A Combined Experimental And Zone Modelling Study Of Multi-layer Cable Tray Fires In A Confined Space

Electrical cable is one of the essential equipments in industrial production and daily life. However, cable has been responsible for lots of important fires over the world due to its combustible materials. If cable fire occurs at a nuclear power plant (NPP), more serious damage will be caused like the failure of safety shutdown, explosion of hydrogen and nuclear accident. Actually, cable fire is attracting more and more attentions. In the present study, the fire behavior of cables in horizontally elevated trays in a confined space was investigated by combining experimental method and computational simulation.The experimental work began with testing the fire spreading characteristics of two types of commonly used cables (ordinary cable and retardant cable). Then the fires of one to four vertical stacks of trays were conducted in order to study the effects of ventilation and multi-layer structure. Results show that, ventilation within the fire compartment can affect the burning of cables significantly. And the burning rates are not proportional to the number of cable trays because of the interactions of each layer’s combustion. According to the measured vertical temperature distribution, the fire compartment can be roughly divided into an upper hot zone and a lower cool zone, which is consistent with the basic assumption of two-zone model.The zone fire model CFAST (Consolidated Model of Fire Growth and Smoke Transport) was used to predict the fires of multi-layer cable trays. Generally, the predictions by CFAST show good agreement with experimental data. However, CFAST underestimates the temperatures in the fire scenario of four-layer trays under no ventilation. The lower oxygen limit used in HRR model accounts for these under-prediction. Then predictions using a modified coefficient show reasonable agreement with measurements.Based on the model validation, fire risks within a typical cable spreading room(CSR) of NPP were analyzed via several parameters, including smoke layer height and temperature, HRR of cables as well as fire lasting time. Ventilation within the CSR, such as the opening of the door and the mechanical ventilation system were considered. Finally, the sensitivity of the cable ignition time to the input variables was calculated in terms of the rank order correlations by combining monte carlo simulation with CFAST. The distance from fire source to cable has the strongest effect on the ignition time. All of these findings provided us a more comprehensive understanding about the cable tray fire and would be useful to fire protection.