Fire Safety Journal 43 (2008) 531–540 On the significance of transient heat release rate excursions above a set limit T. Ohlemiller à , R. Peacock Building and Fire Research Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Received 18 January 2006; received in revised form 6 July 2007; accepted 8 December 2007 Available online 5 February 2008 Abstract When a heat release rate limit for a consumer product is set by a regulatory agency, it is of interest to know whether small excursions above that limit, such as may occur due to production line variability, represent a disproportionate increase in fire hazard. This paper presents a methodology to examine this issue. The heat release rate curve of the object is described by a Gaussian time variation; a perturbation peak, also Gaussian, is added to this main peak. The impacts of the perturbation peak on the build up of hazardous conditions in a room fire (where the object is the only item burning) and on the threat of ignition of secondary items are examined. For the peak heat release rate domain studied here, only the ignition threat is significantly affected by the perturbation peak. The results quantify the trade-off between the height of the perturbation peak and its duration for a fixed percentage of increase in the room area threatened by secondary object ignition. The results show that the increased threat is of the same order as the relative perturbation in heat release rate. Published by Elsevier Ltd. Keywords: Furnishings; Heat release rate; Ignition 1. Introduction It is a common practice among various regulatory bodies to set an upper limit on the acceptable heat release rate (HRR) of various objects as assessed in some appropriate flammability test method. Thus, for example, the Califor- nia Bureau of Home Furnishings (CBHF), in California Technical Bulletin 133, sets an upper limit of 80 kW for the HRR of chairs to be used in public occupancies. The US Consumer Product Safety Commission has recently set an upper limit of 200 kW for mattress/foundation sets used in residences (CFR 1633). These test methods apply one or more gas burners to the surface of the object to assess its HRR response, which is then measured by oxygen consumption calorimetry. It is soft furnishing items such as beds and furniture, which are involved in a substantial fraction of the annual fire deaths in the United States; thus these items are a principal concern of the present study. However, the results should apply more broadly to situations where similar HRR limits are prescribed. Subsequent to localized ignition by the gas burner(s), flames spread over and into the structure of the test object increasing the area that is burning and the overall HRR until, at some point, fuel consumption begins to cut back on the overall burning rate and the HRR declines, heading ultimately to zero. Thus, the burning process is inherently transient and the HRR behavior involves one or more substantial peaks. For various furniture items tested in Ref. [1], the burning durations ranged up to 20 min or more, but the time above a HRR half the peak value was more like 2–5 min. For bed assemblies (with ignition of the bed clothes) based on designs aimed at passage of the CFR 1633 criterion, the time above the half peak value is of the order of 5 min, though here there tends to be two peaks of such duration [2]. It is the highest peak HRR value seen over some test interval, e.g., 30 min, that is required to be no greater than an upper limit value such as one of those mentioned above. ARTICLE IN PRESS www.elsevier.com/locate/firesaf 0379-7112/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.firesaf.2007.12.007 à Corresponding author. E-mail address: Thomas.ohlemiller@nist.gov (T. Ohlemiller).