Smoke without fire
Evolution of protocols for testing the fire safety of electrical cables has led to an important divergence between fire performances provided by electrical cables and fire performances expected and needed. The understanding amongst most specifiers, sellers, installers and users of fire safety cables is that the products they buy and use will provide a level of fire performance in real emergency conditions commensurate with the performance implied by the testing procedure.
Unfortunately, unless exposed to exactly the same fire conditions as in testing this is unlikely to be the case.
Before 1970 there were no common standards for testing electrical circuits for fire survival. Since 1932’s Mineral Insulated (MICC cables) had been used for high temperature resistance and it was considered almost common sense that these cables were the best option available for fire performance.
In the 1970’s new polymeric cable insulations evolved and manufacturers found that with Silicone Rubber or Glass Mica Tape insulation they could make cables to pass some flame tests. In 1970 IEC 331 was born and formed the basis for BS6387:1983 this is still the basis for most flame circuit integrity testing of cables in Europe and Asia.
In America, Canada, Australia, New Zealand, Germany and Belgium simple flame tests on cables are no longer accepted and certification requires furnace testing of full ‘wiring systems’ to the “Standard Time Temperature” protocol ISO834-1, EN1363-1, AS1530 pt4, BS476 pts 20-23 & in USA ASTM E119-75.
It is not commonly understood that fire resistant cables, where tested to common British flame test standards, are not required to perform to the same 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 all tested to the Standard Time Temperature protocol required by BS476 parts 20 to 23 (also known as ISO834-1, EN1363-1 or in America & Canada ASTM E119-75).
Contrastingly, Fire Resistant cable test standards BS 6387CWZ, BS8343-2, BSEN 50200, BS8491 require cables to be tested to standards which have lower final temperatures (than required by BS476 pts 20 to 23) and in ‘flame’ rather than ‘fire’ conditions.
Given Fire Resistant cables are likely to be exposed in the same fire and are needed to ensure that all Life Safety and Fire Fighting systems remain operational. Fire resistant cables are specified to ensure life critical circuits remain functional in a fire. Emergency lighting & power for alarms, sprinkler systems, communication / evacuation systems, ventilation systems etc MUST remain functional until everybody is out alive. They should last at least as long if not longer than fire doors, windows and walls etc.
Cables are installed by many different trades for many different applications, what is not often realized is that the many miles of cables and many tons of plastic polymers which make up the cable insulation and jacketing may represent one of the biggest fixed fire loads (fuel source) in a building.
The common argument that other organic / burnable materials in the building are greater is pointless as only electric cable has the real potential for all three basic requirements needed for a fire:
- Fuel source – materials used
- Air around the cable
- Ignition source – spark from outside cable or short circuit inside
There are two ways a polymeric cable can burn:
- External heat / fire
- Internal heat (overload / short circuit)
Common tests to evaluate flame propagation on cables are:
- IEC 60332-1 BS 4066-1 USA: UL1581, UL2556-pt 9.1, 9.3, 9.4 & NFPA 262
- IEC 60332-3 BS 4066-3 AS/NZS 1660-5.1 USA: UL1666, UL 1685, UL2556- pt 9.6 & IEEE 383
All these test are conducted on cables starting at ambient temperature but in practice they will be likely be at operating temperature 60, 70 or 90°C (if cables were tested preconditioned at operating temperature many would fail !)
There are “NO” flame propagation tests done or required by any Standard or AHJ for cables under short circuit or overload ! (given that reports by Fire Authorities often cite cables as the source of many fires this oversight is concerning !)
Note: once the fire reaches flashover temperatures 300˚C to 400˚C – all polymeric cables burn.
At present all tests on cables are done with flames on outside of the cable. This is only part of the danger and risk to life.
The insulation materials used on conductors are often hydrocarbons, meaning they burn. This includes PVC, Polyetheleen or XLPE, EPR etc. It is important to point out that another well know hydrocarbon is petrol!
As these materials are all oil based they all have a very high fuel element, they all burn.
In overload or short circuit the heat is on the inside – there does not need to be a fire for these cables to burn, as with the Address Hotel fire in Dubai this is often a cause of fire.
Why isn’t there International tests on cables for fire due to overload / short circuit? The evidence suggests there should.
Many polymeric cable manufacturers claim the polymers they use for insulation and jackets are low smoke. They often justify this by claiming compliance to tests like BS EN 61034.
These smoke obscuration tests are dependent on a specific sample weight of cable burned in a specific room / air volume.
These results are not predictive end use simulations. (Smoke generation can be greater on high heating before flame and smoke volume is directly related to amount of material burnt)
The current test is a 3m cube test with a flame, however as discussed polymeric cables burn much easier from the inside so most smoke is actually given off before a fire, this heating up causes a large amount of smoke to be released.
Major incidents like in the Singapore and Washington DC Metro systems, along with exploding manhole covers in London a few years ago and more recently the huge fire at the Address Hotel in Dubai on New Year’s Eve have all led to large evacuations due to great smoke release from polymeric cables…. All these caused by cable short circuits.
How can a BS-EN 61034 low smoke cable give off so much smoke?
PVC gives off more smoke in flame but PE / XLPE (polyethylene) gives of more smoke on heating without flame!
Plasticized UPVC is used to make general electric cable. In flaming and none flaming mode both feature high smoke outputs, indeed very bad as a fire proof cable…
Many leading fire resistant cable brands use polyethylene – In a direct flame it indeed shows as low smoke generation factor…. But under overload, short circuit or internal heat for any reason… 590DM is a huge amount of smoke.
It doesn’t take an actual fire to cause an emergency evacuation, smoke with no fire is more common, extremely costly not to mention a major risk to life!
Whilst provocative this is again factual information. The heat given off if directly proportional to oxygen consumed;
Petrol = 48 mega joules – We know how well it burns, that’s a lot of heat!
PE & XLPE = 46 – A very common cable insulation that shockingly releases almost as much heat as petrol!
If we consider how many tons of XLPE is currently specified and used in today’s buildings it’s not hard to acknowledge that they represent the biggest fire load.
Of course this burning up of the oxygen produces a ton of carbon monoxide too!
95% of deaths in fires are from carbon monoxide poisoning.
Current international standards for fire performance cables must be considered as only bare minimum performance standards in a controlled environment. There is no guarantee of the performance given in real-world scenarios, especially cables only to pass flame based standards.
It is therefore recommended for end-users, consultants and other industry experts to review any specific requirement for fire rated cables thoroughly and specify a solution that will give the required real-world performance. Whist carefully considering ways to reduce the overall risk of fire and its effects on the occupants in the building.
By taking the holistic approach to fire safety we can help improve the safety of our buildings and ultimately contribute to saving lives.
For more information, go to www.temperature-house.com