Electrical circuits supporting life safety and fire fighting equipment
Unlike other cables, fire resistant cables have to work even when directly exposed to fire to keep essential equipment working: Fire alarms, emergency lighting, emergency warning and communication systems (EWIS), fire sprinkler pumps, fireman’s lift sub-mains, smoke extraction fans, smoke dampers and shutters, stair pressurization fans, emergency generator circuits etc.. We call these circuits Life Safety and Firefighting circuits.
British Standards for electrical fire resistant wiring systems are closely adopted in many countries and throughout Asia Pacific with the implementation of tests to BS 6387 CWZ in Malaysia, Thailand, Indonesia and other ASEAN and N.E. Asian countries with Singapore adopting the same test method but with minor voltage enhancements.
Building Regulations in the UK and in most countries of Asia Pacific adopt the standard time temperature protocol of ISO834-1 (EN1363-1) for fire resistance testing of almost all structures, components and systems in buildings. This time temperature test protocol is commonly called the “Standard Time Temperature Curve” or “Cellulosic Curve” and has been used since the early 1900’s as a standard for fire resistance testing. It is specified throughout BS 476 and BS EN 1634 for testing of fire doors and hardware, fire stopping systems for penetrations and in fact is used for every single structure, material, component and product that is required to have a fire resistance rating in buildings. In the Asia Pacific region this same fire test protocol ISO 834-1 is also used for all building elements required to have a fire resistance rating.
It is then both surprising and concerning that the single exception for any component or product which is required to have a fire resistance rating in buildings are the very electric cables which form an integral part of the critical life safety and firefighting systems. These fire resistant cables are needed to enable life safety systems to ensure safe evacuation of people and effective firefighting interventions during emergency. As it stands today these essential cables are allowed by British Standards to be tested to different protocols which are not generally representative of any known building fire profiles.
Logical questions to ask might be: “Why is it that the test methods used for flame testing of electric cables in the UK, Europe and most parts of Asia Pacific, vary so much from the fire test methods for the other fire rated building components and structures?” and “Why should electric cables be tested differently and often at lower final temperatures?” It would seem these questions need more investigation given that electrical cables in practice may be exposed to the same fire time temperature conditions.
It is noteworthy to benchmark what test protocols other developed countries adopt and mandate in their building codes for essential life safety and firefighting electrical wiring systems: Germany, Belgium, Australia, New Zealand, America and Canada all require the electrical cables and wiring systems which need to maintain electrical integrity to essential systems during fire, be tested and certified to exactly the same fire time temperature protocol as every other building element, structure, component or system and this is the Cellulosic Curve of ISO834-1, EN1363-1 and AS/NZS1530pt 4 as specified in BS476 or in the case of USA and Canada ASTM E119-75 and NFPA251 (which is almost identical). Indeed these full scale furnace tests in Australia, New Zealand, America and Canada also include for the mechanical stresses caused by thermal deformation of cable supports and water spray testing to ensure reliability of these circuits when you need them most.
Recent research has identified that for modern above ground cellulosic buildings, todays use of light weight thermoplastic building materials like synthetic foams and fabrics have significantly increased fire loads resulting in time temperature fire profiles well above the original parameters of the existing, early 1900’s test protocol ISO834-1. Underground environments are also known to exhibit very different fire profiles to those in above ground cellulosic environments. Specifically in confined underground public areas like road and rail tunnels, underground shopping centers, car parks etc. fire temperatures can exhibit a very fast rise time and reach temperatures well above those in above ground buildings. If anything this should point toward more stringent testing and especially for those wiring systems to life safety and firefighting systems. British Standard BS8519:2010 today clearly identifies underground public areas such as car parks as “Areas of Special Risk”. In these environments more stringent test protocols for fire resistance including for electric cables systems may need to be specified by designers.
It should be noted that national standards and building regulations are drafted as minimum general requirements and that should a building, project or environment be known to have more demanding emergency conditions, fire risk or long evacuation times than considered in these standards, then simply complying with a minimum code may not indemnify professional engineers, respective project owners or authorities from liability. Reference should be made to Appendix D of BS 476.20:1987 AMD 6487 April 1990 which highlights this exact point: “While the heating regime described in clause 3 (ISO834-1) is the fire exposure condition required by this standard and generally employed for building uses it is recognized that it may be inappropriate for certain special risk requirements”.
There is one cable design which today meets all of the British, German, European, Australian, New Zealand, American and Canadian standards for fire resistant wiring systems. MICC cables made with a copper jacket, magnesium oxide insulation and copper conductors ensure the cable is effectively fire proof. In addition the cable design cannot propagate flame, gives off no smoke during fire, and generates no toxic gasses at all. In addition MICC cables have an indefinite service life because the cables do not age.
For more information, go to www.miccltd.com