Electrical explosion accidents and fires refer to explosion accidents and fires caused by electrical causes. Most of the insulating materials in electrical equipment are flammable substances. During operation, the conductor will generate heat when passing current. When the switch cuts off the current, an arc will be generated. Arcs and sparks may occur due to grounding, short circuit or equipment damage, which will make the surrounding flammable. The object ignites, a fire or explosion accident occurs. This accounts for a large proportion of fire and explosion accidents.
In addition to causing human injuries, electrical fire and explosion accidents may also cause large-scale power outages and cause national economic losses. Therefore, how to prevent fire and explosion is extremely important.
Ignition refers to a phenomenon in which a part of the combustible is subjected to a high-temperature heat source, which leads to the partial combustion of the combustible and gradually expands to the whole. Under normal circumstances, most of the fire is caused by ignition. Sometimes, fires and explosions are accompanied.
Explosion refers to a phenomenon in which a substance undergoes a violent decomposition and oxidation reaction, which causes its temperature and pressure to increase sharply. Explosion is a special burning phenomenon.
Combustible gas, dust and liquid mixed with air will explode when encountering heat source and open flame. The lowest concentration of the mixture is called the lower explosive limit, and the highest concentration is called the upper limit, usually expressed in volume percentage.
Causes of electrical fires and explosions
Electrical fires and explosions, in addition to product defects or improper installation and other design, manufacturing and construction reasons, there are also some heat generated during operation and electric sparks or arcs Wait for the direct cause.
(1) Overheating of electrical equipment
Overheating of electrical equipment is mainly caused by the thermal effect of current. When the current passes through the conductor, due to the resistance of the conductor, a certain amount of electrical energy is consumed when the current passes. This part of the energy is consumed in the form of heat and heats other materials around it. When the temperature exceeds the allowable temperature of electrical equipment and its surrounding materials, it may cause a fire when it reaches the ignition temperature.
The main causes of electrical equipment overheating are as follows:
¢ÙShort circuit: When the line is short-circuited, the current in the line will increase to several times or even dozens of times the normal working current, causing the equipment temperature to rise sharply, especially the connection part Contact resistance, etc. If the temperature reaches the ignition point of combustibles, it will cause combustion.
¢ÚLoad overload: due to the unreasonable selection of wire cross-section and equipment, or the current exceeding the rated value of the equipment during operation, and the long-term allowable temperature of the equipment, it will cause heating.
¢Û Poor contact: Poor contact of movable contacts (switches, fuses, contactors, sockets, bulbs and lamp holders, etc.), and the connection of wire connectors is not secure, which will lead to high contact resistance, and the passing of current will cause the connectors to overheat.
¢Ü The iron core is overheated: the iron core of motors, transformers and other equipment is too saturated, or the non-linear load causes high-order harmonics to cause the iron core to overheat.
¢Ý Poor heat dissipation: The heat dissipation and ventilation measures of the equipment have been destroyed, and the heat generated during the operation of the equipment cannot be dissipated in time and effectively, causing the equipment to overheat. Improper installation or use of some electrical equipment with a large amount of heat may also cause a fire.
(2) Arc and electric spark
Arc and electric spark are a common phenomenon. For example, electric arcs and sparks also occur during normal operation or normal operation of electrical equipment. There will be electric sparks at the sliding contact of DC motor brush and commutator, and the sliding contact between AC motor brush and slip ring during normal operation. A strong arc will be generated when the switch opens the circuit, and the contactor will open the circuit or be unplugged. There will be sparks when plugging. The arc is greater when the circuit is short-circuited or grounded.
Electrical equipment itself can also explode, such as oil-filled equipment such as oil circuit breakers, power capacitors, transformers, and voltage transformers. The space around electrical equipment may explode under the following conditions:
¢ÙThe surrounding area contains explosive mixtures, which may cause an explosion in the space when it encounters electric arcs and sparks.
¢ÚThe insulating oil of oil-filled equipment decomposes and vaporizes under the action of an electric arc, and sprays a large amount of oil mist and flammable gas. Fire and explosion accidents may also occur when encountering electric arcs, electric sparks, or when the ambient temperature reaches dangerous temperatures.
¢ÛIf hydrogen leakage occurs in hydrogen-cooled generators and other equipment, an explosive mixture may be formed. Fire and explosion accidents may also occur when arcs, electric sparks, or the ambient temperature reaches dangerous temperatures.
Electrical fire protection and explosion-proof measures
There are two main reasons for electrical fires and explosions: 1. There are flammable and explosive materials at the scene; 2. There are conditions for ignitors to detonate at the scene.
So preventive measures should be taken from these two aspects to prevent electrical fires and explosions from happening.
In various production and living places, there are widespread flammable and explosive substances, such as combustible dust, combustible gas and fiber. When encountering high-temperature ignition sources such as sparks and arcs generated during the operation of electrical equipment, electrical fires and explosions may occur. Explosions are also the cause of fires.
According to the causes of electrical fires and explosions, fire-proof and explosion-proof measures can start from improving the environmental conditions of the site, trying to reduce various combustible and explosive substances in the air, or reduce the concentration of combustible and explosive substances. At the same time, strengthen the maintenance, supervision and management of electrical equipment to prevent fire and explosion accidents caused by electrical fire sources.
¢ÙExclude combustible and explosive materials.
Maintain good ventilation to reduce the concentration of combustible and explosive dust, gas and fiber to the limit that cannot cause fire and explosion. Strengthen the seal to reduce and prevent the leakage of combustible and explosive materials. The production equipment, storage containers, pipe joints and valves of combustible and explosive substances should be strictly sealed and inspected and supervised frequently.
¢Ú Eliminate electrical fire sources
When designing and installing electrical devices, they must be selected, arranged and installed in strict accordance with the requirements of fire protection regulations. For electrical equipment and devices that can produce sparks, arcs and high temperature hazards during operation, they should not be placed in flammable and explosive hazardous locations. Electrical equipment and devices installed in flammable and explosive places must be sealed explosion-proof electrical appliances. In addition, try to avoid using portable electrical equipment in flammable and explosive places. In places prone to explosion and fire hazards, the rated voltage of the insulated wires and cables of the power line shall not be lower than the rated voltage of the power grid, and the low-voltage power supply line shall not be lower than 500 V. To use copper core insulated wire, the wire connection should be good and reliable, and joints should be avoided as much as possible. In flammable and explosive places, the cross-section and insulation of the working zero wire should be the same as that of the phase wire, and should be in the same sheath or tube. The wire should be laid through a flame-retardant wire (or flame-retardant cable).
Note: In places where sudden power failure may cause electrical fires and explosions, there should be two or more power supplies, and several power supplies can be automatically switched. The metal shell of electrical equipment in places prone to explosion hazard should be reliably grounded (or connected to zero). In operation and management, electrical equipment should be maintained and supervised to prevent equipment accidents.
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