The relationship between busbar shape, current carrying capacity, and heat generation

Principles for selecting busbar specifications for low-voltage switchgear

1) Due to the low insulation voltage, the clear distance of TTA full type test products can meet the insulation requirements as long as 14mm, so rectangular busbars can meet the requirements.

2) Low voltage cabinets generally have high currents, such as 5000, 6300, 7800A, etc. Therefore, more consideration should be given to the effects of skin effect and proximity effect. The larger the cross-sectional area and quantity of copper bars, the greater the impact. At the same time, the influence of eddy currents is also significant. Non magnetic materials should be used as support components, and stainless steel bolts should be used as connections to effectively reduce temperature rise.

The following figure shows the ratio of AC resistance to DC resistance. It can be seen that the K value of 100 × 10mm is greater than that of 100 × 5mm copper bars, and with an increase in the number of busbars, the AC resistance also increases significantly. This is a typical skin effect effect, where the current tends to the surface and the internal copper bar current carrying capacity decreases, leading to an increase in resistance.

3) As a vertical busbar, the opposite sex busbar, such as the C-type, can effectively reduce the impact of skin effect. At the same time, the connector can be directly inserted into the opening of the C-profile, reducing the number of parts and connections, which is a good design solution. A C-shaped row with a length of 35, width of 30, and a wall thickness of 6mm can achieve a current carrying capacity of 1000A.

4) The plug-in connection of low-voltage cabinets and ACB circuit breakers is the area with the highest temperature rise and heat generation, requiring additional copper bars to reduce current density and also serve as a heat sink.

Choose under high current conditions

In the case of high current, rectangular busbars require the use of multiple conductors, resulting in additional losses, large skin effect coefficients, uneven current distribution, and a significant decrease in current carrying capacity. Under the same cross-sectional area, conductors with different cross-sectional shapes are affected by skin effects, such as increasing the number of copper bars and spacing between in-phase busbars, which can reduce the impact of skin effects.

The relationship between the K value and the shape of the conductor in the following figure shows that the circular tube AC resistance has the least impact. And the current carrying capacity is divided into two groups and arranged higher, that is, the two groups are arranged separately, and the maximum current carrying capacity tends to be in the form of pipes.

SCHNEIRDER OKKEN horizontal busbar 7300A is designed with full consideration of skin effect and proximity effect. With more pieces and increased grouping distance, the AC resistance is reduced.

GE AKD20 vertical busbar 6000A, specially designed to ensure heat dissipation area and reduce skin effect. The specific application of the shape with the highest current carrying capacity in the cross-sectional shape above.

MB301M 6300A ACB end connected to copper busbar, 6300A switchgear natural ventilation main busbar: 5-5x200 (5000 square millimeters), ACB inlet and outlet copper busbar connection end: 10-5x125 (6250 square millimeters), adding interval copper busbar as a heat sink

As a precious non-ferrous metal, reducing the usage of copper bars can effectively save resources. By designing multiple copper bars, a large amount of copper bar usage can be saved. For example, the original design of SENPLUS cabinet 6300A used 7-100x10, but after splitting and improving the design with multiple pieces, the copper bars were changed to 12-40x10, saving 30% of copper bar usage; By using a thin copper bar design and adding heat sinks to the ACB end, natural air cooling of the 6300A circuit breaker can also be achieved without the need for a fan for forced air cooling, increasing reliability and reducing costs.

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