Copper Bar Design - Summary Chapter

The design of copper bars in switchgear is a systematic project, and it cannot be simply said that the selection of copper bars based on their current carrying capacity is always appropriate, or that the temperature rise exceeds the standard, or that the margin is large. This is because the copper bars are located inside the switchgear, taking into account the heating power and position of the internal components, the surrounding environment, the effective heat dissipation area on the exterior surface of the cabinet, the area and path of ventilation holes, and so on.  

The temperature rise of the switchgear generally does not exceed the standard at all temperature measurement points such as the busbar and connections, unless the cross-sectional area of the copper busbar is far from sufficient. Generally, it is at a certain point or points in the local area, such as the location of the plum blossom contact in the central cabinet, which is a difficult to control high temperature rise point.  

Generally speaking, the temperature rise of the switchgear is caused by the components, while copper bars generally do not. Simply select the cross-sectional area according to the specifications and there will be no difference. Calculate by using the formula for calculating the air current carrying capacity of a single busbar.  

In the formula, A is the surface area of the busbar heat dissipation (m2), R is the resistance, and the resistance is inversely proportional to the cross-sectional area of the busbar. It can be seen that a larger cross-sectional area and a larger surface area will reduce the temperature rise. If both have a cross-sectional area of 1000mm2, the surface circumference of 10 × 100mm is 220mm, while the surface circumference of 5 × 200mm is 410mm. A circular tube with a diameter of 80mm and a wall thickness of 4mm is used, and the inner and outer surface circumferences of 477mm are higher due to the reduced skin effect coefficient, resulting in a higher current carrying capacity.

The same principle applies to special-shaped busbars, which can also increase their strength, especially aluminum special-shaped busbars.

Regarding the overlapping area of fixed connections, the area generally does not need to be too large, and the overlapping length is 5 times the thickness to fully meet the requirements. The key is the contact pressure and quantity of the connecting bolts. Therefore, M10/M12 bolts are generally used, with bolt spacing of around 25mm for M10 and 30mm for M12. This is because the outer diameter of the gasket is 20mm for M10 and 24mm for M12, leaving a 5mm gap between the gaskets can meet the pressure coverage range of the gasket and achieve maximum connection efficiency.

The core of temperature rise is to solve the problem of excessive local temperature rise, such as the high heating of the circuit breaker itself, whether it is the low-voltage frame or the contacts of the vacuum circuit breaker, which are the parts with high heating, and poor heat dissipation conditions leading to excessive temperature rise.

It is difficult for us to control the high temperature rise of components, so it is very important to choose high-quality circuit breakers. When encountering circuit breakers with high heat generation and poor quality, even if measures such as increasing copper bars and ventilation are taken to reduce the temperature rise, it is still very difficult. There are also pull-out contacts, and the heating is determined by the contact pressure, biting condition, and contact surface, so it is necessary to control the heating of these key points.

The key to solving the problem of temperature rise is to reduce high incidence hotspots through effective measures, such as using cross flow fans to blow against the contacts to reduce temperature rise, or using high thermal conductivity insulation components, insulation heat pipes, phase change radiators, etc. to bring heat to the surface of the cabinet for heat dissipation. These are all developed based on this goal.

Solving temperature rise is a systematic project that also requires consideration of insulation and other issues. It cannot simply rely on a copper bar selection table to solve all temperature rise problems. However, with the high price of copper bars today, the scientific reduction of copper bar usage to reduce costs and increase efficiency, or the realization of high current applications to meet the demand for new energy, all require scientific and technological support.