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Innovative insulation technology improves fire safety01 March 2010Panel builders and electrical system installers will welcome an advance in the materials science related to the properties of cable insulation, which has opened up new possibilities for power and communications cables with outstanding fire resistance. Arnaud Piechaczyk explainsFire-resistant cables are normally specified for 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 national and international standards. 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 1980s, 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.
New customer requirements for improved fire performance, strippability and ease of installation and connection prompted Nexans to develop a new insulation technology dedicated to fire-resistance, which it has dubbed ‘INFIT’. This innovative technology combines, in a polymeric material, the advantages offered by both the tough mica tapes layer and the extruded silicone insulation layer. Thus fire-performance cables can be provided that are both tough and easy to handle, as well as being easy to strip and install.
When INFIT 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 optimise the nano-structure 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 and thus prevent short-circuits.
A ten-year research effort
The new technology is the result of some ten years of development by the Nexans International Research Centre 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 providing 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 also fulfilled 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.
Applications
Depending on the specific cable application, the new insulation material will 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, optimised for each application.
Nexans cables insulated with INFIT can, for example, resist fires reaching temperatures of around 1,000oC, at voltages of up to 1kV, with a high level of char cohesion and electrical insulation. It will be gradually deployed in the coming months across the Nexans fire resistant product ranges, including power, communications, control and LAN cables for use in public building and industrial applications, as well as the marine sector.
Arnaud Piechaczyk is R&D group leader, Nexans International Research Centre
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