Fire properties of electric cables used in buildings

A significant number of cables of different materials and construction is used extensively in building objects increasing their fire load and, therefore, strongly influencing safety in the case of fire. Electric cables and electrical installations constructed from them, despite being important elements of fire safety, are not considered in the general analysis of the fire safety of buil-dings and are usually not assessed as potential fire risks. One of the tasks in the field of counteracting fire hazards in buildings should be to reduce the risk of fires caused by short circuits in electrical installations. In the event of fire, the process of fire propagation involving electric cables should be considered, which, due to the way cables are installed in buildings, can transfer fire over long distances from the origin of the fire and across storeys through installation shafts. The scientific problem of the doctoral dissertation is to determine the impact of significant constructional and material parameters of electric cables on their fire properties by establishing the qualitative, and possibly also quan-titative, relationships between these parameters. The aim of the presented work was to investigate the effect of material and constructional parameters on the fire properties of electrical cables, such as heat release, smoke generation, range of flame spread and amount of toxic fire effluents under various ventilation conditions. To the best of the author’s knowledge, such systematic research has not been published so far. The presented study is original and fills the scientific gap regarding the constructional and material parameters of electric wires and cables, which influence their fire properties. In order to investigate the relationship between the constructional–mate-rial parameters of cables and their fire properties, eighty-three cables (eighty-nine cable samples) were examined by means of a standard experi-mental method. The selection of cable samples included the presence of one distinctive parameter. The conclusions drawn from the experiments were as follows: (1) construction, the number of conductors and the presence of armour or concentric metallic conductors improve fire properties by forming a barrier against flame penetration through the cable; (2) the use of copper conductors resulted in decreasing fire parameters compared to cables with aluminium conductors (maximum average heat release rate parameter almost four times lower for copper cables); (3) construction materials based on plasticised poly(vinyl chloride) significantly reduce the fire properties of cables compared to halogen-free materials (maximum average heat release rate parameter more than 17 times higher for fully halogenated cables), which is due to the decomposition process of the material; (4) no clear relationship between the fire parameters and cable parameter χ was found. The χ para-meter was developed to improve the monotonicity of the reaction to the fire test results obtained and has been used in the selection of cable samples for testing within the same cable family so far. As the investigation showed a significant impact of the number of metallic barriers (conductors) on flame penetration into the inner layers of electric cables and the volume of non-metallic materials on the fire properties of cables, a parameter related to the volume of effective non-combustible content Ω was proposed. The new Ω parameter depends on the non-metallic non-combustible components volume to non-metallic combustible components volume ratio and the effective area of heat transfer within the cable. Increasing Spearman’s correlation factors (close to −1) were obtained for total heat release rate and total smoke generation parameters as a function of the Ω parameter rather than the χ parameter. In order to examine the amount of fire/combustion gases under ventilation-controlled fires, a simple poly(vinyl chloride)-based copper electric wire widely used in buildings was studied by means of a Steady State Tube Furnace. A reference pure polymer unplasticised poly(vinyl chloride) and additionally pure low-density polyethylene were also tested. Decreasing carbon dioxide yields at different ventilation conditions for the poly(vinyl chloride) based copper electric wire were obtained in comparison to three times higher yields for pure poly(vinyl chloride) and two times for low-density polyethy-lene than those received for the tested wire at the same ventilation conditions, which confirms the insignificant contribution of the hyperventilation effect to humans during a cable fire. To the contrary, four times higher values of toxic carbon monoxide yields were obtained for the tested wire rather than for the reference polymer and pure low-density polyethylene. The maxi-mum value of carbon monoxide yield (0.57 g/g) was obtained in the case of 5 l/min of primary airflow, which decreased with increasing ventilation. The measured yields of hydrocarbons were similar to the reference values except for the equivalence ratio = 0.27. The corrosive and toxic hydrochloric acid occurring in fire effluents from the studied wire was independent of the ventilation conditions tested. The reaction between copper and the hydrochloric acid compound, inorganic fillers, and hydrochloric acid decreased the hydrochloric acid content in fire effluents. Analysis using Quintiere's theory showed that the cone calorimeter me-thod can be used in numerical modelling of the cable burning process, and its use can significantly facilitate and reduce the number of cable fire tests, without adversely affecting the final results. Summarising, the analysis of the impact of cable construction is an important element from the point of view of the fire safety of buildings. In the course of the study, it was found that factors such as the shape, number and material used for conductor formulation, as well as the types of mate-rials from which the non-metallic elements of the cables are made, and the presence of armour or concentric conductors significantly influence the fire properties of cables, such as heat release, smoke generation, range of fire spread and fire effluent toxicity, largely reducing the fire properties of cables. The experiments enabled the development of a new cable parameter Ω, which is a better predictive indicator of cable flammability than the standard and commonly used parameter χ. In addition, it was found that it is possible to replace large geometric scale fire tests with a simpler cone calorimeter method by applying Quintiere’s theory of electric cables.