BASEC News

High Performance Fire Resistant Cables

19/11/09
New edition of BS 7846 for fire resistant armoured cables published

Fire engineers are making greater use of active fire protection systems in modern buildings.  At the same time, architects, engineers and designers are taking advantage of new technologies to produce innovative designs incorporating more elaborate layouts, and need more effective fire performance systems.  As high prestige buildings become more complex in this way, the need for higher performance cables has been identified by a number of manufacturers.  Public buildings such as hospitals, shopping centres, office buildings with atriums, sports stadiums and even some high specification residential premises are all nowadays using advanced fire safety engineering design approaches.

Traditional fire protection approaches rely primarily on materials choice and passive compartmentation to provide limitations to fire growth and spread.  Fire safety engineering techniques allow a more open building structure, but they ensure safe evacuation in the event of a fire by a combination of traditional and modern, active fire systems such as smoke control and extraction, phased evacuation, gaseous fire extinguishing and firefighter support systems.  Many of these systems require electrical power supplies and control circuitry to remain fully functional throughout a potentially serious fire lasting many hours.  Examples include power supplies for fire-fighting lifts and smoke extract systems.  Robust fire resistant cables are needed in order to satisfy these needs, cables that have been tested under quasi-real conditions and with proven performance.

Normal power distribution cables such as BS 5467 or BS 6724 have limited performance against sustained fire attack.  Cables which need to remain operational throughout the fire need to be robust to not only fire, but also to impact damage from falling items of building structure and resistant to the effects of water spray from sprinklers or from fire fighting activities.  Some well-established cable types, such as mineral insulated cables to BS EN 60702-1 remain very effective under such circumstances and are usually smaller in diameter than the equivalent armoured cable.  A new cable fire test, BS 8491, has been developed to provide assessment of cables larger than 20mm diameter.  The test incorporates direct fire attack, mechanical attack and water spray, over a variable time up to two hours.  Several cable makers have produced enhanced fire resistant cables to meet this requirements, some incorporating spiral interlocked steel tape armour instead of the usual steel wire armour.

A new edition of BS 7846 for fire resistant armoured cables has just been published.   As well as the traditional steel wire armoured fire resistant cables (category F2), the new type using interlocked steel tape armour has been introduced, classified as categories F30, F60 or F120, reflecting the length in minutes of the BS 8491 fire test.  These cables can maintain full operational performance during such a fire by the use of a mineralised insulating tape over the conductors, normally made from mica.  The armouring retains the structure of the cable and protects against shock.  It is important to remember that cable cleats and other cable mounting techniques also need to survive the fire, shock and water attack.

Mineral insulated cables perform well against a combined fire, shock and water attack, and the BS 8491 test method should soon be extended to cover these generally smaller diameter cables.

Installers are beginning to see these new types of cable specified in high prestige buildings or where critical operations need to continue during a fire, for example emergency power supplies.  Because the armour construction of the interlocked steel armour is different, new cutting and preparation methods will be needed.  New gland types will be used, and traditional cone-type SWA glands may not be suitable.  Installers will also need to make sure they are adequately trained to use such cables.

Further information about BASEC and advice on faulty cables is available at www.basec.org.uk or you can contact BASEC directly on 01908 267300

 

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