuit systems. The outer shell of the busway is metal and will not burn. Even if the insulating material of the copper busbar is burned, the flame will not threaten the outside of the busway.
e) Heat dissipation performance: The insulation and outer sheath of the cable are both an insulating layer and a heat insulating layer. Therefore, in order to maintain the heat dissipation of the cable, when the cable is laid in the bridge, a maximum of 2 layers are allowed. The main reason is to consider the heat dissipation problem. The busway uses air conduction to dissipate heat and dissipates the heat through the tightly contacting steel shell, so its heat dissipation performance is much superior than that of the cable.
f) Operation and maintenance: The maintenance of the bus duct is relatively simple. During routine maintenance of the busway, it is usually to measure the temperature rise of the shell and the core bolts, and the temperature rise of the connector of the inlet box. The core bolts need to be tightened regularly if the 4.8 grade is adopted, and the 8.8 grade high-strength bolt is not required to be regularly tightened. Sturdy. However, due to the limitation of the materials such as easy abrasion, easy aging, short life, etc., cables need to be inspected and maintained regularly, and even replaced.
