Wire and cable in the process of operation, due to the existence of resistance will heat. The resistance of a wire is generally very small, and its heating power can be expressed by the formula Q =I^2R. Q =I^2R indicates that: for a section of wire in actual use (R has been basically constant), the greater the current through the wire, the greater the heating power; If the current is constant, the heating power of the wire is also constant. The heat released during operation will be absorbed by the wire itself, causing the temperature of the wire to rise. In the process of running, although the wire is constantly absorbing the heat released by the current, its temperature will not rise without limit. Because as the wire absorbs heat, it is also constantly releasing heat to the outside world, and it turns out that the temperature of the wire gradually rises when it is energized, and eventually the temperature stays constant at a certain point. At this constant point, the wire is in thermal equilibrium with the same power of heat absorption and release. There is a limit to the ability of conductors to withstand higher temperatures, beyond which there is a danger. This maximum temperature of course also corresponds to a certain maximum current, beyond which the wire is overloaded. The overload of the wire directly causes the temperature of the wire itself and its adjacent objects to rise. Rising temperatures are the most immediate cause of such fires.
Overload destroys the insulation layer between the double conductors and causes a short circuit, which burns the equipment and causes a fire. The double conductors are separated by the insulating layer between them. Overload makes the insulating layer soften and destroy, which leads to the direct contact between the two conductors causing short circuit and burning equipment. At the same time, the high temperature generated by the high current at the moment of short circuit causes the line to catch fire and fuse, and the molten beads fall to the combustible material to cause a fire. Overload temperature rise can also directly ignite nearby combustibles. The heat transfer of overload wire increases the temperature of nearby combustible, and it is possible to ignite the combustible with a low ignition point nearby and cause a fire. This kind of danger is particularly prominent in the storeroom of storing inflammable goods and the building that use easy and combustible to decorate.
Overloading also exposes the connections in the line to overheating, which speeds up the process of oxidation. Oxidation produces a thin layer of oxide film that is not easy to conduct at the connection point, and the oxide film increases the resistance between the contact points, resulting in ignition and other phenomena, causing a fire.
So how do you prevent fires caused by overloaded wires and cables?
1. In the process of line design, the capacity of the site should be accurately verified, and the possibility of additional capacity should be fully considered in the future, and the appropriate type of wire should be selected. For large capacity, thicker wires should be selected. Line design and reasonable selection are the key steps to prevent overload. If the design and selection is improper, it will leave congenital hidden trouble that is difficult to rectify. Some small projects and places are not carefully designed and selected. It is very dangerous to choose and lay lines at will. Add electric equipment, electric equipment should consider the bearing capacity of original line adequately. Where the original circuit does not meet the requirements, it shall be redesigned and transformed.
2. The line should be in accordance with the relevant specifications, let qualified electrician construction laying. The laying conditions of the line directly affect the heat dissipation of the wire. Generally speaking, the laying of the line should not pass through easy and combustible materials and stacking, which will lead to poor heat dissipation of the wire, heat accumulation, the possibility of igniting combustible items around, increasing the risk of fire caused by overload; The lines laid in the ceiling of decoration of public entertainment places should be protected by steel pipes to separate the ceiling from the line. In overload, short circuit and other situations, even if there are molten beads, they will not fall, so as to avoid fire.
3. Strengthen power management, avoid wiring, wiring, cautious use of mobile sockets. Miswiring, miscabling, the use of mobile sockets is actually in a section of the line to add electrical equipment, increase the current may cause overload. There are significantly more mobile sockets than fixed sockets on the wall. If too much electrical equipment is used in the mobile sockets, the original line must be unbearable. Appropriate to the equipment with higher power, electric appliance sets separate line, unfavorable use mobile socket to serve as wiring source.
4. Speed up the updating and transformation of old lines to eliminate fire hazards. Old enterprises, old residential areas and other units, due to the use of time is relatively long, many lines have been aging, more than the use of life. Even if the load capacity of some lines is not large, the aging line can hardly bear such load capacity, and it also has the danger of overload. Especially in the old residential areas, the lines have long been aging, but with the improvement of people's living standards, the increase of household appliances, its electricity consumption is still rising year by year, it is really worse. For old lines, should be timely supervision, coordination, as soon as possible to promote its rectification, in order to eliminate fire hazards, to ensure safety.