Cause analysis of fire caused by contact heating o

2022-08-13
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Cause analysis of fire caused by contact heating of electrical equipment

electric contact heating is one of the important causes of electrical fire, which is more common in fire accident investigation. There are many reasons for the formation of electrical contact traces. Only by thoroughly investigating the causes of the formation of electrical contact traces can we make a scientific and meaningful conclusion

the electric contact Joule heat that can cause fire is caused by the thermal effect of current, which releases heat according to the law of I2R. The formula of Joule heat shows that heating is mainly composed of two factors, namely current and resistance, which are electrical physical quantities. After the electric contact weld mark is found in the on-site investigation, the cause of the formation of the weld mark and the objective conditions should be further found. A conclusion can be made only when the trace phenomenon and the cause of the trace formation are basically not contradictory and meet the objective conditions. For example, in a fire investigation, it was found that half of the two plug-ins on the plug were melted, the entire socket, wooden slot board and wire were burned, and the two nearby instruments were also burned. From the trace, it can be concluded that the plug was in poor contact, the contact resistance was too large, and the fire was caused by overheating. However, in the search, it was found that the power line was connected to the voltage regulator, and the choke in the voltage regulator was burned. At the same time, it was found that the choke was overheated and short circuited due to an electronic tube fault, and the abnormal current of the short circuit excited the pin that was already in a seriously deteriorated state to overheat and catch fire. In this case, the fire caused by the heating and melting of the plug and socket contact is determined by one or both of the current and resistance factors. In the actual

exploration process, this trace material evidence is often attributed to poor contact of plugs and sockets, or excessive contact resistance and overheating, while ignoring the factor of current. In this case, the melting trace of the plug is the result of the high current of the short circuit. Therefore, the trace material evidence found in the scene cannot be easily concluded. A comprehensive and detailed investigation must be carried out to find out the reason for the automatic strain result by dividing the length change of the formed sample by its original length, so as to draw an objective and correct conclusion

the factors affecting the heating of contact resistance are analyzed below:

1 contact resistance

contact resistance RJ is composed of two parts, namely shrinkage resistance Rs and surface facial mask resistance RB. Shrinkage resistance refers to the phenomenon that when the current flows through the electrical contact area, it suddenly turns from the conductor with large cross-section to the contact point with small cross-section. Taking PET bottles as an example, the current shrinks violently. The additional resistance presented by this phenomenon is called shrinkage resistance. The surface facial mask resistance is that the contact surface of electrical contact is covered with a layer of substances with poor conductivity due to pollution, which is another part of the contact resistance - film resistance. Many on-site investigators' habits of burning plugs and sockets can be attributed to poor contact and excessive contact resistance. In fact, there are many reasons for the increase of contact resistance

1.1 contact form

the form of contact resistance can be divided into three categories: point contact, line contact and surface contact. The influence of contact form on the shrinkage resistance Rs is mainly reflected in the number of contact points. In general, the number of contact points n of surface contact is the largest and RS is the smallest; For point contact, n is the smallest and RS is the largest; Line contact is in between. The influence of contact form on membrane resistance RB mainly depends on the pressure f borne by each contact point. Generally, when the applied pressure f on the contact is the same, the point contact form n is the smallest, and the pressure F1 per unit area is the largest, which is easy to damage the surface facial mask, so it is possible to minimize RB; On the contrary, F1 of surface contact is the smallest, the destructive force to RB is the smallest, and the value of Rb is likely to be the largest. In practice, it is necessary to integrate the above two factors to make a specific analysis and judgment on the size of contact resistance

1.2 contact pressure

contact pressure f has the greatest influence on the shrinkage resistance Rs value and surface facial mask resistance RB value. The increase of F increases the effective contact area of the contact point, that is, the number of contact points n increases, thus reducing rs. When increasing f exceeds a certain value, the number of gas molecular layer adsorption films on the contact surface can be reduced to 2 ~ 3; When the yield pressure of the material is exceeded, plastic deformation occurs, and the surface facial mask is crushed and cracks appear, thus increasing the contact area. This makes the shrinkage resistance Rs decrease due to the reduction of the surface facial mask resistance Rb, and RS and Rb decrease at the same time, so that the contact resistance is greatly reduced. On the contrary, when the contact pressure f decreases due to poor contact and the loss of elastic deformation of the contact contact contact, the contact area decreases, the shrinkage resistance Rs increases, and the surface facial mask resistance RB is weakened or not affected by F, so that the surface facial mask resistance RB increases. At the same time, due to the increase of Rb, the contact area is reduced, so that RJ is increased. The comprehensive monthly pricing of the two in South China has been gradually introduced, which makes the contact resistance rise as a whole

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