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Choosing the Right Cable Accessories
Posted: 14/01/13 11:21:05

Choosing the Right Cable Accessories

 Life-safety systems, such as emergency escape lighting, sprinkler systems and disabled refuges directly help fire-fighters and those evacuating a building during an emergency, such as a fire.

Considering that one third of fires start inside buildings, it is essential that these systems are designed and installed correctly to ensure speed and effectiveness of evacuation. Vital to these systems are the electrical cables that can allow them to function in the event of fire. Fire resistant cables provide the power and control to such systems and they must be of a certain quality themselves. However, it’s also vital that consideration is given to exactly how these cables are fixed to the building structure.

Any fire resistant cable that is itself not up to the job, or, inappropriately supported, can present a hazard. If a fire resistant cable fails or falls down earlier than anticipated, then the consequences could be disastrous. Fire-fighters could become entangled in falling cables and the evacuation could be hindered and tragic examples exist of this.

Fortunately, clear guidance exists on which cables to use for emergency, detection and life safety systems and exactly how to attach them, via British Standards.

BS 8519 is a complete revision of BS 7346-6, which has been withdrawn, expanded to comprehensively include life safety systems. The standards for design, installation, commissioning and maintenance of fire alarm and detection systems and emergency lighting are laid out separately in BS 5839-1and BS 5266-1, respectively.

BS 8519 separates cable requirements into three categories dependant on fire survival time. Category 1 cables must have a survival time of 30 minutes, Category 2 is 60 minutes and Category 3 is 120 minutes. Test conditions are to BS EN 50200, which was recently amended to include Annex E, incorporating the UK requirement for exposure to water spray. This requirement means cable must prove resistance under simultaneous exposure to fire, mechanical shock and water spray.

 

Further to this, it is recommended that cable fixings and fixing centres should meet similar testing requirements to the cable in order to ensure circuit integrity. Category 1, 2 and 3 refer to the test requirements of the cable specifically; however BS 8519 recommends that cable accessories used in life-safety systems test comparably to the requirements laid out in these categories.

Some large and complex buildings require robust armoured cables in life-safety systems. Due to the weight of these cables, BS 8519 recommends they are fixed direct to the building structure, or, on a cable management system. These management systems are also required to function under fire, impact and water jet conditions similar to that of the cable.

BS 5266-1, the code of practice for emergency escape lighting, offers cable support guidance consistent with BS 5839-1which is the standard for fire detection and fire alarm system installation. Again, the precedent is the use of supports that offer the same durability afforded by the cable used. Further recommendations exclude plastic cable clips, plastic ties or plastic conduit and trunking as the sole means of support for cable with an intrinsically high resistance to attack by fire.

Fire-resistant cable manufacturers may offer both fire-resistant cable accessories and comprehensive recommendations for specifying their use. For instance, Prysmian Cables provides FP cable data sheets for installers choosing the appropriate cable accessories in recognition of the importance of support integrity. The fire resistant cable accessories they produce meet or exceed the requirements of the tests to comparable fire-resistance as the cable they support.

Fire resistant cable supports should be copper, steel or cast iron with a specified non-combustible coating: they should not be plastic or even aluminium as these materials burn or melt at the cable test temperatures.

Especially vital cables, like the mains supply cable to life-safety systems, have separate guidelines for appropriate installation. BS 8519 recommends that this cable should directly enter fire rated switch rooms rather than pass through the building. Where this is not possible, the cable should be protected by a concrete trench or enclosed within a passive fire protection material with a Category 3 fire survival time of 120 minutes.

 

Fire and mechanical damage, or indeed physical collapse, of the mains cable would be potentially disastrous for both fire-fighters and those evacuating, both of whom are reliant on life-safety systems in the event of fire.

In the case of supports and fixings for fire-resistant cables the guidelines seem pretty clear. Those specifying the installation of life-safety systems, fire alarms and escape lighting should apply the same assessments of suitability to the cable and the accessory. This creates a balance in durability that maintains circuit integrity in the wiring system. The fixings can then do their fundamental job; supporting cable systems and helping to ensure “life-safety”.


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