There is a general belief that all fire-resistance tests for essential electrical cables around the world are equivalent and comparable. This significant misunderstanding has resulted in numerous cases where substitution from one standard to another has occurred without a clear understanding of the significant impact to system performance and life safety.
Most concernedly are the cases of substitution where fire-safety standards like NFPA 130 (Mass Rapid Transit Systems) and NFPA 502 (Limited Access Highways – Road Tunnels) and NFPA 72 (Fire Alarms) are adopted. All these fire-safety standards specify the use of UL 2196 performance for fire-resistant cable systems and stringent water-resistance requirements, but more often than not, outside North America, this UL 2196 ‘wiring system’ fire performance is unwittingly substituted for the minimum ‘product’ requirement of local codes: often BS 6387 CWZ, IEC 60331, or in the UK BS 8491, BS EN 50200, BS 8434-2 and the water-resistance requirements are often overlooked.
Correspondingly NFPA 130 requires cables comply with NEC for a WET listing (UL 44) which requires cables to be tested for 12 weeks in water at 90°C with stringent insulation resistance and capacitance requirements. This is necessary as conduits in tunnels are often subject to flooding; however, this requirement is often ignored and not met by commonly used LSZH cables.
What is inadvertently unnoticed with substitution to lower-performance cable standards is that the respective NFPA standards used as an overarching safety protocol have factored the time and reliability that life-safety and firefighting systems must operate for to safely evacuate people. This reflects in evacuation-route design, dimensions and equipment. Substitution to lower-performing cables and wiring systems, without consideration of other factors, may have a major consequence on the ability of the whole safety system to achieve the objectives intended or required.
This article looks at the differences between cable product tests and wiring system tests along with their respective performances, so a more comprehensive understanding of the impacts to overall life-safety system integrity can be made where substitution might be considered.
Mineral-insulated or MICC/MIMS copper-sheathed cables have traditionally been a good solution for fire-circuit integrity; however, mineral-insulated cables do need special tools and fittings to install, and the magnesium oxide insulation, commonly used, is often hydroscopic leading to reduced insulation resistance where moisture or humidity is present.
The first flexible fire-resistant electrical cables were made using copper conductors insulated with silicone rubber. In flame tests, this design seemed effective because even though the silicone rubber degraded in the flames, the remaining ash, albeit brittle, maintained some structural and dielectric functionality. The residual silicone rubber ash (left after the initial degradation in the flame) was not physically strong and movement, vibrations and water could disturb the ash resulting in circuit failure.
In 1970 a standardized test method was developed by the International Electrotechnical Committee: IEC 331. A flame temperature of 750°C was chosen and a proviso to limit the diameter of the cable under test to 21mm diameter due to practical limitations of the test setup.
During the 1970s, cable manufacturers introduced new designs of fire-resistant cables using glass-mica tapes (GMT) wrapped onto conductors with ethylene propylene rubber (EPR) or polyethylene (PE/XLPE) insulations. These cables could also survive the flame test of IEC 331:1970 but often gave more consistent performance in fire tests when subjected to movement, vibration and even some water spray.
British Standards adopted the basic IEC 331 flame test method into BS 6387:1983 but increased the flame temperature to 950°C (test C) and included two additional tests: A 15-minute fire with mechanical shock (test Z) and a 30-minute fire with water spray (test W), although the water test is conducted with a flame temperature at only 650°C.
These modifications were claimed to simulate the anticipated disturbance to electric cables by deformation of mountings, supports, falling debris as well as water spray from firefighting interventions; however, the water flow rates in testing are a long way from outputs of even small domestic sprinklers.
Today, IEC 60331-1/2/3, BS 6387 CWZ (SS299) and the more recent British Standards BS EN 50200, BS 8434-2 and BS 8419 cable flame test methods are widely adopted in many countries around the world as the required test methods for certifying fire-resistant electrical cables.
What should be understood is that these tests have a lower flame temperature requirement than is required by these same countries’ Building Regulations for qualifying all other building components, systems and structures which need an FRL (Fire Resistance Level) greater than 60 minutes.
British Standards: BS EN 50200, BS 8434-2 and BS 8491 fundamentally use the same burner set-up for fire-resistance testing of cables as IEC 331 which was first published in 1970 and BS 6387 CWZ in 1983. This is a basic test rig with a 500mm-long gas ribbon burner and small test specimen tested on a laboratory benchtop, but surprisingly tests at even lower temperatures than BS 6387 C and often for shorter test durations. With these ‘open-air’ flame tests, the set flame temperature does not always equal the temperature of the full cable on test, which can be at a lower temperature due to heat loss by convection, conduction, radiation, physical size of the cable and mounting configuration of the cable specimen. To highlight this point, two Malaysian cable manufacturers today are making fire-resistant cables passing the BS 6387 CWZ 950°C flame test with aluminium conductors and claim LPCB certification. Given aluminium melts at 650°C this anomaly points to not only some inadequacy of the BS 6387 CWZ test methods but also to potential deficiencies in BS 8434-2, BS EN 50200 and BS 8419 as they use even lower test temperatures.
Using small-scale benchtop flame tests stands in stark contrast to moves by other developed countries – Germany, Belgium, Australia, New Zealand, USA and Canada – who now use furnace tests for these critical cables. Furnace testing ensures the full cable specimen on test is exposed to the full furnace temperature, which is the same requirement for every other building element needing a Fire Resistance Level (EN 1363-1, ISO 834-1 or ASTM/UL 263 in USA).
We do note that a British test standard does exist for furnace testing fire-resistant cables (BS EN 50577:2015). This test is not adopted by life safety and firefighting equipment application standards nor is referenced in BS 8519, maybe because it has some deficiencies in that no mechanical or firefighting/water spray test is included, or that cables are not tested in realistic scale or even allowed to be fixed to cable tray supports during testing (purportedly to allow movement, which is not representative of actual installation practice). This test is also not a ‘cable system test’ and only tests cable as a stand-alone product.
Technical note: The insulation resistance of almost all known insulating materials decreases with increasing temperature; thus, the actual cable specimen test temperature is critically important.
Many modern buildings today are significantly bigger, taller more complex, more interconnected, multi-use, with higher fuel loads and often higher population densities than they were when IEC 331 and BS 6387 were introduced respectively 52 and 40 years ago, and by definition the newer British Standard iterations of these tests: BS 8434-2, BS EN 50200 and BS 8419. It is clearly in the public interest for safety to review and update these legacy test protocols.
BS 6387 CWZ is not a prerequisite for certifying fire-resistant cables used in the UK. BSI maintain this 40-year-old test standard because other countries around the world still use it. Given the questionable efficacy of this test standard, consideration should be given to withdrawing it.
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.
