There are many causes of electrical fires and explosions. In addition to equipment defects, improper installation and other design and construction reasons, the heat generated by the current and sparks or arc is the direct cause of fire and explosion. 1. overheating electrical equipment overheating is mainly caused by the heat generated by the current. Although the resistance of the conductor is very small, but its resistance is always objective. Therefore, the current through the conductor to consume a certain amount of electrical energy, this part of the electrical energy is converted into heat, so that the conductor temperature rises, and make its surrounding other materials by heat. For motors and transformers and other electrical equipment with ferromagnetic materials, in addition to the heat generated by the current through the conductor, there is heat generated in the ferromagnetic material. Therefore, the iron core of such electrical equipment is also a heat source. When the insulation of electrical equipment is reduced, the leakage current through the insulating material increases, which may lead to an increase in the temperature of the insulating material. From the above analysis, it can be seen that the operation of electrical equipment is always to heat, but the design, construction and operation of the correct electrical equipment, its maximum temperature and its temperature difference with the surrounding environment (i.e., the maximum temperature rise) will not exceed a certain permissible range. For example: the maximum temperature of bare wires and plastic insulated wires generally does not exceed 70 ℃. In other words, the normal heating of electrical equipment is allowed. However, when the normal operation of electrical equipment is damaged, the heat generation to increase, the temperature rises to a certain condition, may cause a fire. The abnormal operation that causes electrical equipment to overheat includes the following broadly: (1) short circuit. When a short circuit occurs, the current in the line increases to several times or even dozens of times the normal, so that the temperature of the equipment rises sharply, greatly exceeding the permissible range. If the temperature reaches the spontaneous combustion point of combustible materials, it will cause combustion, which will lead to fire. The following is caused by a short circuit of several common situations: the insulation of electrical equipment aging and deterioration, or by high temperature, humidity or corrosion of the role of the loss of insulation capacity; insulated wire directly winding, hooked on nails or wire, due to wear and tear and rust corrosion of iron, so that the insulation is damaged; improper installation of the equipment or negligence, so that the insulation of the electrical equipment is mechanically damaged; lightning strikes and other over-voltage effect, the insulation of electrical equipment may be Being broken; in the installation and maintenance work, due to wiring and operation errors, etc. (2) overload. Overloading will cause electrical equipment heating, resulting in overloading of the causes of the following two situations: one is the design of the selection of lines or equipment is unreasonable, so as to produce overheating under the rated load; the second is the use of unreasonable, that is, the line or the load of the equipment exceeds the rated value, or the continuous use of too long a period of time, more than the line or the equipment's design capacity, which results in overheating. (3) Poor contact. The contact part is a key part of the occurrence of overheating, easy to cause localized heating, burning. There are the following kinds of cases easy to cause poor contact: non-removable joint connection is not secure, poor welding or joints mixed with impurities, will increase the contact resistance and lead to overheating joints; removable joints are not tightly connected or due to vibration loosening, will also lead to joints heat; movable contacts, such as the contacts of the gate switch, the contacts of the plug, the contact between the bulb and the lamp base of the contact and other movable contacts, if there is no sufficient contact pressure Or the contact surface is rough and uneven, will lead to contact overheating; for copper and aluminum joints, due to the different electrical properties of copper and aluminum, the joints are prone to corrosion due to electrolysis, which leads to overheating of the joints. (4) Iron core heating. Iron core of transformers, motors and other equipment, if the core insulation is damaged or subjected to prolonged over-voltage, eddy current loss and hysteresis loss will increase, so that the equipment overheating. (5) Poor heat dissipation. All kinds of electrical equipment in the design and installation should be considered to have a certain amount of heat dissipation or ventilation measures, if these parts are damaged, it will cause equipment overheating. In addition, electric furnaces and other electrical equipment that work directly with the heat of the current, the working temperature is relatively high, such as placement or improper use, may cause a fire. 2. electric sparks and arcs General electric sparks are very high temperature, especially the arc, the temperature can be as high as 3,000 ℃ 6,000 ℃, therefore, electric sparks and arcs can not only cause combustible combustion, but also make the metal melting, splashing, constituting a dangerous source of ignition. In places with explosion risk, electric sparks and arcs are a very dangerous factor causing fires and explosions. Electrical sparks include work sparks and accident sparks two categories. Work sparks refers to the normal operation of electrical equipment or generated during normal operation. Such as switch or contactor opening and closing sparks, pins pulled out or inserted when the sparks and so on. Accidental spark is the line or equipment failure occurs. Such as the occurrence of short-circuit or grounding sparks, insulation damage when the flash, wire connection loose sparks, fuse blown sparks, over-voltage discharge sparks, electrostatic sparks, and repair work caused by faulty operation of the sparks and so on. In addition, there are mechanical sparks caused by collision; when the bulb is broken, the hot filament has a dangerous effect similar to sparks.
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