Prysmian Cables & Systems take a closer look at the specification of cables for fire detection and fire alarm systems in order to comply with the relevant British standards
During the design of buildings, it is vital to be prepared for emergencies such as fire. When you consider that a third of fires start inside buildings, it’s clear that the correct and efficient installation of life safety, emergency and fire detection systems is paramount.
Fire fighters and those evacuating a premises affected by fire rely on systems such as emergency lighting and sprinkler systems. And, in turn, the systems themselves rely on the functionality of the electrical cables used to power and control them.
A fire resistant cable may be able to endure for a prolonged period in the environmental conditions that a fire presents. However, if these cables are inappropriately supported they can become a hazard. If the cables fall down during the fire it is possible that fire fighters could become entangled.
In order to minimise this risk, clear guidance to correct installation of life safety systems can be found by consulting the relevant British standards.
Standards
BS 7346-6 has been withdrawn and replaced by BS 8519 ‘Selection and installation of fire resistant power and control cable systems for life safety and fire fighting applications’. It is a full revision expanded to include life safety systems. The standards for fire alarm and detection systems and emergency lighting are laid out separately in BS 5839-1 and BS 5266-1, respectively.
BS 8519 categorises cables depending on their fire survival time in test conditions defined by BS EN 50200. This standard was recently amended with the addition of Annex E which incorporates the UK requirement of exposure to water spray into the test conditions.
The criteria of the test now means that, to pass, cables must prove resistance to simultaneous exposure to fire, water spray and mechanical shock.
There are three categories of fire survival time – cables in Category 1 must have a survival time of 30 minutes, Category 2 requires 60 minutes and Category 3 cables must endure for 120 minutes.
These categories refer specifically to the cable used and these standards should be adopted when choosing cable accessories. BS 8519 recommends that cable accessories used in life safety systems test comparably to the cable they support.
The standards differ depending on the application. BS 8519 recommends that this kind of cable is fixed directly to the building structure but where this is not possible, a cable management system can be used as long as it functions under similar fire, impact and water conditions as the cable.
Guidelines
There are also separate guidelines for installation of the power supply cable for life safety systems. The desired installation would be direct entry into fire rated switch rooms. However, BS 8519 recommends that if this cable has to pass through the building it should be protected by a concrete trench or enclosed within a passive fire protection material which has the Category 3 fire survival time of 120 minutes.
BS 5266-1:2011, the code of practice for emergency escape lighting, offers cable support guidance consistent with BS 5839-1:2002, which is the standard for fire detection and fire alarm system installation. Again, both standards recommend the use of supports that offer the same resistance as the cable. Plastic cable clips, plastic ties or plastic conduit and trunking are excluded as the sole means of support for cable with an intrinsically high resistance to attack by fire.
A starting point for correct installation would be to choose a fire resistant cable manufacturer that produces fire resistant cable supports. Prysmian Cables is one such company that manufacture cable supports and fixings that meet or exceed the requirements of the tests for fire resistance met by its fire resistant cables.
Prysmian also provides FP cable data sheets and BS guidance documents to aid installers in choosing the appropriate cable accessories.
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 below the cable test temperatures.
Summary
Considering the requirements laid out in the British Standards, the guidelines for choosing appropriate cable accessories for life safety systems, emergency lighting systems and fire detection systems seem very clear. The same assessments of suitability should be applied to both the cable and the accessory. By choosing cables and cable supports that are comparable in fire resistance, circuit integrity can be maintained in these vital systems.
Prysmian Cables & Systems
T: 0238 029 5029
