Where did halogen-free flame retardant cables come from and why are they important? Mark Froggatt, Technical Director at BASEC, explains more.
Often overlooked, the electric cable is a fundamental ingredient in any Infrastructure project globally. The greater need for more power, more data transmission and more of everything, in general, has driven significant development of cable technology over the years.
In the 1880s, the first cables were insulated with gutta percha, a natural latex material produced from the sap of trees of the same name. In the late 19th century, paper cables were becoming more popular and could operate at voltages up to 10KV. However, in the 1930s in Germany, the evolution of the numerous polymeric insulation materials in use today started to gather pace, and by the 1950s, PVC cables were more commonplace. Other materials, such as chlorosulphonated polyethylene (CSP), were developed.
From this point onwards, the development and progression of cable technology to fulfil the ever-growing requirements of power and a data-hungry world have continued. In the 1970s, using XLPE as an insulating material significantly developed in the power transmission and distribution sector, replacing paper-insulated cables with a much improved dielectric property. At this time the use of PVC and XLPE was widespread, however due to the growing uses of cables in a wide range of applications with high power requirements, the suitability of some of these materials was brought under scrutiny.
Tragedy driving change
There have been several high-profile tragedies that have driven change, such as HMS Sheffield going down in flames during the Falkland war (1982), the Kings Cross station disaster in 1987 which resulted in the loss of 31 lives, as well as the Dusseldorf airport fire in 1996.
In all these instances, the presence of choking black smoke and acidic gases were deemed to be a significant contributor to the loss of life due to the inability to evacuate and the respiratory issues caused by the smoke and fumes, with burning PVC cabling cited as one of the contributory factors in the smoke and fume production.
It has been established that when combusted PVC evolves, significant amounts of choking black smoke and acidic fumes are in the form of hydrochloric acid. This is often the case when it comes to in internal cabling, specifically in areas of high human traffic. As a result, an alternative to PVC was required.
The evolution of halogen-free flame retardant cables
The detrimental effects of PVC and other halogenated materials in the form of the evolution of acidic, choking, dense smoke-producing fumes and gas in the event of fire have led to the development of materials which offer a much cleaner burn. This is particularly important in areas where public evacuation in the event of a fire is of the utmost importance, and hence there has become greater regulation and de-facto regulation in airports, high-rise buildings, and hospitals, to name a few. This material evolution has been a journey, though, with several advances.
The primary reason for this is that halogenated flame-retardant materials chemically disrupt the production of free radicals and effectively shut down the combustion process. Halogen-free materials rely on inorganic materials, such as aluminium trihydrate ATH or magnesium dihydrate MDH, which works by withdrawing heat from the plastic during its decomposition into metal oxide and water. The water vapour propagates and reduces the fuel supply to the flame.
This creates formulation problems in that the polymer is required to accept a large amount of mineral filler and maintain some mechanical and electrical properties – mutually exclusive, as adding the filler inherently reduces both.
When it comes to the maintenance of electrical properties, the inorganic materials significantly reduce the dielectric properties of materials such as polyethylene, considered the best polymeric insulator available. This makes using halogen-free low smoke compounds as insulators more difficult, as the primary function of the insulator is compromised.
The current state of play
Over the years, there have been many advances in the properties of halogen-free materials in cables. The dichotomy between electrical/mechanical performance and flame retardancy has been addressed in a number of ways, with chemical additives allowing increased use of mineral fillers, polymer composition and the development of “thermoplastic elastomers” such as Ethylene Vinyl Acetate Copolymer (EVA).
These materials allow significantly higher amounts of mineral filler to be incorporated into the compound to give a higher flame-retardant performance. This does, however, reduce the dielectric properties. Due to this, the usual mechanism for reduced flammability in halogen-free cables adopts the approach that the cable is protected from fire from the outside.
High-performance, low-flammability cable sheaths are employed to prevent the interior of the cable from becoming compromised by the fire. In this way, better dielectric materials such as polyethylene and cross-linked polyethylene can be used as insulation – allowing for better data transmission in data cables (where data speed and integrity are becoming more prominent in a data-driven world). Likewise, smaller wall cables for medium voltage cables are used in buildings – which is becoming increasingly popular.
So, why is third-party approval important?
High-performance flame-retardant materials allow an improved reaction to fire performance, although it is critical to the cable performance that these are used in suitable applications.
The fact that the technology employed in these materials comes at a higher cost means that the manufacturer must use more expensive materials in their cable designs. In today’s ultra-competitive world, this leads to the typical discussion around ‘golden samples’, where high-end materials are used for type approval tests, and a cheaper, less effective material is used for ongoing production.
This potential risk can be mitigated with a BASEC product approval, where all cable designs are tested annually, and materials are fingerprinted to ensure that the approved materials are used on an ongoing basis. This type of product approval, along with the CPR/UKCA approach to product selection, means that building designers and specifiers can have peace of mind when it comes to cable selection of BASEC approved products.
Learn more about BASEC cable testing and certification here