Cable fire performance standards BS8519, BS8491 and BS7846

BS 8519:2010, which is intended for designers, contractors, fire engineers and authorities (including building control bodies) and health and safety inspectors, is a Code of Practice about the selection and installation of fire-resistant power and control cable systems for life safety and fire-fighting applications. BS 8519:2010 replaces BS 7346-6:2005, which is now withdrawn.

High-rise and complex buildings have become ever bigger and higher, and have also become more complex in terms of active fire protection, so fire engineered solutions have been developed. Such solutions need high performance from the building services components, which includes the electrical supplies. This is the background to BS 8519, which was introduced in February 2010 and became effective immediately.

BS 8519 guidance and recommendations:

BS 8519 gives guidance and recommendations on the selection and installation of power and control cable systems, covering the systems that are required to maintain their circuit integrity for life safety and fire-fighting applications. It also gives specific recommendations for electrical system design for such applications, and recommended limits for survival times.

In addition, BS 8519 identifies those electrical loads defined as life safety and fire-fighting loads. It lists the factors to be considered by the engineer when selecting and specifying the performance requirements of the electrical distribution system needed to maintain integrity under defined fire conditions for a specified period, referred to as the fire survival time.

Furthermore, BS 8519 makes reference to the recommendations identified in BS 9999, with regard to the design and installation of the electrical distribution systems for life safety and fire-fighting equipment.

It also makes reference to three categories of circuits required to maintain their circuit integrity under defined fire conditions for varying fire survival times of 30 minutes, 60 minutes and 120 minutes. Appropriate cable tests are identified for each category derived from applicable British Standards that assess cable performance under conditions of fire as might be expected in an actual fire incident.

Coverage:

This standard aims to ensure that the level of circuit integrity is not compromised by other components of the whole electrical distribution system, including cable glands, terminations, joints and cable support systems. It covers:

• The source of supply
• The distribution voltage [high voltage (HV) or low voltage (LV)]
• The appropriate location of the main intake enclosures, HV switchrooms, LV switchrooms, transformer rooms, generator rooms, risers, fire life safety plant rooms and fire-fighting/evacuation lift motor rooms/shafts.

It also identifies the need for dual redundant electrical supplies run via diverse cable routes, installed within separate fire compartments, and the need to incorporate automatic changeover devices, located within the same fire compartment as the life safety and fire-fighting equipment.

What changes have been made to the standard?

BS 8519:2010 replaces BS 7346-6:2005, which is now withdrawn. The main changes to this standard include:

• Change of identifier
• Change of scope
• Change from a specification to a code of practice
• Removal of Annex B (now published separately as BS 8491)
• Expansion of content to include all life safety/fire-fighting systems (not only smoke venting and fire-fighting cores as in the previous edition).
• The inclusion of new and revised technical guidance relating to the selection and installation of fire-resistant cables and systems for life safety and fire-fighting applications.

BS 7846:2009:

BS 7846:2009 refers to electric cables, thermosetting insulated, armoured, fire-resistant cables of rated voltage 600/1000V, having low emission of smoke and corrosive gases when affected by fire. It provides the specification and was published in September 2009.

BS 7846:2009 now incorporates a second, higher specification type of fire resistant armoured cable. This cable type is intended for use in special protected circuits for essential fire safety systems, and has a requirement for testing to BS 8491. This test incorporates fire, water and direct shock attacks for up to two hours. Further information on this can be obtained from BASEC.

The old British Standard for instrumentation and control cables, BS 5308, has been withdrawn because of a conflicting newly-published CENELEC standard. However, there remained a market need for a particular specification for these cables. Consequently, the British Cables Association has published a new version, PAS 5308, through the British Standards Institution. Certification to both the withdrawn BS 5308 and to the new PAS 5308 standards is available from BASEC.

A good example of cable meeting BS 7846:2009 is Draka's Zero Halogen, Low Smoke (OHLS) cable with stranded copper conductors and a protective armour layer. This cable has been tested and approved by the Loss Prevention Certification Board (LPCB) and BASEC, and it offers the advantages of an armoured 600/1000V rated, zero halogen, low smoke cable with circuit integrity. This cable is intended for use in installations where vital circuits are required to continue operation in the event of the outbreak of fire.

BS 8491:2008:

BS 8491:2008 concerns the method for assessment of the fire integrity of large diameter power cables for use as components for smoke and heat control systems and certain other active fire safety systems.

Published in January 2008, BS 8491:2008 describes a method for assessment of the fire integrity of large diameter power cables for use as components for smoke and heat control systems as given in BS 7346-6, and certain other active fire safety systems. It is applicable to cables of rated voltage not exceeding 600/1000V and of overall diameter greater than 20mm.

Cables approved to the requirements of BS 8491 including the maximum 120 minute rating, are already being used and are subject to growing interest for use in large and complex buildings where the superior level of resistance to fire with direct impact and water jet is appropriate.

The test method in BS 8491:2008 includes subjecting the cable under test to radiation via direct impingement corresponding to a constant temperature attack of 842 °C, to direct mechanical impacts corresponding to a force of approximately 10 N, and to direct application of a water jet simulating a water fire fighting jet. The test method given in this standard includes three different test durations to allow testing of cables intended for different applications.

BS 8491:2008 contains:

• Normative references
• Terms and definitions
• Test chamber
• Apparatus
• Verification procedure for the source of heat
• Test specimens
• Test procedure
• Test report.

It is emphasised that fire tests do not assess a fire hazard, nor can the results of fire tests alone guarantee safety. They only provide information to assist in the assessment of the suitability of a cable for a given application.

An example of cable meeting this standard is provided by Prysmian, with its enhanced performance fire resistant power cable, FP600S. This has been certified by the LPCB for compliance with BS 7346-6:2005, and the new test standard BS 8491:2008 for smaller cross sections. This means that the most popular cable sizes in the FP600S range all now have independent third party approval.

Large cross section, four core cables in the FP600S range gained approval from LPCB last summer. Now two-, three- and four-core FP600S cable with cross sections of between 10-70mm² also hold certification from LPCB, as does FP600 two, three and four core 4mm² and 6mm² cable.

Approved Document B of the Building Regulations states in Clause 5.38: 'In large or complex buildings there may be fire protection systems that need to operate for an extended periods in a fire'. By choosing FP600S cable, specifiers can be confident that the cable meets all the necessary requirements of Approved Document B of the building regulations as it complies with BS 7346-6. The standard ensures that the cable complies with the requirements for 'protected power circuits' given in the 2006 edition Approved Document B Fire Safety of the Building Regulations 2000.

FP600S, with its galvanised steel wire armour, is designed specifically to meet the more onerous fire survival requirements now being requested by regulators and specifiers for life safety systems, property protection and fire fighting systems in large and complex buildings.

Fire testing:

Responsible manufacturers would warn against installing fire performance cable that is not compliant with British Standards or approved by a third party such as BASEC or LPCB. It is during the development of these high performance products that fire testing facilities become essential.

The main tests conducted on fire performance cables are fire resistance, impact, flame propagation and smoke emission tests:

• The fire resistance test - This measures the time that the cable continues to function under set conditions, retaining its electrical state.
• Impact tests - These simulate falling materials in a building with water blasts used to replicate sprinklers or fire fighting hoses in a fire situation. A metal rod repeatedly impacts the cable simulating falling materials in a building.
• Flame propagation tests - These examine how far a cable burns in a vertical position before it stops burning. Cable is fixed so that it runs vertically inside a fire testing chamber to simulate installation within a building.
• Smoke emission tests - These involve burning cable in a smoke cube, which contains a light source at one end and a light sensor at the other. The cable is then set alight and emits smoke into the chamber. The percentage drop in the amount of light travelling from the source to the sensor is calculated and must not drop below a certain percentage depending on the test being applied.

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