In the course of operation, the wire and cable will be heated due to the existence of resistance. The resistance of wire is generally very small, and its heating power can be expressed by formula q=I^2R. Q=I^2R shows that for a wire used in actual use (R is basically constant), the greater the current through the wire, the greater the heating power; if the electric flux is constant, the heating power of the wire is constant. The heat released during the operation will be absorbed by the conductor itself, causing the temperature of the conductor to rise. During the operation of the conductor, although the heat absorbed by the electric current is continuously absorbed, the temperature will not rise unlimitless. Because the wire is endothermic, it is also constantly releasing heat to the outside world. The fact shows that the temperature rises gradually after the wire is electrified, and the final temperature is constant at a certain point. At this constant point, the conductor has the same power of absorption and exothermic, and the conductor is in a thermal equilibrium state. The ability of a conductor to operate at relatively high temperatures is limited, and there will be danger beyond a certain maximum temperature. This maximum temperature naturally corresponds to a maximum current, which is overload when the conductor exceeds this maximum current. The overload of wires directly causes the temperature of the conductor itself and its vicinity to rise. Temperature rise is the most direct cause of such fires.

Overload causes the insulation between two wires to cause short circuit, burning equipment and causing fire. The double conductor wires are insulated by the insulation layer. The overload causes the insulation layer to soften and destroy, resulting in direct contact between two wires, causing short circuit and burning equipment. At the same time, the high temperature generated by the short-circuit current causes the line to fuse and fuse, and the molten beads fall to the combustible to cause a fire. The overloading temperature rise can also directly ignite the nearby combustibles. The heat transfer of the overload wire increases the temperature of the combustible nearby. It is possible to ignite the fire caused by the low combustible nearby. This danger is particularly prominent in storehouses where flammable goods are stored and in buildings that are easy to use and combustible.

The overload also makes the connection in the circuit under the condition of overheating, which accelerates the oxidation process. Oxidation makes a thin layer of oxide film that is not easy to conduct at the junction, and the oxide film increases the resistance between the contact points, resulting in fire and other phenomena.

So, how to prevent fire caused by overload of wires and cables?

1. In the process of line design, the capacity of the site should be accurately checked, and the possibility of new capacity should be fully considered, and the suitable type of conductor should be selected. Large capacity, should choose the thicker wire. Line design and reasonable selection are the key steps to prevent overload. If the design type is improper, it will leave a hidden danger that is difficult to rectify. Some small projects and sites are not seriously designed and selected. It is very dangerous to choose and lay out lines at will. New electrical appliances and electrical equipment should fully consider the bearing capacity of the original lines. If the original line does not meet the requirements, it should be redesigned and reformed.

2, the line should be in accordance with the relevant specification requirements, so that electrical qualification personnel construction. The laying condition of the line directly affects the heat dissipation of the conductor. Generally speaking, line laying should not pass through easy, combustible material and stacking, which will lead to the failure of heat dissipation, heat accumulation, the possibility of igniting the combustible materials around the lead, increase the risk of fire in the case of overload; the installation of line in the decoration ceiling of public entertainment places should be put through the steel pipe protection to make the ceiling and the ceiling. If the line is separated, it will not fall off even if there are molten beads in overload and short circuit conditions, so as to avoid fire.

3, strengthen the management of electricity consumption, avoid random wiring, random line, and use mobile socket cautiously. Random wiring, random connection, and using mobile socket are actually adding electrical equipment on a certain line, increasing the current quantity, which may cause overload. The socket of mobile socket is obviously more than the fixed socket on the wall. If the mobile socket is equipped with too many electrical equipment, the original circuit will be difficult to bear. For larger power equipment and electrical appliances, a separate line should be set up, so the mobile socket should not be used as the wiring source.

4, speed up the upgrading of old lines and eliminate fire hazards. Old enterprises, old residential areas and other units, because of the relatively long service life, many lines have been aging, exceeding the useful life. Although the current carrying capacity of some lines is not large, the aging lines are also difficult to withstand such carrying capacity, and also have the danger of overload. In particular, the old residential areas, the line has long been aging, but with the improvement of people's living standards, the increase of household electrical appliances, its electricity consumption is still rising year by year, it is really worse. For old lines, they should be urged and coordinated in a timely manner so as to promote their rectification as soon as possible so as to eliminate fire hazards and ensure safety.