The UK Building Regulations, Approved Document B adopts the standard time temperature protocol of ISO834-1 in BS476 pts 20 to 24 for fire resistance testing of all building elements such as fire doors, fire penetrations, structural elements, fire walls and partitions, in fact every material, component and product used in a building that is required to have a Fire Resistance.
Often overlooked is that this time temperature protocol for fire resistance testing (ISO834-1 / EN1363-1 aka the standard time temperature curve) was developed 100 years ago when buildings and contents were commonly made of masonry, wood and fabric. At this time plastics and synthetic materials did not exist1. Buildings of this age were mostly not very tall or very large. Today our built environment is more complex, from small domestic, industrial/commercial buildings to super high rise, mega-interconnected transportation, healthcare and retail facilities many with significant below ground environments. Across these buildings types we have a large range of evacuation times and where egress times are very long, engineers need to seek alternative solutions for evacuation & protection of occupants such as; reducing fire loads, lift evacuation or protect in place refuges.
Research2 has identified that in most modern buildings, use of light weight polymeric building materials, plastic contents, synthetic foams and fabrics with high calorific values can significantly increase fire loads resulting in fire profiles well above the original parameters of the existing, early 1900’s test protocol of today’s UK Building Regulations. Underground environments also exhibit very different fire profiles to above ground built environments3. In confined underground public areas like road tunnels, underground shopping centers, car parks where a high fire load is present, fire temperatures can exhibit a very fast rise time and reach temperatures well above those in standard model above ground buildings. British Standard BS8519:2010 and BS EN12485 recognise underground public areas like car parks, loading bays and large basement storages as “Areas of Special Risk” with potential for fire temperatures well above the current requirements of the building regulations. In these environments more stringent test requirements for essential wiring systems routed through them may be appropriate.
Life Safety & Fire-fighting systems often depend on reliable function of electric cables during emergency. If these essential cables fail the critical equipment they enable also fails. This means firefighter’s lifts, fire sprinklers, hydrant pumps, smoke & heat extraction equipment, pressurization fans, communication and alarms might stop working during evacuation putting occupants, emergency response workers and property at risk. It is therefore concerning that the only exception in the UK Building Regulations for fire resistance testing to BS476 pts 20-24 (already “sub-optimal” for some buildings) is for the very electric cables required to power emergency life safety and firefighting systems. This contradiction allows these essential cables to be certified to alternative flame tests which have little or no relevance to real building fires and at much lower final temperatures than for all other fire resistant elements of the building.
This testing anomaly has occurred in the UK Building Regulations because British Standards for testing fire resistant electric cables (Protected Circuits) allow this exception. It is interesting to note that other developed countries including America, Canada, Australia, New Zealand Germany and Belgium do require testing of these essential cables to the same fire time temperature protocol as every other building element, which is the same as used in BS476 pts 20-24 i.e. ISO 834-1/EN 1363-1.
A close look at the evolution of test methodology in the UK suggests only incremental test method adjustments over time rather than a holistic evaluation of suitability and best practice benchmarking.
- BS 6387:1994: Performance requirements for cables required to maintain circuit integrity under fire conditions. (now withdrawn – revision 2013)
- BS 7629:1997: Parts 1 & 2: Specification for 300/500V fire resistant electric cables: (BS7629-1 reissued in 2008, superseded BS7629-1:1997 and BS 7629-2:1997 now withdrawn)
- BS EN 50200:2000: Method of test for resistance to fire of unprotected small cables: PH30, fire temperature 850°C 15 min. then 15 min with mechanical shock and water spray. PH120 fire temperature 850°C, 60 min. further 60 min. with mechanical shock and water spray. (now replaced by BS EN 50200:2006)
- BS7846/F2:2000: 0.6/1kV armored fire resistant cables: (adopts tests of BS 6387 for cables up to 20mm diameter) Cat C fire temp. 950°C for 3 hours. Cat W fire temp. 650°C for 15 min then water spray 15 min. Cat Z fire temp. 950°C for 15 min with mechanical shock
- BS 5839-1:2002: Clause (26.2 d&e) Fire detection and alarm systems for buildings: Included additional testing for “Enhanced” fire performance.
- BS8434-1:2003: Method of test for resistance to fire of unprotected small cables: (Similar to BSEN 50200 adds water spray). Fire temperature 850°C 15 min with mechanical shock then 15 min with mechanical shock plus water spray. (withdrawn as incorporated into BS EN50200 with Annex E)
- BS8434-2:2003: Method of test for resistance to fire of unprotected small cables: Fire temperature 930°C 60 min with mechanical shock plus an additional 60 min 930°C with mechanical shock and water spray. (Similar BSEN50200 but flame temperature to 930°C and water spray)
- BS 7346-6:2005: Components for smoke and heat control systems – Specification for cable systems: LS 30 (minutes), LS60, FF120 (identical with BS8491:2008 withdrawn with by BS 8519:2010)
- BS EN 50200:2006: Method of test for resistance to fire of small unprotected small cables for use in emergency circuits. PH15 (minutes), PH30, PH60, PH90, PH120 flame temperature 850°C duration with mechanical shock every 5 minutes. Optional fire with water spray test with fire and shock for 15 minutes then fire with shock and water spray for 15 minutes.
- BS 8491:2008: Method of assessment of fire integrity of large diameter power cables: Fire temperature 850°C 115 minutes with direct mechanical shock on the cable – then water jet for 5min.
- BS 8519:2010: Selection and installation of fire-resistant power and control cables for life safety and fire-fighting applications. (Test methods to BS EN50200:2006 or/and BS8491:2008 for 30, 60 or 120 minutes)
- BS 6387:2013: Test method for resistance to fire of cables required to maintain circuit integrity under fire conditions. Now of only three component protocols designated C, W and Z. (test methods unchanged from 1994 edition). Categories A, B, S, X and Y now obsolete.
Today we have a wide range of built environments so adopting a “one size fits all” protocol for fire resistance testing of cables may not be appropriate. Economic factors must also be considered. Many of the current products and test regimes, arguably even current tests for electric cables, may provide an adequate level of protection in small or low rise buildings where evacuation times are short. The concern is; are these same products and standards going to provide the required performance reliability and duration in new large high rise, metros, mega projects and large healthcare facilities where long evacuation times and secure ‘protect in place’ refuges are unavoidable?
British Standards and The Building Regulations themselves are only minimum requirements. Whilst often mandatory to meet these minimum codes it does not preclude the design of buildings and systems with higher performances. Professional engineers are accountable for the use of ‘reasonable skill and care’ and most often build to code, but the ‘fit for purpose’ criteria often remains the responsibility of the building owners and/or the installing contractor. It has been a surprise to some building owners and contractors to find out that simply following the professional consultants specifications may not absolve them from liability if it was reasonably known that a higher performance than the minimum code was required4.
Looking at global best practice for fire resistance testing of critical electrical wiring systems, the American UL 2196 test method is more relevant. This test is done in a full scale 6.6 x 7 meter vertical furnace where cables, fixings and accessories are all tested together in the mounting configuration they will be actually installed. The most demanding installation configuration is for vertical runs of cables, a common and unavoidable installation condition for all tall buildings and which current British and IEC standards do not address. UL2196 requires cables are tested with 5 samples of small to large sizes in each mounting configuration. These circuits are mounted horizontally and vertically in 3 meter lengths with bends and joints if needed. The cables are energized and subjected to the fire time temperature protocol of ANSI/UL 263 which is virtually identical with ISO 834-1. During testing the cables, fixings and supports experience significant mechanical stresses caused by expansion and contraction. After 2 hours at a final temperature of 1,020°C the cables are immediately subjected to a full pressure firefighter’s hose test which imparts huge thermal mechanical stresses on the wiring system. All samples must survive in working condition and certification is given independently for horizontal and/or vertical mounting.
Fire testing electrical integrity of cables in full scale is significantly more demanding and representative than testing short laboratory specimens of horizontally mounted circuits because the sheath and insulation of flexible polymeric cables will burn away in fire so that the cable clamps holding the cables vertically can no longer support them. Bare Mineral Insulated cables do not have this problem.
Whilst the time temperature regimes of ANSI/UL 263 (used in UL2196) and ISO834-1 (used in BS476 pts 20-24) may not be fully representative for all built environments where modern building materials with high calorific values are used, nor for the potentially higher/faster time temperature profiles of fires in ‘areas of special risk’, the American UL2196 test is certainly more representative of real life installed practice. Unlike the British tests for Protected Circuits, UL2196 is aligned with the test protocols required for all other fire resistant building elements and therefore is a far more appropriate test protocol.
Given most buildings and occupants rely on functional and reliable life safety and firefighting systems to protect life and property it is logical that “protected circuits” should not be allowed to be tested to different standards than every other fire resistant component or structure because logically they will be in the same fire. It might be better if they were subjected to even higher standards in order to ensure that the critical life safety and firefighting systems remain functional during evacuation.
Professional engineers, who design our buildings and the systems in them, are accountable for the use of ‘reasonable skill and care’5 but often avoid the obligation of ‘Fit for Purpose’ design. In turn this means ‘Fitness for Purpose’ frequently remains the responsibility of the unknowing project owner or installing contractor and as such there can be a critical gap in ownership. A similar finding on responsibility is identified in the Grenfell Fire Independent Review6.
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Reference
- A Short History Of The “Standard” (Cellulosic) and Hydrocarbon Time/Temperature Curves (2000) Paul Mather Technical Engineering Manager Fire & Insulation Products, International Coatings Limited
- Fire Safety of Buildings Based on Realistic Fire Time-Temperature Curves (2013). Ariyanayagam, Anthony Deloge & Mahendran, Mahen Queensland University of Technology.
- Recent achievements regarding measuring of time-heat and time –temperature development in tunnels (2004). Haukur Ingason and Anders Lönnermark SP Swedish National Testing and Research Institute
- Fenwick Elliott Annual Review 2014/2015 Understanding your design duty – “reasonable skill and care” vs. “fitness for purpose” – mutually incompatible or comfortably coexistent?
- Johnson Winter & Slattery ”Managing Design Risk through ‘fit for purpose’ warranties” (March 2017) Stephen Byrne, Isabelle Whelan
- Building a Safer Future: Independent Review of Building Regulations and Fire Safety (Dec 2017) (UK). Follows Grenfell fire disaster: Interim Report Dec 2017)
About the Author
Richard Hosier has over 35 years in the electro-technology industry, working for a variety of tier 1 international electrical cable companies. He consults to EDF on fire performance of cabling systems on the Hinkley Point 2 Nuclear Power Station. In these roles he has extensively researched our changing built environment, particularly in relation to the increasing fire risks arising from higher fire loads in large modern buildings, longer egress times and higher population densities. He currently works as the Director, Buildings and Infrastructure with Marmon IEI, operating in Asia Pacific and Middle East.
