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Security & Safety

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Low-smoke cables reduce hi-rise risks

SIMON HOPKINS of the Prysmian Group looks at how specifying the right cable can contribute to the safety of a building’s occupants.

01 August 2014

SPECTACULAR architecture is a characteristic of many modern, dynamic Middle Eastern cities. Awe-inspiring buildings such as the Burj Khalifa, the world’s tallest building, are pushing the limit of what is possible in both architecture and engineering. And, with even more spectacular buildings planned for the region, it is important to ensure that such buildings are as safe as they are awe-inspiring.

One vital aspect of building safety is the issue of fire. All modern high-rise buildings will be designed with fire detection and active fire suppressant systems and emergency lighting systems, but one element that can be overlooked is the contribution that the electrical cabling itself can make to the safety of the building. This is not an area that can be compromised, so it is essential to understand the different options available and to acknowledge the importance of specifying the right quality product.

Statistics show that the majority of fatalities in buildings occur through gas and smoke inhalation, rather than directly from the fire itself. One way of reducing the risk of smoke is for engineers to design systems using cables made of materials that can reduce the risks caused by fire, particularly in highly populated, enclosed public areas.

Prysmian’s Afumex range of low smoke zero halogen (LSZH) flame-retardant cables provides the power and control where it is needed without making the fire worse and with significantly reduced smoke output. As a low smoke range of cables, these have a low smoke zero halogen sheath which, in the event of a fire, will emit very low levels of smoke and non-toxic levels of poisonous halogen gases. These cables, commonly called ‘LSOH’ or ‘OHLS’ have distinctive thermoplastic and thermoset materials rather than the traditional PVC (polyvinyl chloride) for their insulation, bedding, fillers and sheathing.

Traditional PVC materials can react adversely in the event of a fire; they contain halogens which can release hydrogen chloride (HCl) gas which, in the presence of water produces hydrochloric acid. HCl on the skin can cause severe burns and inhalation of HCl fumes can be fatal. Other issues associated with traditional PVC cables are that they can ignite readily and fuel the fire, producing thick black smoke, which can obscure fire exits and escape routes.

When specifying a LSOH cable, it is imperative to ensure the cable is genuine. Industry initiatives have uncovered instances of PVC cables falsely claiming to have LSOH characteristics. The consequences of installing such a cable could be fatal should a fire break out.

Confusion has also arisen because cables are being marked with different acronyms and phrases. Low smoke, LSF (low smoke and fume) or LSOH are just a few. This terminology can be misleading and can result in products being installed which may not meet the performance expected of them.

Many countries have strict standards to which fire-resistant cables must comply. British Standards is one such globally recognised standards authority. A genuine low smoke zero halogen cable must adhere to a series of stringent tests. “Low smoke” has to adhere to BS EN 61034-2 smoke density test, also known as the three-metre cube test. In this, one-metre samples of cable are burnt and must achieve in excess of 60 per cent light visibility through the smoke. The “zero halogen” test, in accordance with BS EN 50267-2-1, must emit less than 0.5 per cent halogen acid when the sample cable is burnt.

The selection of fire-resistant cables for emergency critical systems – such as fire detection, fire alarm, emergency escape lighting and emergency voice systems – is particularly important because these systems must remain fully operational in the event of a fire. One of the characteristics for enhanced fire-resistant cables, which should be specified for these systems, includes demonstration of circuit integrity for two hours at 950 deg C. These are tested against fire, water and impact and, therefore, provide the highest level or performance to help manage a fire hazard.

It is not only the cable itself that is significant, it is also the supports used to fix it. In the UK, investigations into recent fatalities in building fires have led to a change in regulations and a heightened awareness of the risks posed by cables that fall down during a fire. These include the risk to people trying to escape the fire as well as to the circuit integrity.

The British Standard BS5839-1:2013 ‘Fire Detection and Fire Alarm Systems for Buildings, Code of Practice’, considers the danger posed by unsupported cable as well as the integrity of the alarm circuit. Manufacturers such as Prysmian already manufacture fire-resistant fixings that help designers comply with this standard.

In addition to fire critical circuits, in the UK regulations are set to be amended later this year to ensure that all cables in public areas – not just fire alarm cables – are supported by fire-resistant cable supports. The objective is to reduce the risk of people becoming entangled in fallen cable during a fire.

As a general rule, fire-resistant cables can also be fixed with copper, stainless steel or galvanised cast iron fixings, such as Prysmian’s Bicon range. These are suitable for most electrical cable installations and will be the most appropriate fixings for cables in public spaces that are likely to be included under the proposed changes to the wiring regulations. It must be noted though that aluminium is not recommended because of its relatively low melting point.

If there is any doubt about the compatibility of a cable with a particular fixing, Prysmian recommends specifiers speak to the cable manufacturer for advice. Further details on fire-resistant cables can be downloaded from Prysmian’s website (http://www.fpcables.co.uk/supporting_documents.html).