Switchboards have formed the heart of the distribution systems and an optimum design of Switchboards has a big impact on the life cycle costs of the electrical systems. The busbar material for Switchboards has been a topic for eternal debate. I went through numerous papers and articles written by reputed manufacturers and thought of sharing a summary of the same.
Before we proceed further, let us have a comparison between physical properties of the two .
Property | Unit | Aluminium | Copper |
Density | g·cm-3 | 2.7 | 8.94 |
Resistivity | nΩ·m | 28.2 | 16.78 |
Coefficient of expansion | µm·m−1·K−1 | 23.1 | 16.5 |
Tensile Strength | Mpa | 200 | 380 |
The above figures might imply that Copper (Cu) is both a better conductor of electricity as well as mechanically stronger than Aluminium (Al). However, going in depth in it shows a different picture .
1) As Al has a lesser conductivity, almost 68% more cross-section is required for the same amperage as compared to Cu. But having a density almost 1/3rd of Cu , the total weight per ampere comes to almost 50%.
2) Also cost per kg for Al being almost 1/3rd as compared to Cu, cost per ampere comes to 1/6th as compared to Cu .
Thus from a pure economic point of view, Al beats Cu by a large margin. But there are other factors that need to be given a consideration before reaching any conclusion.
a) Al is much less malleable and soft as compared to Cu. It undergoes plastic deformation over time and under stress which may result in failures. Only recently, new age Al alloys have been developed to improve its tensile strength even matching that of Cu. Care needs to be taken to select the right material based on the mechanical stress the system faces .
b) Al tends to get oxidised immediately when in contact with Air. It forms a thin film of Al oxide which acts as a protective layer. However, this makes the Al-Al connection a bit unreliable. This might be dangerous for higher current ratings. Some manufacturers use plating of Cu or Tin (Sn) on the Al Busbar to overcome this difficulty .
The use of Cu in many products can help is significant size reduction . All the manufacturers these days are using Cu terminals in their Switchgear which has made it more and more compact. This sometimes creates a concern terminating Al busbars especially on higher rated breakers. Hence, we have been recommending Cu busbars for ratings beyond 2000A .
For ratings higher than 2000A, it has been observed that multiple busbars are stacked for Al and the space for air circulation is reduced which in turn tends to heat the panel . In a similar case, Cu (despite smaller surface area) tends to perform far better in terms of reliability. Higher temperatures result in more expansion for Al as compared to the steel shell and can cause stress at the clamps and joints . Deformations are observed over a period of time in such cases.
To cut short the debate , we recommend Al busbars up to 2000A and Cu beyond it . Our observations are limited to Switchboards . Busways and Rising mains have different considerations which are well addressed by the manufacturers.
The facts and figures are taken from the datasheets and white papers published by reputed manufacturers.
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