WCA January 2016

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. 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 continuously 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. 4 Fire Protection Strategies Fire protection is not only a cabling issue. There must be a general fire protection concept regarding all construction elements of a building. This need is taken into account in the European Constructive Products Regulation [6] . We have seen there are two aspects of fire protection: fire avoidance and reduction of fire impacts. How do these aspects correlate to the regional differences in fire protection strategies?

Flame retardant

Low smoke

Halogen free

Material

PVC

X

–

No

Polyolefin

– – – –

X X X X

Yes

Polyurethane

? ?

TPE

– +

FRNC

Yes

Fluorpolymer No! ❍ ❍ Table 2 : Typical fire performance properties of cable compounds 4.1 Fire Avoidance The common fire protection philosophy in America is to prevent fire at any cost. There are high amounts invested into research and investigation and the results are very challenging regulations regarding the fire performance of indoor cabling in terms of self ignition, flame propagation and fire resistance. To meet these requirements there is no other way than to use halogens as flame retardants in cable compounds, as well as in building materials. This approach risks the threat of personal injury by acid fumes and of emergency exits being hidden by dense smoke. 4.2 Reduction of Fire Impacts It seems to be complete nonsense to reduce potential fire impact but to do nothing to avoid fire. And it really is, because there are so many and various impacts of fire. There may be very specific situations where such a scenario makes sense, but such an exotic application shall not be discussed in this paper. Nevertheless we do not know any regulations which just support this approach. Reduction of fire impact cannot be a fire protection strategy itself but it should be an important part of a combined strategy, as it is European standard. 4.3 Diverse Redundancy It is said that Europeans and especially Germans have a preference for multiple safety. We are happy to know there is a second protection instrument if the first protection instrument should fail. Combined safety strategy is well-known in many technologies relevant to safety. So in safety discussions regarding nuclear power plants the idea of diverse redundancy is a basic approach. This means there must be an additional safety procedure which works completely independently from the first one in case the basic safety procedure does not work. So in Europe it is the consensus to avoid fire as much as possible but at the same time to keep low the consequences on health or goods if a fire happens. Due to physical reasons a better reduction of the effects is achieved by reduced fire avoidance. But in total the risk according to Equation (1) is significantly lower. This is also shown in Figure 2 . + +

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Wire & Cable ASIA – January/February 2016