The reason why aluminum wires in
cables are more likely to catch fire than copper wires
It is well known that the fire danger of aluminum wire is far greater than that of copper wire, and the fire of aluminum wire is not connected to the aluminum wire by the aluminum wire itself. There are several reasons for the fire hazard of aluminum wire connections compared to copper wire.
1. The surface of the aluminum wire is easily oxidized in the air
The conductor surface has more or less sheet resistance. If the film resistor overheats the junction, the overheating can also lead to increased film resistance and poor conduction, and this overheating is especially severe in aluminum wire wiring. This is because even if the surface of the aluminum wire is scraped, it only needs to be exposed to the air for a few seconds, and it will immediately oxidize into an aluminum oxide film. Although only a few microns thick, it has high resistivity and exhibits large sheet resistance. Therefore, when the aluminum wire is connected, after scraping the aluminum wire, the conductive glue should be used to separate the aluminum wire from the air in contact with the surface, otherwise the contact resistance will increase.
2. High expansion coefficient
The expansion coefficient of aluminum is 23 × 10-6/C, and a larger 39%. Copper is 97% larger than iron. When an aluminum wire is connected to two metal conductors and an electric current is passed, the connection point is heated by the presence of contact resistance. All three conductors expand, but aluminum expands more than copper and iron, which pushes the aluminum wire out. After the wire is cut off, the aluminum wire is slightly broken, not completely restored to its original state, the connection is loose, and an alumina film is formed when the air enters, and the contact resistance increases.
The next heating will be more intense, making the situation worse. In serious cases, it may ignite due to abnormal high temperature or sparks. For this reason, transition joints should be arranged when large-section aluminum conductors are connected to copper-iron conductors. Small cross-sectional area (not more than 2), the connection of the aluminum wire should use a spring pressure cap, so whether the connection is energized, whether there is heat or no contact surface is under the pressure of the spring, so that air and moisture cannot enter, so as to maintain a good electrical conductivity of the connection sex.
3, prone to electrolytic corrosion.
If there is an acidic or alkaline liquid between two metals at different potentials, the two metals will form a kind of local battery. Potential for aluminium - 0.78v, copper and local battery - 0.17V, formed with water salts between aluminium conductors and copper conductors. Ionization can cause corrosion of low-potential aluminum conductors to increase contact resistance.
4. Easy to be corroded by hydrogen chloride
There may be another problem with PVC insulated wire and cable. In order to prevent the decomposition of hydrogen chloride gas in the PVC insulation, a stabilizer to prevent the decomposition of hydrogen chloride is added to the PVC insulation. However, when the line temperature exceeds 75 degrees, such as line overload or high temperature junctions, the stabilizer will no longer prevent the formation of hydrogen chloride, which can corrode aluminum, which also increases contact resistance and fire hazard.
