Almost all Life Safety & Fire Fighting systems depend on the reliable function of electric cables during emergency. If these essential cables fail during a fire event, the critical equipment they enable also fails. This could mean that firemen’s lifts, fire sprinklers, hydrant pumps, smoke & heat extraction and pressurization fans, emergency communication, alarms and lighting systems stop working during evacuation putting occupants, emergency response workers and property at risk.
Today the UK Building Regulations, Approved Document B adopts the standard time temperature protocol of BS476 pts 20 to 24 for fire resistance testing of all building elements such as fire doors, fire stopping systems for 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 rating. It is therefore concerning that the only exception to this test are the very electric cables required to power all emergency life support systems. This contradiction within the UK Building Regulations allows these essential cables to be tested to alternative flame tests which have little or no relevance to real building fires and at lower final temperatures than required for all other fire resistant elements of the building. Why this anomaly has occurred in the BSI standards adopted by the Building Regulations is equally concerning, especially as other countries like America, Canada, Australia, New Zealand Germany and Belgium have for years, required testing of these essential cables to the same fire time temperature protocol as every other building element.
In light of recent major fire events in the UK and around the world, having British Standards years behind global best practice for electrical wiring systems enabling life safety and firefighting systems is unacceptable. Clearly more emphasis is needed on the fire safety and integrity of electrical systems and especially the reliable performance of all critical systems during emergency to facilitate safe egress, but before we simply seek to defend current standards or clamor for a change in line with global best practice, we should perhaps also look to the reality of fire testing any element of building construction.
What is often overlooked is that the time temperature protocol for fire resistance testing which BS476 pts 20-24 adopts is the ISO834-1 (EN1363-1) standard time temperature curve. This curve was developed almost 100 years ago when buildings and contents were primarily made of wood and fabric and when plastic or synthetic materials did not exist. Further, buildings of this age were mostly not very tall or very large. Today our built environment consists of both large and small buildings but critically is far more complex with super high rise and mega-interconnected transportation, retail, commercial structures with significant below ground environments. In these buildings we have a much larger range of evacuation times and where these egress times are very long, designers and engineers need to look for alternative more innovative solutions for evacuation or protection of occupants such as reducing fire loads, lift evacuation or protect in place refuges.
Recent research has identified that in most modern buildings the use of light weight and polymeric building materials, plastic contents, synthetic foams and fabrics with high calorific values can significantly increase fire loads resulting in time temperature fire profiles well above the original parameters of the existing, early 1900’s test protocol as used in BS476 (ISO834-1 EN 1363-1) and as mandated by the Building Regulations for fire resistant building elements. Underground environments can also 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. British Standard BS8519:2010 and BS EN12485 clearly recognise underground public areas such as car parks, loading bays and large basement storage as “Areas of Special Risk” with potential for fire temperatures to 1,200°C. In these environments more stringent requirements for fire resistance testing maybe needed and especially for any electric cables supplying power to life safety equipment which are installed in or run through these locations.
It is therefore both logical and urgent that the fire testing protocol for essential electric cables (called Protected Circuits in the Building Regulations) be reviewed because it is critical that these electric circuits keep working during fire emergencies. It is also logical that these protected circuits should be tested to perform to at least the same standard as everything else because logically they will be in the same fire. It might be better if they were subject to to even higher standards in order to ensure that the critical life safety and firefighting systems remain functional during evacuation.
Whilst we recognise that the standard fire test protocol adopted by BS476 (ISO834-1 EN 1363-1) may not be fully relevant for all built environments, it may remain appropriate for the majority of small and domestic, low rise buildings with a short evacuation times. For large projects such as high rise, mega interconnected retail and transportation projects, public buildings like hospitals, airports and theaters with long evacuation times, the current fire resistance testing protocols for essential electric wiring circuits may be inadequate due to the need for the life safety and firefighting systems to work reliably for longer thus facilitating successful evacuation.
For this reason we believe adopting a “one size fits all” protocol for fire resistance testing wiring systems may not be appropriate anymore. Clearly economic factors must also come into play and as it stands, many of the current products and test regimes, including those 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 reliability in performance and duration for the large high rise, underground and mega projects where long evacuation times are needed?
It is correct to say all British Standards and indeed The Building Regulations themselves are only minimum requirements, so whilst it is mandatory to meet this minimum requirement it does not preclude the design of buildings and systems with higher performances. It is the professional engineers who design our buildings and systems that are responsible to ensure products and systems specified are ‘fit for purpose’. It has been a surprise to some to find out that simply designing to code may not absolve from liability if it was reasonably known that a higher performance than the minimum was required.
Looking at global best practice for fire resistance testing of electrical cables it is clear the American UL 2196 test method is more relevant. This test is done in a large 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, which in most buildings and high rise is an unavoidable and common installation condition. UL2196 requires that cables are tested at their rated voltage (rather than just the operating voltage) and with a minimum 5 samples across a range of small to large sizes. All these circuits are mounted both horizontally and vertically in 3 meter (10 foot) lengths with bends. The cables are energized and samples are subjected to the fire time temperature protocol of ASTME-119-75 which is virtually identical with BS476 (ISO834-1). 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 fire hose stream test which not only imparts huge thermal stresses on the wiring system but also significant mechanical stresses. All 5 samples must survive in working condition and certification is given independently for horizontal and/or vertical mounting. (Note, it is well established that fire testing representative lengths of vertically mounted cables for electrical integrity is significantly more demanding than testing short lengths of horizontally mounted circuits).
Australia and New Zealand have a similar test ASNZS3013:2005 which subjects the cable, supports and fixings samples to the same time temperature protocol as BS476 (ISO834-1 EN 1363-1) for 2 hours in a 1 meter by 1 meter horizontal furnace. This test only requires 1 sample from selected groups and samples are energized only at operating voltage. This test allows a 2 out of 3 pass criteria should the first sample fail. Frankly, we feel a 2 out of 3 performance isn’t good enough for life safety and firefighting systems and this without any representative vertical test component leaves this fire test method questionable (albeit arguably better than the current British Standards tests).
Whether BSI should encompass the test protocol of UL2196 into a new British Standard or look to the Australian AS/NZS3013 or German DIN4102 test methodology (which doesn’t test for water spray), they must address the current test standards which we believe are inadequate, contradictory to all other building elements needing fire resistance and unrepresentative of any known building fire profile.
It would also appear an urgent case for a review of The Building Regulations; Fire Safety ‘Approved Document B’ is needed to harmonize fire resistance test protocols to include protected electrical circuits for all buildings but to also consider a more representative, robust and reliable fire test protocol based on world best practice for protected circuits in modern tall, large, underground and complex public buildings with long evacuation times.
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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.
