EuroWire November 2015

Technical article

3.3.2 Flame Propagation Cables are connecting devices. So cables carry the threat that a fire may propagate along the cable from one building part into another. This is the effect of a fuse cord. To determine the flame propagation (or fuse cord) properties of cables, IEC 60332 defines test methods on several levels (eg IEC 60332-1-2, Figure 3 ). The common idea of all these tests is the same: a burning cable in a defined position shall extinguish before the flame has propagated a specified distance. The sample position may be horizontal or vertical, and the sample may be a single cable or a cable bundle. Flame propagation parameters are related to both fire avoidance and impact reduction. 3.3.3 Fire Resistance Especially for cables used in fire protection application there are specific fire resistance requirements defined in IEC 60331. This means a cable in a fire shall maintain its function at least for a defined time. These cable types are used, as an example, for fire exit lighting, alarm and warning devices and similar purposes. The test purpose is to prove that no short circuit appears when a defined burner flame works on the cable for the required time. Fire resistance is more an aspect of impact reduction. 3.3.4 Smoke Exhaustion The quantity of smoke and fume is an important indicator. Smoke reduces the sight of escaping people and rescue teams, thus the translucence of smoke emissions is tested according to IEC 61034. A high quantity of exhaustion as well as very dense fumes reduces the light transfer. The reduction of fire impacts is clearly the purpose of smoke exhaustion parameters. 3.3.5 Absence of Halogens There are different test methods described in IEC 60754: to detect the acidity of smoke the quantity of halogen carboxylic acid is determined (IEC 60754-1). The electrical conductivity of smoke indicates the quantity of acid radicals. This is to be tested according to IEC 60754-2. Another test procedure in this standard is the toxicity of smoke, measured by the pH value which indicates the acid content of the smoke in a liquid solution.

This paper does not dive into statistics to investigate the probability of failure nor into economic sciences to quantify financial impacts of any damages. The focus here is the fire protection strategy concerning in-house cabling. 3.3 Cable Fire Performance Cables are important elements in fire protection concepts of buildings. There are different kinds of ingredients which increase the fire performance of cable compounds. Halogenated polymers are self-extinguishing by chemical reactions, but in case of fire they generate toxic gases. Halogens are elements of the 7 th principal group: Cl, Fl, Br, J. In the oxidation process they react to acid radicals, which generate acids by reaction with hydrogen. When halogens are burning at low temperatures, dioxins are generated. The consequences of personal injury or damage of goods are described above. , prevent fire propagation by catching oxygen. The chemical reaction generates water which gives an additional extinguishing and cooling effect. These mineralic flame retardants generate very little smoke when burning, and the fumes are non-toxic and contain no acids. But this class of material is not the perfect solution, either. To achieve a very good fire performance one must use these ingredients in higher concentration. This reduces the mechanical performance of that cable, causes embrittlements or reduces the operating temperature range. Several cable fire-testing procedures are defined by national and international standardisation bodies. Each of them alone represents just one of the different threats of fire. An overview is given in Table 1 . 3.3.1 Self Ignition Cables should be designed in a proper way so that neither voltage peaks nor high ampacity should lead to self- ignition. Voltage and ampacity testing determines the ability of a cable regarding self-ignition. The parameter self-ignition is connected to the probability of fire. Halogen-free flame retardants, for example Mg(OH) 2 or Al(OH) 3

▲ ▲ Figure 3 : Test set-up flame propagation

All the tests and parameters to prove a cable to be halogen free are aspects of impact reduction, too. 3.4 Cable Construction Aspects In cable construction a lot of parameters affect the cable fire performance. The selection of materials is of main importance. Thus Table 2 gives an overview for some common compounds for cable insulation and jacketing regarding the fire characteristics. This deals with the basic material. Of course compound engineering improves con- tinuously and by use of specific additives there are materials of the same family available with far better fire performance. Nevertheless we should remain realistic and keep in mind that there will never be the perfect material. The addition of mineralic fire retardants keeps the material halogen free and reduces flame propagation, but it also reduces mechanical properties such as elongation and elasticity. But not only material affects the fire performance of cables. A lot of detailed construction parameters are important. So for example the tightness of a jacket should be taken into account. Interstice filling jackets provide more combustive material to a fire, but they prevent the air flow inside the interstices and reduce the oxygen available to the flame. A jacket extruded as a tube has a similar effect as a funnel when the cable is burning, especially in vertical fire tests.

▼ ▼ Table 1 : Fire performance parameters and their correlation to fire risk elements

Parameter Self-ignition

Avoidance

Impact reduction

4 Fire Protection Strategies

X X

– X X X X

Flame propagation

Fire resistance

(X)

Fire protection is not only a cabling issue. There must be a general fire protection concept regarding all construction elements of a building.

Smoke exhaustion

– –

Halogen free

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November 2015