Protection of electrical fire caused by the hottes

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Protection against electrical fire caused by ground fault

Abstract: the newly released GB "installation and operation of residual current operated protective devices" emphasizes that the installation of residual current operated protective devices not only plays a protective role in preventing personal electric shock accidents, but also plays an important protective role in preventing electrical fire accidents caused by ground fault

key words: grounding fault electrical fire protection the newly released GB "installation and operation of residual current operated protective devices" emphasizes that the installation of residual current operated protective devices not only plays a protective role in preventing personal electric shock accidents, but also plays an important role in preventing electrical fire accidents caused by grounding faults. The introduction, 3.26 in terms and definitions and 4.4.5 in hierarchical protection in the standard all emphasize the role of installing residual current protection devices in preventing electrical fires caused by grounding faults, and clearly stipulate the installation of residual current operated fire monitoring devices and action parameters in buildings, In the appendix, the mode diagram of installing residual current operated fire monitoring device in the way of removing obstacles in the pendulum strike circle of the shelving wood machine with graded protection is listed. Based on the reality, the revised standard puts forward that the overcurrent protection cannot protect the electrical fire caused by the grounding fault due to the small fault current when the grounding fault occurs, and the residual current protection device makes up for this deficiency. 1. The multiple occurrence of electrical fire accidents in recent 10 years, the occurrence of electrical fire in China ranks first in the total number of fire accidents, accounting for about 30% of the total. In the electrical fire, the fire accidents caused by electrical short circuit account for more than half. There are two kinds of electrical short circuits: one is the direct contact between conductors, such as the short circuit between phases and between phases and N lines. The short circuit point is often short circuited by the metal of high-temperature fusion welding, which is called metallic short circuit; The other is short circuit of charged conductor to ground, which is an arc short circuit with arc as the path. The short-circuit current of the former is calculated by several kiloamperes, the metal core produces high temperature and even incandescence, and the insulation is violently oxidized and spontaneous combustion. The fire risk is very large, but the large short-circuit current generated by the metal short-circuit can make the circuit breaker act instantaneously to cut off the power supply, and the fire is often avoided. In the latter, because the short-circuit current is affected by impedance, the arc lasts for a long time, and the local temperature caused by the arc can be as high as 3000~4000 ℃, which is easy to burn nearby combustible substances and cause a fire. However, due to the small short-circuit current caused by the grounding fault, it is not enough to trip the general circuit breaker and cut off the power supply, so the fire risk caused by the arc short circuit is much greater than the metal short circuit. 2 danger of grounding fault in the fire caused by short circuit of electrical circuit, the fire caused by grounding fault arc is far more than that caused by metal short circuit between live conductors. Firstly, this is because the probability of arcing grounding fault is much greater than that of short circuit between live conductors. Once a ground fault occurs, the probability of dangerous arc caused by it is much greater than that between live conductors, which can be illustrated in Figure 1. In the figure, a, B, C and D are the connection terminals of phase line, neutral line and PE line respectively. a. B if the two terminals are poorly connected or non-conductive, the equipment will not operate or operate abnormally, which can be detected and repaired in time to avoid accidents. However, the non-conductive or poor conductivity of terminals C and D of PE line is not easy to detect, because the equipment can still operate as usual, at this time, the poor connection of terminals C and D will continue to exist as a potential accident hazard. In case of grounding fault of the shell as shown in Figure 1, if the C and D terminals are not conductive, the equipment shell will cause an electric shock accident due to the voltage to the ground phase. If the C and D terminals are poorly conductive, electric sparks or arcs (continuous and concentrated electric sparks are electric arcs) will burst out at the terminals, which is easy to cause fire. Therefore, in the TN-C-S system where the grounding fault circuit is all metal conductors, the poor conductivity and loss of grounding protection will not affect the normal operation of electrical equipment, so it is not easy to find. However, once a ground fault occurs, the impedance of the connection point limits the short-circuit current and cannot make the circuit breaker act. The occurrence of the above arc short circuit is not only due to technical advantages. As for TT system, the grounding protection of power supply and the grounding protection of equipment shell are connected in series in the grounding fault circuit. The two grounding resistances cause great impedance of the circuit itself, which is more prone to arc short circuit. It can be seen that the circuit impedance of grounding fault is large, which makes it easy to appear in the form of arc short circuit, which is also an important reason why single-phase grounding short circuit fault is easy to cause fire. Short circuit occurs in the power line due to mechanical damage. For example, when the conductor contacts with metal pipe components without casing protection, the insulation is damaged due to long-term friction. This short circuit is mostly caused by single-phase grounding fault, which is prone to arc short circuit. Generally, arcing grounding faults caused by insulation damage of electrical equipment include: aging of insulation of wires and electrical equipment; The insulation around the wiring terminals of electrical appliances or motors is carbonized due to long-term heating; Motor overload and turn to turn short circuit; Electrical equipment is affected by moisture or serious condensation; There are conductive dust deposits in electrical equipment. This kind of fault will cause grounding arc short circuit and cause fire. In this case, the heating power generated by the leakage current is about 60~100 W, and the price plunges due to the leap in production capacity. Lithium diaphragm enterprises have to fight for technology to break through. If this power is released in a few square millimeters, as long as there are combustible materials around, it will cause a fire. When the insulation temperature of the line exceeds the maximum allowable working temperature due to overload, and the insulation aging accelerates, the insulation level drops below the specified value. If there is no external cause to trigger, the short circuit will generally not occur. If there are external triggers, such as transient overvoltage caused by lightning, operating overvoltage of adjacent high-power equipment, and transient overvoltage caused by grounding fault at the high-voltage side of the substation, the aging insulation will be broken down under this large value overvoltage impulse to form an arc short circuit. The overvoltage disappears in an instant, but the power frequency short-circuit arc can last for a long time, because the high impedance of the arc limits the short-circuit current, making it impossible for the circuit breaker to act. This kind of overvoltage mostly occurs between the live conductor and the ground, so this kind of short circuit is also mostly single-phase grounding short circuit. Electrical short circuit is mostly single-phase grounding fault, and arc grounding is the most serious risk of electrical fire. It should also be noted that the impedance of the ground fault circuit is large, which is an important reason for the formation of arc short circuit. No matter TN system or TT system, the impedance of grounding fault circuit is greater than that of short-circuit circuit circuit of electric conductor in the industry that the State encourages to develop

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