Unlike other cables, fire resistant cables have to work even when directly exposed to the fire to keep essential Life Safety and Fire Fighting equipment working: Fire alarms, Emergency Lighting, Emergency Communication, Fire Sprinkler pumps, Fireman’s Lift sub-main, Smoke extraction fans, Smoke dampers, Stair pressurization fans, Emergency Generator circuits etc.
In order to classify electric cables as fire resistant they are required to undergo testing and certification. Perhaps the first common fire tests on cables were IEC 331: 1970 and later BS6387:1983 which adopted a gas ribbon burner test to produce a flame in which cables were placed.
Since the revision of BS6387 in 1994 there have been 11 enhancements, revisions or new test standards introduced by British Standards for use and application of Fire Resistant cables but none of these seem to address the core issue that fire resistant cables where tested to common British and IEC flame test standards are not required to perform to the same fire performance time-temperature profiles as every other structure, system or component in a building. Specifically, where fire resistant structures, systems, partitions, fire doors, fire penetrations fire barriers, floors, walls etc. are required to be fire rated by building regulations, they are tested to the Standard Time Temperature protocol of BS476 parts 20 to 23 (also known as ISO834-1, ASNZS1530pt4, EN1363-1 and in America and Canada ASTM E119-75).
These tests are conducted in large furnaces to replicate real post flashover fire environments. Interestingly, Fire Resistant cable test standards like BS 6387CWZ, SS299, IEC 60331 BS8343-1 and 2, BS8491 only require cables to be exposed to a flame in air and to lower final test temperatures (than required by BS476 pts 20 to 23). Given Fire Resistant cables are likely to be exposed in the same fire, and are needed to ensure all Life Safety and Fire Fighting systems remain operational, this fact is perhaps surprising.
Contrastingly in Germany, Belgium, Australia, New Zealand, USA and Canada Fire Resistant cable systems are required to be tested to the same fire Time Temperature protocol as all other building elements and this is the Standard Time Temperature protocol to BS476pts 20-23, IS0 834-1, EN1363-1 or ASTM E119-75 in USA.
The committees developing the standard drew on the guidance given from the International Fire Prevention Congress held in London in July 1903 and the measurements of furnace temperatures made in many fire tests carried out in the UK, Germany and the United States. The tests were described in a series of “Red Books” issued by the British Fire Prevention Committee after 1903 as well as those from the German Royal Technical Research Laboratory. The finalization of the ASTM standard was heavily influenced by Professor I.H. Woolson, a Consulting Engineer of the USA National Board of Fire Underwriters and Chairman of the NFPA committee in Fire Resistive Construction who had carried out many tests at Columbia University and Underwriters Laboratories in Chicago. The small time temperature differences between the International ISO 834-1 test as we know it today and the America ASTM E119 / NFPA 251 tests likely stemmed from this time.
The curve as we see it today (see graph above) has become the standard scale for measurement of fire test severity and has proved relevant for most above ground cellulosic buildings. When elements, structures, components or systems are tested, the furnace temperatures are controlled to conform to the curve with a set allowable variance and consideration for initial ambient temperatures. The standards require elements to be tested in full scale and under conditions of support and loading as defined in order to represent as accurately as possible its functions in service.
This Standard Time Temperature testing protocol (see graph right) is adopted by almost all countries around the world for fire testing and certification of virtually all building structures, components, systems and elements with the interesting exception of fire resistant cables (exception in USA, Canada, Australia, Germany, Belgium and New Zealand where fire resistant cable systems are required to be tested and approved to the Standard Time Temperature protocol, just like all other building structures, elements and components).
It is important to understand that application standards from BS, IEC, ASNZS, DIN, UL etc. where fire resistive cables are specified for use, are only ‘minimum’ requirements. We know today that fires are not all the same and research by Universities, Institutions and Authorities around the world have identified that Underground and some Industrial environments can exhibit very different fire profiles to those in above ground cellulosic buildings. Specifically in confined underground public areas like Road and Rail Tunnels, Underground Shopping centers, Car Parks fire temperatures can exhibit a very fast rise time and can reach temperatures well above those in above ground buildings and in far less time. In USA today electrical wiring systems are required by NFPA 502 (Road Tunnels, Bridges and other Limited Access Highways) to withstand fire temperatures up to 1,350 Degrees C for 60 minutes and UK British Standard BS8519:2010 clearly identifies underground public areas such as car parks as “Areas of Special Risk” where more stringent test protocols for essential electric cable circuits may need to be considered by designers.
Of course all underground environments whether road, rail and pedestrian tunnels, or underground public environments like shopping precincts, car parks etc. may exhibit different fire profiles to those in above ground buildings because In these environments the heat generated by any fire cannot escape as easily as it might in above ground buildings thus relying more on heat and smoke extraction equipment.
For Metros Road and Rail Tunnels, Hospitals, Health care facilities, Underground public environments like shopping precincts, Very High Rise, Theaters, Public Halls, Government buildings, Airports etc. this is particularly important. Evacuation of these public environments is often slow even during emergencies, and it is our responsibility to ensure everyone is given the very best chance of safe egress during fire emergencies.
It is also understood today that copper Fire Resistant cables where installed in galvanized steel conduit can fail prematurely during fire emergency because of a reaction between the copper conductors and zinc galvanizing inside the metal conduit. In 2012 United Laboratories (UL®) in America removed all certification for Fire Resistive cables where installed in galvanized steel conduit for this reason:
UL® Quote: “A concern was brought to our attention related to the performance of these products in the presence of zinc. We validated this finding. As a result of this, we changed our Guide Information to indicate that all conduit and conduit fittings that come in contact with fire resistive cables should have an interior coating free of zinc”.
It would seem that some Standards authorities around the world may need to review the current test methodology currently adopted for fire resistive cable testing and perhaps align the performance of Life Safety and Fire Fighting wiring systems with that of all the other fire resistant structures, components and systems so that Architects, building designers and engineers know that when they need a fire rating that the essential wiring system will be equally rated.
For many power, control, communication and data circuits there is one technology available which can meet and surpass all current fire tests and applications. It is a solution which is frequently used in demanding public buildings and has been employed reliably for over 80 years. MICC cable technology can provide a total and complete answer to all the problems associated with the fire safety dangers of modern flexible organic polymer cables.
The metal jacket, magnesium oxide insulation and conductors of MICC cables ensure the cable is effectively fire proof. Bare MICC cables have no organic content so simply cannot propagate flame or generate any smoke. The zero fuel-load of these MICC cables ensures no heat is added to the fire and no oxygen is consumed. Being inorganic these MICC cables cannot generate any halogen or toxic gasses at all including Carbon Monoxide. MICC cable designs can meet all of the current and building fire resistance performance standards in all countries and are seeing a significant increase in use globally.
Many engineers have previously considered MICC cable technology to be “old school’ but with the new research in fire performance MICC cable system are now proven to have far superior fire performances than any of the newer more modern flexible fire resistant cables.
For further information, go to www.temperature-house.com
About the Author
Jamie Groves is Sales Director for MICC Ltd. headquartered in the UK. MICC and sister company TRM are manufacturers of the World’s largest range of Mineral Insulated Cable products.
