Life-safety, firefighting and critical emergency systems depend on the unfailing reliability of electric cables during a fire. Should these essential cable systems fail, the critical equipment they supply also fails, putting occupants, emergency responders and property at risk.
A fragmented ‘product only’ approach to compliance testing, rather than a more holistic testing of full-system performance, can lead to system failures. This combined with the minimum code requirements of building regulation generally taken as a prescriptive requirement, leaves any question of liability difficult to define.
Building codes often include a requirement that emergency equipment provided will continue to operate for the period of time necessary to ensure that the intended function of the equipment is maintained. It is a common expectation that these ‘systems’ are ‘fit for the purpose intended’, as well as being compliant with the minimum code.
This article explores if current BS and IEC cable testing methods can satisfy this requirement.
Background
Building codes are mostly minimum requirements referencing product and test standards. In the case of essential cables, manufacturers test the cables alone, letting installers provide supports, fixings and joints with the assumption that the combined parts of the ‘wiring system’ will together provide the needed performance. Regrettably there are instances where this approach has proved ineffective.
Observations
The problem of code compliance as it relates to essential cables, lies mainly in the test and performance requirements of product and test standards. These are adopted into Building Codes and enforced by regulators.
Certification bodies in many countries test, audit and then certify companies and their fire-performance cable products to specific performance standards on a ‘component only’ basis. This means that despite the marketing claims of manufacturers and even accreditation by the most reputable certification bodies, this does not validate or imply any ‘installed system performance’ nor any ‘Fitness for Purpose’.
For validation of any ‘system’, the whole system, inclusive of all components and installation methods must not only be tested and certified together but this system approach must be required to follow on to the installation. This is particularly true for ‘wiring system’ fire performances because the different components and materials of the wiring system can interact at fire temperatures to produce unexpected and interference results.
A review of current test standards for fire-resistant cables
British and IEC standards have proven over the years to be reputable and reliable technical documents which are used, trusted and adopted by many countries. Standards are proposed and written by technical committees made up from industry representatives with broad stakeholder interest and experience. Standards are reviewed and updated or confirmed on a regular basis adapting to changing needs of stakeholders and for the introduction of new technologies.
Historically, standards organisations make incremental changes to stay relevant, but evolutionary development of some British and IEC fire-resistant cable test standards, seem to have stalled when compared with the needs of the market and emerging global trends for higher and more reliable performances. Today there is a growing movement towards ‘wiring system’ performance rather than testing the ‘product only’ performance.
British and IEC fire-resistance testing of cables employs laboratory-scale testing of cable specimens in a designated flame source at ‘arguably’ non-representative fire temperatures (Fig. 1). Incremental changes have been made by adding fire with mechanical impact and fire with water impingement test requirements over the years, but there has been no change to the laboratory-scale flame testing of short cable specimens as stand-alone products (rather than full wiring systems). In defence, some British fire-resistant cable test standards do include a statement that the fire test methods described ‘do not assess a fire hazard, nor can the results of these fire tests alone guarantee safety’.
Of note, fire-resistance testing and classification for all non-cable products, components and systems of building construction in the UK and in most other countries around the world, require full-scale furnace testing at higher temperatures than is required by BS and IEC standards for testing fire-resistant cables (Fig. 1). Why this contradiction exists is an interesting question. The time temperature protocol used for almost all ‘non-cable’ testing and certification is ISO 834-1 (as defined in BS 476 pts 20–24 and EN 1363-1 codes). This standard time temperature protocol has a long history dating back over 100 years and is widely adopted around the world. The American standard time temperature test protocol ANSI/UL 263 is almost identical and originates from the same beginnings in the early 1900s.
Understanding life-safety and firefighting ‘wiring systems’ are essential, several countries have now upgraded the testing of fire-resistant cables and wiring systems to adopt and harmonize with this more realistic standard time temperature curve. The logic being that ‘essential’ circuits must at least be able to survive the same fire intensity as all the other fire-rated elements and components of the building exposed in the same fire. Germany, Belgium, Australia, New Zealand, United States and Canada have all subsequently adopted furnace testing for fire-resistant cables.
Comparison of fire circuit integrity testing of cables
BS 6387, BS 8491, BS EN 50200, BS 8434-2
British standards for testing fire-resistant cable does not require cables to be installed on supports or with fixing components, fixing distances or methods which are often used during installation. Notably, fixing distances during testing is between 200mm and 400mm, which is not generally replicated in manufacturers’ installation instructions. The mounting configuration for specimens under test is largely horizontal and with only short laboratory-scale cable lengths. These cable tests do not require testing of cables in full scale nor in realistic vertical or inclined configuration. This is important because the insulation and sheath materials of many polymeric cables may quickly burn away when exposed to fire temperatures, leaving the cable fixings with no way to support cables weight in vertical installations (Fig. 2).
The test-flame source is generally a 500mm- or 600mm-long ribbon burner with a flame temperature as specified in the test standard. Due to convection and conduction of heat, the cable itself and the cable components (fixings and supports) may not be exposed to the full test-flame temperature across the whole test specimen. Finally, the fire with water resistance tests are not representative of realistic firefighting interventions from high-pressure sprinklers or firefighting hoses.
These ‘flame only’ British and IEC tests are designed to test cables as stand-alone items and not as full wiring systems. There is no follow-on, mandatory requirement to ensure that the same fixings, fixing distances, cable supports as tested are used during installation as part of a tested system, and it is left to individual manufacturers to suggest installation methods, often which have not been fire tested together as a system.
These laboratory scale, ‘flame only’ bench test methods do not test full system performance, nor do they imply that the cables tested will perform as tested when installed in buildings as part of a complete fire-resistant wiring system.
German (DIN 4102), Belgium (NBN 713020), European test EN 50577, Australian (ASNZS3013)
In recognition of this contradiction in testing essential cables differently to every other component, structure and element of building construction required to have a fire rating, Germany, Belgium, Australia and New Zealand have now moved to furnace testing the circuit integrity of essential cables. These furnace tests use the standard time temperature protocol, and therefore are more realistic. These tests, however, still have important deficiencies as cable specimens are only mounted horizontally and are mostly supported by the cable tray or supports during the test. This ignores the critical effect of mass and gravity on the cables, supports and fixings in vertical installations.
The ‘product only’ approach to life-safety and firefighting equipment cables, where components are individually tested, then combined together in the hope that somehow the assembled products will provide a reliable ‘system’ performance is fundamentally flawed and introduces a potentially dangerous unknown.
It is important to understand the interactions between different materials under real fire conditions. This was clearly demonstrated when the reaction between zinc galvanization in conduits and copper conductors of fire-resistant cables was identified, resulting in random and premature cable failures. UL investigated and subsequently withdrew all certifications for fire-resistant cables in June 2012 pending a full review of all testing.
The result of the complete review by UL changed the testing protocol in the USA from testing cables to testing full wiring systems together including the cable, cable supports, couplings, boxes/conduit bodies, optional splices/joints, vertical supports, earths, pulling lubricants, cable tray/ladder and metal conduits.
A similar finding was exposed by Factwire in Hongkong where 9 out of 12 BS 6387 CWZ cables installed in Hong Kong buildings were found to fail the test when installed in conduits.
Today UL 2196 is the only test protocol anywhere in the world that requires testing of fire-resistant cable systems in full scale as a complete system, in both horizontal and vertical configurations and where the system as tested, with all its tested components, mounting and fixing configurations are listed. UL also requires cable circuits to be tested in groups of five from smallest to largest in full-scale horizontal and vertical installations. All five specimens in each vertical and horizontal configuration must pass the fire test with no failures including the subsequent full-pressure firefighter’s hose test to be certified and listed as a 2-hour fire-rated wiring system (unlike BS6387 and AS/NZS3013, which allow a two-out-of-three test pass criteria).
New trends for fire-performance cables
Cables together with their respective wiring system components must be certified together if test results and certifications are to have any practical meaning. Underwriters Laboratories UL 2196 takes a ‘full system’ approach rather than a ‘product only’ approach to test compliance. This ensures that the full wiring system tested will most likely be ‘fit for the purpose intended’ during real fires.
Underwriters Laboratories list several types of cables and their associated wiring systems that meet and have been certified to the UL2196 stringent FHIT 2-hour fire resistance furnace test immediately followed by a full pressure firefighter’s hose stream test. These cables include Metal Clad (MC) cables and Mineral Insulated Copper Clad (MICC) cables.
For more information, go to https://www.comtrancorp.com/vitalink-a-fit-for-purpose-fire-resistant-wiring-system/
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