There are a large number of electrical equipment control cabinets in the power, metallurgy, petrochemical and other industries. These control facilities are arranged in an array in the control room. There are a large number of printed circuit boards (PCB boards) and cables in the cabinet. Although most of the PCB board substrates currently used are FR-4 type, the main component is brominated epoxy resin impregnated alkali-free fiberglass cloth, and the flame retardant grade reaches UL94V 0 level 1121. However, for various reasons, the control cabinet is inside. Cable fire accidents have occurred. The hot flue gas generated by the cable fire will cause pyrolysis of components on the PCB circuit board and the circuit board, and various toxic and corrosive gases will be diffused into the room through the ventilation grille at the top of the control cabinet, and the corrosion is not threatened by fire. The equipment is not working properly, and under certain conditions, the open flame will spread to the outside of the control cabinet, igniting the adjacent control cabinet, and the local fire develops into a global fire.
Since such places are usually unattended or unattended, the number of casualties is lower than that of civil buildings and public places, but it is caused by the interruption of production and commercial operation compared with the direct economic loss caused by the burning of a large number of equipment. The indirect economic losses are even more devastating. Norman Alvares.L.Lee SchultzNeilO.KeshkiRahkonen and others have conducted research on the causes and spread of cable fires in communication systems, petrochemical enterprises and nuclear power plants. Mans et al. conducted a full-scale simulation test of electrical equipment control cabinet fires. 71. All of the above studies are based on qualitative research, so it is necessary to carry out quantitative research on cable fires in electrical equipment control cabinets in order to achieve early division of fires in such places. Alarms and fast and effective fire suppression provide a more effective basis.
1, every fire.
Table 2 shows typical flame heights as shown.
(a) Ventilation rate of 0.34m3/min (b) Ventilation rate of 0.17m3/min Under different air exchange rate conditions, cable fire spread temperature curve flame height changes with time using the flame front to appear signal peaks after passing two temperature measurement points The time interval and its average distance As, combined with the flame height captured by the camera, the average speed of the flame spread can be obtained by calculation. Finally, the average speed of several measurement intervals is taken as the average flame spread speed. Table 3 shows the different working conditions. Lower average flame spread speed.
Table 3: Average flame spread speed under different air exchange rate conditions cm/s air exchange rate 0 The electrical equipment control cabinet applies a certain air exchange rate mainly for the purpose of heat dissipation, and the air flow rate per unit area is low, at 0.4m3/min (for For example, gas 1 time/min), the air flow rate is 0.02 m/s, so the change of the gas exchange rate has no obvious effect on the fire propagation speed.
2.2 The temperature change law of the central part of the control cabinet under different air exchange rates is as shown. Compared with the fire spread rate, the air exchange rate is increased, the convective heat transfer in the control cabinet is enhanced, and the temperature at the upper end of the axis is obviously increased. 9 thermocouples The measured maximum temperature exceeds 400C, at which temperature the PCB will produce a very strong pyrolysis phenomenon, causing the control device to collapse. The CO generation rate and O2 consumption rate at the top of the control cabinet are directly proportional to the ventilation rate. When the CO reaches the peak and the O2 consumption reaches the maximum, the open flame of the cable in the control cabinet is extinguished (see). The reason is the fire during forced ventilation. The speed of the smoke moving upwards is accelerated, and the cable fire is a typical Class A fire. Although the open flame has been extinguished, the possibility of internal smoldering cannot be ruled out, and then the flue gas is discharged through the ventilation grille at the top of the control cabinet, and there is a new one. The oxygen concentration is gradually reduced, and the O2 concentration is gradually increased until the normal value.
Although the cable fire in the electrical equipment control cabinet is relatively slow, the hot flue gas generated by the fire will cause strong pyrolysis of the rcB circuit board in the cabinet, causing the whole device to collapse, and the flame will be pulled out through the ventilation grille at the top of the control cabinet. Burning adjacent equipment and cable trays, local fires can easily develop into a global fire.
Under the common gas exchange rate in the industry, the speed of cable fire spread does not change substantially, but the temperature field in the electronic equipment control cabinet changes greatly. The higher the air exchange rate, the faster the temperature rises in the central control cabinet, and the harm to electronic equipment. The larger the volume, the greater the amount of toxic gas produced.
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