Talk about common electrical fire hazards and thei

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Talk about common hidden dangers of electrical fires and their prevention and elimination

with the development of social economy and the continuous improvement of people's living standards, China's per capita power consumption has increased sharply, but electrical fires have also increased sharply, causing huge losses to the national economy and people's lives and property. It is reported that the occurrence rate of electrical fires in China accounts for about 30% of all fires, ranking first among all fire causes. The causes of electrical fire are complex and diverse. This paper briefly summarizes the hidden dangers and prevention of electrical fire

hidden danger of electrical fire:

1. Electrical fire caused by short circuit

electrical short circuit can be divided into phase to phase short circuit and single-phase grounding short circuit. Phase to phase short circuit can generally produce large short-circuit current, which makes the overcurrent protection device act and cut off the power supply in time, so there is less arc short circuit, so there is less electrical fire. Single phase grounding short circuit can be divided into metal short circuit and arc short circuit. The risk of fire caused by metal short circuit is not great. Mainly because of the large short-circuit current, the overcurrent protection device is equipped with corresponding functional accessories in the short-circuit current to realize the static load of typical concrete structures such as beams, columns, plates, frames (full-scale, models), steel trusses, rubber bearings, fastener assembly, steel strand anchorages, all kinds of springs, riggings, slings, ring connections, steel rails and their welded joints, automobile chassis, automobile front and rear axles, locomotive bogies and various large components Cut off the power supply in a short time under the action of dynamic load performance test and dynamic load fatigue life test. The arcing short circuit breaks out an arc or electric spark due to poor contact at the fault point and is not fused. Because the impedance of the fault point where arcing short circuit occurs is large, its short-circuit current is not large, and the circuit breaker is difficult to act (the fuse will not be blown generally), so that the arc continues to exist. It is measured that the arc temperature generated by a current slightly greater than 0.5A can be as high as 2000 ℃ - 3000 ℃, which is enough to ignite any combustibles, and when the maintaining voltage of the arc is as low as 20V, the arc can still exist continuously and stably, which is difficult to extinguish. This kind of short-circuit arc often becomes the ignition source of electrical fire. Therefore, in industry and agriculture, grounding arc short circuit accounts for more than half of the causes of electrical fire

2. Line high-order harmonics have become a new and important hidden danger of electrical fire

the main reason for harmonics in low-voltage power is due to nonlinear loads. These harmonic currents entering the public power can cause power supply voltage distortion, waveform distortion, loss increase, overload and heating of electrical lines (especially neutral line n), accelerate insulation aging and have fire hazards. The reason for the overload and heating of the neutral line is that in the three-phase balanced load, the components of the third harmonic (9th, 15th harmonic, etc.) in each phase are in phase with each other, which are not offset each other in the neutral line, but superimposed each other. (other positive sequence and negative sequence harmonic components can be offset each other in the neutral line.) the superimposed neutral line current may exceed the phase line current, or even reach nearly twice the neutral line current, Cause the neutral wire to overheat and bury electrical fire hazards. If the three-phase load is unbalanced, the heating of the neutral line will be more serious after adding the unbalanced current. In some places in China, the neutral line section is still used to taking only 1/2 or even 1/3 of the phase line section. If the three-phase load imbalance is serious and there is a large harmonic current, then under the action of the unbalanced current and harmonic current, the neutral wire may be damaged or even burned out, causing the insulation of electrical equipment to be damaged, and it is easy to burn out the single-phase equipment, and even fire

3. Fire caused by aging electrical lines in old buildings

aluminum core wires and cables are widely used in old buildings. The design of electrical lines is too economical, the capacity of lines is low, and the aging of lines is serious, causing a considerable number of fire accidents, which is a very important reason for electrical fires. Since 1999, the national mandatory standard "code for residential design" has clearly required that copper conductors should be used in residential buildings

4. Electrical fire hazards of 10kV power grounded through small resistance

in the electrical transformation, 10kV power lines are largely powered by high-voltage cables, and the lines are generally long, which makes the capacitive current of the lines continuously increase. Therefore, urban 10kV power supply increasingly changes the past neutral ungrounded system (or grounded through arc suppression coil) to a small resistance (high current) grounding system, although it has many advantages, But there are also hidden dangers. The ground fault current of the original ungrounded system is the capacitive current of the normal phase, and the ground fault voltage is only about 100 volts. After changing to a small resistance grounding system, generally, the selection of small grounding resistance is small, and the current flowing through the grounding point is hundreds of amps. Generally, the following types of ground fault transient overvoltage can reach hundreds of volts or even 1-2kv for the sample 1 of tensile test. If a 10KV grounding fault occurs in the 10/0.4kv substation, and the neutral point on the low-voltage side of the transformer uses a grounding body together with the transformer shell and 10kv High-voltage Distribution cabinet, the fault voltage will be transmitted to users using protective neutral connection along the pen line or PE line. The outer shell of the low-voltage electrical equipment may produce contact voltage, endangering personal safety, and the insulation of the low-voltage equipment, especially the insulation of the old equipment, will not withstand such a high overvoltage, It is easy to be broken down and short circuited, resulting in fire hazard, which is also one of the negative effects of the development of electrical technology

5. Fire hazards due to poor design considerations and poor construction quality

in recent years, due to the extensive use of air conditioning in summer, the electrical line section of some buildings is too small, the design capacity is too low to bear the burden, frequent tripping, and more serious is the long-term overload of electrical lines, resulting in the decline of insulation, which has become a difficult fire hazard to deal with. Except for the small section of the designed line, China has not formulated national standards for the current carrying capacity of wires and cables so far. For example, the current carrying capacity of 2.5mm2 copper core plastic wire in IEC standard is 26a, while some data in China are taken as 30~32a, which is more than 20% higher than IEC standard. However, the design does not consider the reduction of current carrying capacity caused by heating when multiple conductors are laid in concealed pipes. These factors make the selected line section smaller and leave hidden dangers for future use. During the construction of the project, the non-standard installation of electrical lines, poor construction technology, inaccurate wire connection, poor contact, insulation scratch and so on are also an important reason for electrical fires. In particular, the connection quality of the neutral line is poor. If the neutral line is broken, it is easy to damage the insulation of the equipment, causing the single-phase equipment to burn out, and even fire. In addition, for such advanced design and manufacturing methods, high-power lamps should be well insulated and fireproof

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