Effect of Wall Characteristics on Thermal Exposure in Fire Resistance Tests of Power Cable, 01-R9851Printer Friendly Version
Inclusive Dates: 09/08/08 01/08/09
Background - Many building components must undergo standardized fire tests for code approval. These tests are often performed on assemblies or elements that are exposed to a fire in a large-scale furnace. Some components must endure a more severe fire test exposure if they are to be used in certain locations where a more severe fire may occur, such as in an offshore oil rig. Components that are crucial to maintaining control of a fire, such as power cables and suppression system controls, must be subjected to this severe "hydrocarbon fire" exposure. With this in mind, the Insulated Conductors Committee (ICC), part of the Institute of Electrical and Electronic Engineers (IEEE), set forth to create a fire test standard for power cables to supplement other IEEE electrical tests and requirements for power cables. Currently, only a single test method exists, UL 2196, Tests for Fire Resistive Cables. This method requires the use of an unnecessarily large test specimen and has inherent flaws in the procedure that make it virtually impossible for a test laboratory to perform. SwRI sought to provide technical input for the ICC to help develop a repeatable and realistic standardized test method for the power cable industry with the intention of obtaining more business in this market.
Approach - To demonstrate the flaws in the UL standard, SwRI constructed two wall assemblies that acted as representative base assemblies that a power cable would be mounted to during a standardized fire resistance furnace test. SwRI performed a fire resistance test on each wall using the hydrocarbon fire exposure. The required hydrocarbon exposure conditions were unreachable in either test. SwRI therefore recommended to the ICC that a smaller scale sample should be tested. A smaller test furnace could then be used with the smaller sample, and the hydrocarbon fire exposure could be recreated in the smaller furnace environment. In this process, SwRI collected valuable data that characterizes the furnace environment using standard probes such as radiometers and thermocouples and more advanced probes such as differential flame thermometers and plate thermometers. This information can be used to validate boundary conditions in transient heat transfer models.
Accomplishments - SwRI performed full-scale fire resistance testing on a gypsum-protected, steel stud wall and a concrete masonry unit block wall. Data was collected and subsequently presented at the biennial ICC meeting in San Antonio. The draft test method for fire testing of power cables was subsequently altered to incorporate a smaller sample size. Temperature and heat flux data from the furnace exposure was collected and may be used in future modeling efforts. The furnace exposure data reinforced a relationship between incident heat flux and wall characteristics such as emissivity of the exposed surface and thermal conductivity, specific heat capacity, and density of the construction materials.