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BSEE-AUG21-P14 Flexicon_Layout 1 22/07/2021 15:33 Page 14


BSEE


DATA CENTRES & CABLE MANAGEMENT


Specifying the correct cable protection for data centre fire performance


The UK wiring regulations (BS 7671) specify that a datacentre must take appropriate measures to ensure that all cable installations meet low fire hazard specification; by using appropriate cable protection, such as flexible conduit.


To assess how flame retardant a material is, the normal test method is to measure the Limiting Oxygen Index (LOI) according to BS EN ISO 4589-2. This determines the percentage of oxygen that needs to be present to support combustion. The higher the LOI percentage, the greater the flame retardancy of the material.


Another method is to use a glow wire test, BS EN IEC 60695-2, which applies a glow wire to a plaque of material at 7500C, 8500C or 9600C.


Low smoke emission


If the conduit is involved in a fire, the smoke generated may obscure the vision of people trying to escape, or the firefighters trying to extinguish the flames. It is important that the flexible conduit specified provides superior protection if this scenario occurs, to ensure staff are afforded sufficient time to exit the building safely.


manufacturer, Flexicon will explain some of the common misconceptions and advise on areas to consider when specifying suitable conduit to ensure compliance.


I The standard


BS 7671, commonly referred to as the 18th edition wiring regulations refers to EN 61386 for flexible conduit performance requirements, including fire. However, this standard for conduit systems only addresses non-flame propagation (self-extinguishing) – and no other fire performance properties such as enhanced flame retardancy, smoke and toxic fume emission are included.


The has meant that traditionally, halogen free conduits have been specified, often based on the common misconception that they offer comprehensive fire protection performance. Although such a conduit may prevent the generation of toxic gases in some settings, it does not mean necessarily mean that it is also flame


There are a number of fire tests, where a specified sample of material is burnt under controlled conditions in a given size smoke chamber and the smoke obscuration of a defined beam of light is measured.


n this article, Alex Smith, Technical Director at flexible conduit


retardant or have low smoke properties and may still be flammable if exposed to a heat source.


This can lead to a potentially dangerous situation where cables can be laid in flexible conduit that, by its standard, only needs to clarify if it is self-extinguishing and does not offer a comprehensive level of fire performance.


Risk factors


In a data centre environment, factors such as high temperatures in confined server rooms should be considered. Also conduit systems containing halogens, such as PVC, will give off chlorine acid gas in a fire that can destroy electronic equipment in another part of the building.


Almost all applications will require non-flame propagating (self- extinguishing) as called for within the UK wiring Regulations (BS 7671) and tested by means of the flame propagation test in EN 61386 as a bare minimum. Many customers will assume that this basic requirement will be met by any flexible conduit they specify, but this is not always the case.


Meeting low fire hazard specification


For a product to be classified as low fire hazard, it must display four characteristics: high flame retardancy, low smoke emission, low toxic fumes and halogen free. All metal conduit systems are inherently low fire hazard as there is no plastic to burn. However most composite conduit systems (for example, a metallic core with a plastic coating) or all-plastic conduit systems should be assessed for fire performance. Traditionally, halogen free conduits have been specified, often based on the common misconception that they offer comprehensive fire protection performance. Although such a conduit may prevent the generation of toxic gases in some settings, it does not mean necessarily that it is also flame retardant or have low smoke properties and may still be flammable if exposed to a heat source.


Flame Retardancy


There are a number of established test methods to prove performance. Flammability – the measure of how difficult it is to ignite the conduit if it is exposed to a heat source - is often referred to. The minimum requirement is that the product is self-extinguishing, according to conduit system standard EN 61386. Here a vertical sample of conduit is exposed to a 1kW burner and must extinguish within 30 seconds of the removal of the flame with no flaming droplets.


Low toxicity


The generation of toxic gases may incapacitate people trying to escape from the fire, so appropriate protection must be provided at all times.


To test for toxicity, a specified sample of material is burnt under controlled conditions in a given size smoke chamber and the fumes are analysed for various gases. The concentration of each gas is then multiplied by its toxic potency to give a toxicity index.


If halogens, sulphur or phosphorus are present in a material, it is unlikely to pass the low toxicity tests.


Halogen free


One of the main misconceptions is that a halogen-free material is automatically a low fire hazard product. A material cannot be considered as low fire hazard if it contains halogen, but as we have learnt, without the accompanying low toxicity, low smoke and flame- retardant properties, it will not meet the full criteria.


Typical halogens are fluorine, chlorine, bromine and iodine. Chlorine is the most common in PVC, fluorine is present in fluoro-polymers and bromine appears in flame retardants. All of them produce highly toxic fumes and thick smoke if exposed to a naked flame; another reason why operators may have tended to rate this area of performance above other fire hazard properties.


www.flexicon.uk.com 14 BUILDING SERVICES & ENVIRONMENTAL ENGINEER AUGUST 2021 Read the latest at: www.bsee.co.uk


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