By Arnaud Piechaczyk, R&D Group Leader, Nexans International Research Center
Fire-resistant cables play a crucial role in applications where it is essential to ensure the integrity and continuity of vital safety circuits during the critical building evacuation and fire fighting periods required by stringent national and international standards. In many cases, the need for cables that can deliver the required levels of performance, reliability and safety has forced designers and installers of electrical systems to compromise on other desirable properties regarding ease of handling and installation.
Now though, a major advance in the materials science related to the properties of the cable insulation – called Infit – is making it possible to produce new families of electrical power and data cables with outstanding fire resistance using classical extrusion methods. The result is a user friendly cable that offers the ‘best of both worlds’.
Eliminating the need to chose between insulation technologies
Until now, the cable industry has mainly relied on two major technologies to insure the integrity of flexible cable insulation during a fire: XLPE/Mica taping and ceramic forming silicone rubbers.
Each of these technologies presents a number of advantages. The classical insulation taping based on Mica, and largely used since the 80s, can easily be implemented on an industrial scale to provide a tough, effective electrical insulation when overlaid with a cross-linked polyethylene (XLPE) coating. It is strong but stiff to handle, so can present some installation difficulties.
On the other hand, silicone rubber insulation can be extruded directly on to the conductors, and offers a good compromise between fire performance and ease of installation thanks to its flexibility. It is, however, vulnerable to cuts and tears.
Increasing customer demands for improved fire performance, together with strippability, ease of installation and connection prompted Nexans to search for a new insulation technology that could offer all these benefits.
Infit transforms into a tough insulating ceramic layer
Infit is a unique, proprietary innovative technology that combines, in a polymeric material, the advantages offered by both the tough mica tapes layer and the extruded silicone insulation layer. This now enables the manufacture of fire-performance cables that are both tough and easy to handle as well as being easy to strip and install.
Infit technology offers enhanced fire-performance because when the insulation is exposed to fire, it transforms from a flexible, plastic covering to a tough insulating ceramic layer, hardening like clay in a potter’s oven to form a protective shield. The key to the success of the new insulation has been in using advanced materials and polymer science to optimize the nanostructure of the primary insulation materials. A combination has been found that reduces the occurrence of cracks or breaks in the insulation to preserve the operational integrity of the circuit i.e. preventing short-circuits.
Extensive laboratory tests have shown that cables with Infit insulation will to continue to deliver power in the event of a fire, long after the plastic sheath and insulation have burnt away. This means, for example, exit lighting, smoke and heat exhaust ventilators, fans or pumps will still function reliably, even in areas directly affected by fire, ensuring safe evacuation.
The science part
The new technology is the result of some ten years of development by the Nexans International Research Centre based in Lyon, France, working in close partnership with the Australian Nexans R&D Centre. The successful outcome of the project has been based on fundamental studies that especially highlight the synergy between ceramic science and the latest polymer science.
In classic ceramic science, a well defined curing process is followed to form a high performance ceramic. Yet, in the case of accidental fire, the temperature increase is sudden and unmanaged. So the first challenge was to develop a ceramic forming system able to react and form an electrical insulating shield in a very short time across a wide range of temperature increases, while also exhibiting a high level of electrical insulation.
The second parallel challenge was to achieve this performance using an extrudable formulated polymeric material, rather than a powder, that was also suitable for the very demanding standards of the cable industry. As a result, Infit technology is principally based on filled copolymers of polyolefins (like polyethylene). This kind of polymeric matrix is well adapted to the extrusion process, and well known in the cable industry - but it is also intrinsically highly combustible. However, Infit uses the synergy between this combustible matrix and a mixture of inorganic fillers to create a new insulating material that offers superior fire-performance.
Depending on the specific cable application, the new insulation material can be offered in either a cross-linked or thermoplastic version. This will enable cables to offer the ideal combination of fire, mechanical, electrical or thermal properties optimized for each application.
Infit is a proprietary Nexans technology, and can be implemented in compliance with many worldwide cable standards, and according to the most rigorous product quality and safety criteria. Nexans cables insulated with Infit can, for example, resist fires reaching temperatures of around 1,000°C, at voltages up to 1kV, exhibiting a high level of char cohesion and electrical insulation.
Infit is gradually being deployed across the Nexans fire resistant product ranges. This will include power, communications, control and LAN cables for use in public building and industrial applications and it is expected to be of particular interest to the marine sector.