1 Causes of electrical line fires
1.1 Electrical circuit short circuit
Short circuit of electric circuit has two forms: phase-to-phase short circuit and phase line to ground short circuit. However, no matter which kind of short circuit, the short circuit will generate a large current, the current will produce an instantaneous high temperature, and the high temperature will burn adjacent flammable substances and cause a fire. Moreover, the organic insulating materials of the line, such as polyvinyl chloride, polyethylene, neoprene, etc., can release flammable gases due to thermal decomposition. When the temperature reaches 300 degrees Celsius or more, combustion occurs, and the fire spreads along the line, thereby increasing the fire hazard. . Another form of short circuit is an arc short circuit. Arc short circuit refers to a short circuit caused by arc or electric spark between two points of different potentials at a short circuit. The voltage between the two conductors breaking through the air gap is at least 30KV/cm, and the two short-circuit conductors are separated. If the arc is pulled out, the sustain voltage only needs 20V/cm, and only 2A-10A arc current can generate 2000°C-4000. The local high temperature of °C, 0.5A arc current can cause fire. On the other hand, the arc itself has a large impedance, which limits the short-circuit current, makes the overcurrent protection device inoperable or difficult to operate in time, and provides sufficient time for the arc to ignite the nearby combustible material, which causes the arc to be short-circuited. More harmful.
1.2 Contact resistance is too large
When the electrical line is connected or the live line is connected to the ground, the contact at the contact is not tight. The voltage between the contacts is enough to break through the gap air to form air conduction. If the gap between the contacts is slightly larger, it coincides with the peak voltage fluctuation. Pull the arc between the air. From this, it is understood that the contact resistance is widely present in the electrical contact region. If the joint gap is small, the end point under the action of voltage, even if it will break through the air bonfire, whether it is a slamming spark or a pulled arc, it is enough to ignite the nearby combustibles to form a fire. The fire caused by excessive contact resistance is very concealed, and its development is also very rapid, which occupies a considerable proportion in electrical line fires, and its harmfulness is more serious.
1.3 Insulation damage caused by fire
A variety of ways can cause insulation damage. The causes of insulation damage are: insulation loses its original electrical and mechanical properties due to excessive use of aging; overvoltages such as lightning strikes can cause damage to insulation breakdown; corrosion and wear of chemical substances due to prolonged overheating, Moisture and mildew, insect bites and damage can cause damage to the insulation. In addition, the deposition of conductive dust or fibers on the surface of the insulator will destroy its surface insulating properties resulting in leakage or short circuit. When bakelite insulation is subjected to an electric arc, its surface may be carbonized and thus cause a more severe arc short circuit. After the insulation is damaged, two situations may occur depending on the degree of damage: (1) Leakage. If the insulation is only partially damaged, it will cause local heat generation at the leakage. If the local temperature is too high, it may directly cause a fire, or the insulation may be further damaged to form a short circuit, causing a fire. (2) Short circuit. Complete destruction of the insulation will cause the line to be short-circuited. When the short circuit flows, the current flowing through the line increases to several times or even tens of times the normal operating current, and the heat generation increases sharply, which may cause a line fire in a short time.
1.4 overload
If the overload time is too long, the distribution line will cause the wire to overheat and may ignite nearby items. Causes of overload: (1) overload caused by a large number of private electrical equipment; (2) due to the designer's mistakes did not fully consider the development needs, the overload caused by the margin design is too small. Although the electrical line is allowed to be overloaded for a short period of time, the overload time and overload must be strictly controlled; (3) The selected conductor cross section is small.
1.5 Harmonic current of electrical lines
With the development of electrical and electronic technology, there are an increasing number of non-linear load electrical equipment such as televisions, computers, microwave ovens, etc. The load current of such equipment contains multiple harmonic currents, which can overload and accelerate the electrical lines, especially the neutral line. Insulation aging causes a short circuit fire.
2 Measures to prevent fires in electrical lines of civil buildings
Generally speaking, measures to reduce electrical line fires are a comprehensive system problem, involving many technical theoretical and practical issues, as well as many management measures and implementation of management systems, mainly in the following aspects:
2.1 Human factors have a large component in electrical line fires. Raising people's awareness of disaster prevention is the fundamental way to reduce electrical fires. Many major electrical fires are caused by human negligence and unsafe behavior. Therefore, it is necessary to strengthen the education of people, especially to strengthen the technical training of key personnel, so that they fully understand the hazards and preventive measures of electrical fires, enhance the awareness of fire prevention, and improve their consciousness of doing things according to scientific laws.
2.2 The quality of electrical design and construction directly affects the subsequent use, and may also be the cause of future line fires. The careful design and careful construction of electrical engineering according to the specifications is the most fundamental measure to prevent electrical fires. Before designing an electrical circuit system for a civil building, the designer should make a survey of the use of the building, including predicting the possible changes in the use of the building in the near future and even in the long-term. Select electrical parameters, rationally and scientifically arrange all equipment of the electrical circuit system, select products that pass the national product quality inspection, and do a good job in testing the products before use. During the construction process, it shall be constructed in strict accordance with the drawings, and the installation and commissioning personnel shall be those who have passed the training examination. After the completion of the project, relevant parties should carefully organize the inspection and acceptance, and then fully qualified before they can be put into use to ensure the construction quality.
2.3 Strengthen the use of management and strengthen the system construction. Establish an electrical safety post responsibility system to ensure that the civil building electrical line system is always in good operating condition. Establish and improve the safe electricity use system, strict fire supervision and inspection. Found problems to urge rectification.
2.4 Using the latest monitoring and testing equipment, use the automatic fire alarm system in the building, use advanced detection means, such as infrared detection and ultrasonic detection, to detect and notify the fire in time, and take effective measures to eliminate the fire in the bud.
2.5 Do a good job of fire protection of the partition
2.5.1 The electrical shaft of a high-rise civil building is relatively long. Once a fire occurs, the shaft becomes a ventilation duct, which will produce a chimney effect. Therefore, it is necessary to properly handle the ground of each layer of power distribution and weak electric shafts. The gaps of various electrical circuit holes should be sealed with materials with the same fire resistance rating as the structural members to form a fireproof seal between the floor shafts.
2.5.2 When the distribution line runs through the refractory wall, it should be carefully considered according to the requirements of the fire compartment. Especially for the distribution lines of flammable insulating materials, when cable slabs, busbars, etc. are used to penetrate the refractory wall, the holes of the same burning grade should be used to block the holes tightly. The cable trays running on both sides of the partition wall shall be covered with fine sand, and the length of the wall shall be 1m, so as to prevent the fire from expanding from a fire zone to the line channel of another fire zone through the line channel.
2.5.3 When a cable with flammable insulating material is used as the distribution line, the flammable circuit should be completely enclosed in the fire-resistant cable trough of the outer casing brush fire-retardant coating.
2.5.4 Casings for cables and wires (except G25), the ends of the nozzles shall be blocked with materials of the same fire resistance as the surrounding.
3 Summary
After understanding the diversity of the causes of building electrical line fires, it is necessary to combine technical, management, training and other aspects of work to prevent building electrical line fires and reduce the occurrence of building fires.
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