Fire Hazard in a Room Due to a Fire Starting in a Plenum

Abstract

An issue of interest is the contribution to fire hazard in a room from products in a plenum space above it. This contribution can result from two scenarios: fire in the room or fire in the plenum. The products being addressed here are PVC electrical products contained in a plenum. The first scenario involves a fire starting in the room. Three room dimensions and two ceiling materials were analysed; the products were PVC conduit (rigid and ENMT, semi-rigid; 100 m of either) and PVC wire coating (400 m). It was found that the amount of energy needed for the room fire to cause thermal decomposition of the PVC products in the plenum was larger than that needed to take the room to flashover. Furthermore, if the PVC products did eventually decompose or burn, somehow, they would cause a lethal smoke concentration only significantly later than a lethal (by toxicity) atmosphere had already been created by the fire itself. Thus, the PVC products did not add any significant fire hazard to that caused by the room fire. The next scenario is more complex: it involves a fire starting in a plenum and has been analysed only for wire coating. Calculations were made, for many fire scenarios, in which the fire hazard model F.A.S.T. was used to simulate hazard to occupants of a standard room following a fire starting in a plenum above it. A total of 400 m of PVC wire coating was assumed to be present in the plenum. Its decomposition was made a function of the plenum temperature achieved. The fire ranged between RHR of 50 and 500 kW; the heat of combustion of the fire varied between a standard value (20 kJ/g) and that of methane (57 kJ/g). Various vent connections between compartments and surroundings were used. The plenum temperature was never enough to decompose all the PVC wire coating. If the plenum was vented to the surroundings, almost no smoke entered the room. In an unvented plenum smoke entered the room but the fire burnt for a short period only: the level of oxygen was not enough for full combustion. In extreme cases the fire generated enough heat for an untenable atmosphere in the room. In almost all single plenum cases studied the smoke flowing into the room was insufficient to generate a concentration lethal to man. Therefore, using such low heat release PVC wire coating products did not cause a significant increase in the fire hazard to occupants.