Being an Electrical Engineer we may often face situations where copper and aluminium electrical joints are required. Such joints are required in various scenario. For example, in Switchyard the material of buses are of Aluminium whereas that of P1 &P2 connector of CTs are of copper. You may have seen this or can observe when you get a chance to visit. Various such connection are also present in medium voltage (6.6 kV) Switchgear. Such connection of Aluminium and Copper are very crucial and needs a great care else the joint may get damage due to overheating and eventually may lead to damage of equipment.
There are various factors which must be known before making electrical joint / connection of Aluminium and Copper. In this post, some of them will be discussed.
Unlike aluminium to aluminium or copper to copper direct joints, a direct joint / connection of Aluminium and Copper suffers from corrosion. The corrosion of joint basically take place due to Galvanic Corrosion of Aluminium and Copper and reaction with atmospheric oxygen. Aluminium may react with atmospheric oxygen to make aluminium oxide. The conductivity of this aluminium oxide (corrosion product) is very poor when compared to aluminium or copper. This leads to increased contact resistance.
Increased contact resistance causes more ohmic loss (I2R) and hence heating of joint. Excessive and prolonged overheating will definitely lead to damage of contact and hence may result into plant outage. There is one another point which should be considered. This is different thermal expansion coefficient of Aluminium (69×10-6 m/mK) and Copper (50×10-6 m/mK). Since the volume expansion coefficient of Aluminium is more than Copper, the volume expansion of Aluminium will be more than that of Copper. Again during cooling period (when the load current reduces, the heating in the joint will be less), the contraction in Aluminium will be more. Such periodic heating and cooling of joint will lead to loosening of joint and joint tightening nuts & bolts. Subsequently to loose connect joints, sparking will take place. This spark may spread to cause fire in the Switchgear.
From the above discussion, it is clear that Copper and Aluminium should not be connected directly. Then how should we make a connection or joint of Aluminium and Copper?
Aluminium Copper bimetallic should be used for making joint or bimetallic lugs should be used for making connection of aluminium and copper wire. Figure below shows a typical bimetallic of copper and aluminium.
Carefully observe the figure above. Note that inner surface of bimetallic is of copper while outer surface is of aluminium. Above bimetallic is used for connecting the Current Transformer Primary (connectors made of copper) to the aluminium made bus. While making connection, copper side of bimetallic is mate with copper side of bimetallic and aluminium side with outer face of bimetallic. Thus similar metal make contact by suing bimetal. This eliminates Galvanic Corrosion.How? You are requested to read Galvanic Corrosion to understand how bimetallic eliminates Galvanic Corrosion. The same philosophy applies for all connection using bimetallic.
In some applications where copper to aluminium joint is not supposed to be exposed to atmospheric condition like in indoor Switchgear, individual conductors i.e. Aluminium and Copper may be tin plated or coated before making joint. Tin is very resistance to corrosion with atmospheric oxygen. Thus tin plating prevents corrosion of the joint. But full load heating test must be performed to ensure that tin coated contact joint is not overheating.