Summary
Cable fire risk analysis is important for fire protection design in nuclear power plants, where multiple horizontal cable trays are mostly located on the walls. Fire experiments using three cable trays with different cable arrangements were conducted in a confined room to investigate the burning behavior of a cable tray on a wall. A corner was formed by the side wall and the cable tray. Hot smoke emitted from the burning cable was trapped in the corner and then ignited the cable on the bottom surface of the upper cable tray. It is found that for cables densely packed together, spread of flame on the bottom surface of cable tray was clearly observed and increased the mass loss of cable burning during the growth stage of a cable tray fire. For cables arranged further apart, vertical propagation from the bottom tray to the top tray was fast and dominated the mass loss of cable burning.
Electric cable fires in nuclear power plants could be disastrous and have to be studied carefully for safety and economic considerations. Based on the results of previous work on large-scale and bench-scale cable fire testing, the Flame Spread over Horizontal Cable Trays model was modified and improved to estimate the heat release rate of large-scale cable fires using bench-scale measured data. The heat release rate per unit area measured in the cone calorimeter experiment is taken as the input, to avoid introducing any prediction uncertainties caused by inconsistent values of the heat of combustion and char yield of the cable. Cable fire experiments with vertical stacks of trays with one to three layers of cables were conducted in open space to assess the accuracy of the improved model. In comparing with the experimental results, predictions using the improved model are encouraging. The local error of prediction is less than 15% and the global error lies between 19.2% and 35.7%. In addition, three cable tray fire experiments with data available in the literature were used to validate the improved model. It is shown that the improved model had good predictions for these cable tray fires.
Fire hazard analysis of multiple-layer cable tray is an important part of nuclear safety analysis. Large-scale cable fire experiments with a three-layer horizontal cable tray were conducted in a closed compartment. The vertical temperature profile in the middle of the room was acquired. Distinctive stratification phenomena were found in the vertical temperature distribution. The interface between the upper and lower layer was located at approximately the height of the top cable layer. Heat transfer between the smoke and compartment walls occurred mainly above the smoke interface. A modified non-steady energy balance model in a closed compartment which included the effect of smoke interface height was used to estimate the compartment temperatures. Compared with the experimental results, the modified model for the multiple-layer cable tray fire in a closed compartment provides better estimation than the original model.
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