Why can't copper and aluminum wires be directly connected?

2020-03-03 11:15:25 1045

1. The resistivity of aluminum wire and copper wire are different.

2. Good ductility: the elongation of copper alloy is 20 ~ 40%, the elongation of copper for electrical use is more than 30%, and that of aluminum alloy is only 18%.

3. According to the safe operation specification, aluminum wire cannot be connected with copper wire.

Consequences of direct connection of copper and aluminum wires

1. When the copper and aluminum conductors are directly connected, the contact surface of these two metals is very easy to form an electrolyte under the action of moisture, carbon dioxide and other impurities in the air, thereby forming a primary battery with aluminum as the negative electrode and copper as the positive electrode , Cause galvanic corrosion of aluminum, resulting in increased contact resistance at the copper and aluminum connections.

2. In addition, because the elastic modulus and thermal expansion coefficient of copper and aluminum are very different, after running multiple cold and heat cycles (power on and power off) during operation, a large gap will be created at the contact point, which will affect the contact. Contact resistance is also increased. Increasing contact resistance will cause temperature rise during operation. Corrosion and oxidation will intensify at high temperatures, resulting in a vicious cycle, which will further deteriorate the quality of the connection, and eventually cause the contact point temperature to be too high and even smoke, burnout and other accidents.

Copper and aluminum wires must be transitionally connected

1. For single-strand small-section copper and aluminum wires, copper wires should be tinned and then connected to aluminum wires.

2. When multi-strand large-section copper and aluminum wires are connected, copper-aluminum transition connecting pipes or copper-aluminum transition wire clamps should be used.

3. If the aluminum wire is connected to the copper terminal of the switch, a copper-aluminum transition nose should be used.

Precautions for copper and aluminum connections

1. Copper conductors should be tinned in dry rooms. Copper or aluminum transition boards should be used outdoors or indoors where the relative humidity is close, and the copper ends should be tinned.

2. Correspondingly, copper and aluminum connecting pipes can be used when connecting copper cables and aluminum cables, and copper and aluminum terminals can be used when connecting copper cables and aluminum wires. The copper ends should be tinned.

Advantages and disadvantages of copper and aluminum wires

1. Advantages and disadvantages of copper wire:

(1) Low resistivity: The resistivity of aluminum cable is about 1.68 times higher than that of copper cable.

(2) Good ductility: the elongation of copper alloy is 20 ~ 40%, the elongation of copper for electrical use is more than 30%, and that of aluminum alloy is only 18%.

(3) High strength: The allowable stress at room temperature is that copper is 7-28% higher than aluminum. Especially the stress at high temperature, the difference is even greater.

(4) Anti-fatigue: Aluminum is easy to break after repeated bending, but copper is not easy. In terms of elasticity index, copper is also about 1.7 to 1.8 times higher than aluminum.

(5) Good stability and corrosion resistance: Copper wire is resistant to oxidation and corrosion, while aluminum wire is susceptible to oxidation and corrosion. (6) Large current carrying capacity: Due to the low resistivity, the copper wire cables with the same cross section are about 30% higher than the current carrying capacity (maximum current that can be passed) of aluminum wire cables.

(7) Low voltage loss: Because the resistivity of the copper wire cable is low, the same current flows in the same section. The voltage drop of the copper cable is small. Therefore, the same transmission distance can ensure higher voltage quality; or, under the condition of allowable voltage drop, the copper cable power transmission distance reaches a long distance, that is, the power supply coverage area is large, which is conducive to network planning and reduces Set number of power points.

(8) Low heating temperature: Under the same current, the copper wire cable with the same cross section generates much less heat than the aluminum wire cable, making the operation safer.

(9) Low energy consumption: Due to the low electrical resistivity of copper, compared to aluminum cables, copper cables have lower power losses, which is obvious. This is conducive to improving the utilization rate of power generation and protecting the environment.

(10) Anti-oxidation and corrosion resistance: The performance of the connector of copper wire cable is stable, and no accident will occur due to oxidation. The connector of aluminum wire cable is unstable, and often the contact resistance increases due to oxidation, which causes heat and accidents. Therefore, the accident rate is much higher than that of copper cable.

(11) Convenient construction:

①The copper wire has good flexibility and small allowable bending radius, so it is convenient to turn and easy to pipe.

② The copper wire is resistant to fatigue and is not easy to break due to repeated bending, so the wiring is convenient;

③ The copper wire has high mechanical strength and can withstand large mechanical tension, which brings great convenience to construction and laying, and also creates conditions for mechanized construction.

(12) The high price of copper wire and the large proportion of copper wire determine the high cost of copper wire. There are many advantages in performance as a cable, which is high cost!

2. Advantages and disadvantages of aluminum wire:

(1) Cheap: Copper rods are 3.5 times the price of aluminum rods, and copper's specific gravity is 3.3 times that of aluminum. Therefore, aluminum cables are much cheaper than copper cables, which is suitable for low-cost projects or temporary electricity.

(2) The cable is very light: the weight of the aluminum cable is 40% of the copper cable, and the construction and transportation costs are low.

(3) Anti-oxidation and corrosion resistance: Aluminum reacts with oxygen in the air to quickly form an oxide film, which can prevent further oxidation. Therefore, aluminum wire is a necessary material for high-voltage, large-section, long-span overhead power transmission.

(4) High failure rate, easy to break, easy to generate heat.

Why can't copper and aluminum wires be directly connected?