Electrical wiring is a very important part of the circuit transformation, the current market of household electrical wiring is mainly divided into two kinds of copper wire and aluminum wire. But in the electrical line aluminum wire is often more likely to catch fire than copper wire, and the danger of aluminum wire fire is much greater than copper wire, this is why?
First, the surface of aluminum wire is easy to oxidize in the air
Conductor surface more or less membrane resistance. If the membrane resistance leads to connection overheating, overheating will increase the membrane resistance, the conductivity will be worse, this overheating in the aluminum wire connection is particularly serious. This is because even if the surface of an aluminum wire is even scratched and polished, it only needs to be exposed to air for a few seconds before it oxidizes and immediately forms a thin film of aluminum oxide. Although it is only a few micrometers thick, it has a very high resistivity and thus presents a large film resistance. Therefore, when connecting aluminum wire construction, conductive paste should be applied immediately after the surface of the wire is scraped clean to cut off the contact between the surface of the wire and the air, otherwise the contact resistance will increase.
Second, high expansion coefficient
The coefficient of expansion of aluminum is as high as 23×10-6/°C, which is 39% larger than copper and 97% larger than iron. When aluminum wire is connected to these two metal conductors and current is passed through them, heat is generated at the connection point due to contact resistance. All three conductors expand, but aluminum expands more than copper and iron, which causes the aluminum wire to be crushed. When the circuit is cut and cooled, the aluminum wire is slightly squashed and does not fully return to its original shape, leaving gaps loose in the connection where air enters to form a thin film of aluminum oxide, increasing the contact resistance.
The next time the power is turned on, the heat becomes more intense, making the situation worse. Serious cases may be due to abnormal high temperature or sparks and ignition fire, for this reason, in the connection of large cross-section aluminum conductor and copper, iron conductor should be configured with transition joints, small cross-section (not more than 6mm ²) aluminum wire connection should be used spring crimp cap, so that no matter whether the connection is energized or heated, the connection of the contact surface are subjected to the spring pressure so that the air and moisture can be no gap into the connection to maintain a good electrical conductivity.
Third, prone to electrolytic corrosion effect
If the potential is different between the two metals there is an acidic or alkaline liquid, the two metals will form a local battery. Aluminum potential of -0.78V, copper potential of -0.17V, when the aluminum conductor and copper conductor between the presence of salt-containing moisture, the formation of this type of local battery, ionization will corrode low-potential aluminum conductor and increase the contact resistance.
IV. Susceptible to corrosion by hydrogen chloride
For PVC insulated aluminum core wire and cable, there may be another problem: in order to prevent hydrogen chloride gas decomposition from PVC insulation, PVC insulation added to prevent the decomposition of hydrogen chloride stabilizer, but when the circuit temperature exceeds 75 ℃, such as circuit overload or for other reasons connected to the temperature is too high, the stabilizer is no longer able to prevent the formation of hydrogen chloride, hydrogen chloride will corrode the aluminum, which will increase the contact resistance and fire hazard.
Unqualified wires and cables not only affect our use, but also directly affect the safety of our lives, so we must keep our eyes open and use quality assurance to choose high-quality cables.
