Because aluminium has good conductivity, it has been used in North America since the 1960s. At first, it used pure aluminium as high-voltage overhead wires. Later, it was widely used in 8000 Series of aluminium alloys.
1. Origin of Aluminum Alloy Cable Materials
During 1965-1971, fire accidents occurred frequently in some buildings using pure aluminium core cables in the United States. Through analysis, the main reason is that instead of designing special connectors for aluminium-core cables, copper-core cable connectors are directly used, resulting in relaxation, micro-damage and arcing between aluminium wires and sockets or switches, resulting in the increase of contact resistance and the occurrence of overheating, resulting in fire.
In order to reverse this situation, the cable industry in North America has made improvements in cable products and connectors. The following measures have been taken.
In terms of materials, 8176, 8030 and other 8000 series aluminium alloys were invented by American Southern Line and Canadian Aluminum Industry, which improved creep properties.
In connection with the wire connection, various methods are adopted to improve the connection performance, including the use of tinned terminals, the addition of special gaskets between aluminum and copper terminals, the application of antioxidants at connection points, and the design of special sockets.
In terms of standards, through the tests of UL Company and National Fire Protection Commission (NFPA), in 1985, NEC Electric stipulated that 8000 Series of aluminium alloys should be used in certain types of conductors below 2000 V. In 1988, ASTM promulgated the B800 standard, including 8000 Series Aluminium Alloys of six brands in the cable conductor standard.
From the history of the development of aluminium alloy cables in the United States, there are the following characteristics.
One is that some accidents occur because of the problem of connecting terminals between pure aluminium and copper.
The second is 8000 Series Aluminium alloy, which is a new type developed on the basis of pure aluminium material. The main purpose is to improve the creep performance of ordinary aluminium cable, mainly to solve the problems of joint terminals.
Thirdly, in 1985, American Electrical Regulations stipulated that 8000 Series Aluminium Alloy should be used in certain types of conductors below 2000 V. It can be seen that in the United States, aluminum alloy cables have never entered the medium-voltage field, but only in the low-voltage part of the field.
Fourthly, since the B800 standard was promulgated in 1988, the use of aluminium alloy cables in the United States has been less than 30 years up to now.
Properties of 2,8000 Series Aluminum Alloys
From the above two standard tables of aluminium alloy, it can be seen that 1350 electrical pure aluminium belongs to pure aluminium, its aluminium content is 99.50%; 8000 Series Aluminium alloy belongs to other alloy series, its aluminium content is more than 98%, adding trace elements such as Si, Cu, Fe, Me, totally 1%-1.5%. That is to say, 8000 Series Aluminium alloy is only added a little trace element on the basis of pure aluminium. The purpose is to improve the creep properties of aluminium, which is used to make aluminium alloy cables, but not to improve other properties of aluminium.
Therefore, compared with 1000 series pure aluminium, the properties of 8000 series aluminium alloys have not been fundamentally improved, but some improvements have been made in creep resistance.
The tensile strength of 3,8000 series aluminium alloys is worse than that of pure aluminium alloys.
The mechanical tensile strength of 8000 Series Aluminium alloy is 30% lower than that of pure aluminium and 100% lower than that of copper according to the index of aluminium alloy product standard. Therefore, if the cable of 8000 Series Aluminium alloy is treated improperly in construction, it will easily lead to fracture and crack marks, which are not found. Suspicion is a hidden danger in the stable operation of cable.
4. Comprehensive Comparisons of the Performance of Cables Made of Various Materials
The performance of aluminium alloy cables, pure aluminium cables and copper cables can be compared comprehensively by product standard indexes.
In terms of conductivity, copper is the best material, followed by pure aluminium, and 8000 Series Aluminium alloy is inferior to pure aluminium.
In terms of elongation/softness at break, copper is the best material, followed by 8000 Series Aluminium alloy, and pure aluminium is inferior to 8000 Series Aluminium alloy.
In terms of tensile strength, copper is the best material, followed by pure aluminium, and aluminium alloy is 30% worse than pure aluminium.
In terms of corrosion resistance, copper is the best material, followed by pure aluminium, and aluminium alloy is also inferior to pure aluminium.
By comparing the properties, it can be seen that the cable made of 8000 Series Aluminium alloy is better than that made of pure aluminium in creep resistance and fracture elongation, and the performance of other aspects is not improved, even worse than that of pure aluminium. Since the price of 8000 Series Aluminium alloy is higher than that of pure aluminium, can we consider using pure aluminium instead of 8000 Series Aluminium Alloy to make cables?
Compared with copper cables, aluminum alloy cables are weaker than copper cables in all aspects of use, so it is unrealistic for aluminum alloy cables to replace copper cables in an all-round way.
Generally speaking, the development of aluminium alloy cables provides users with a third choice besides copper cables and pure aluminium cables. However, according to the actual situation of China's power system, the higher the complexity of the system, the greater the risk, the higher the maintenance cost, and the more spare parts. Taking these factors into account, the EU countries have adopted a more realistic policy, stipulating that the cable should be made of copper and pure aluminium instead of aluminium alloy. Is this systematic method of thinking worthy of our consideration?
(Note) Data sources: The Consumer Product Safety Commission of the United States conducted fire surveys in several cities in 1974.
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