WCA January 2013

❍ ❍ Picture 1 : Metallic and all dielectric cable version

❍ ❍ Picture 2 : IEC 60331-25 fire test

❍ ❍ Picture 3 : EN 50200 fire test

The typical fibre organisation in an optical cable is based on multi-loose stranded plastic tubes; the present trend is to increase the fibre count, reducing or at least not increasing the size of the final cable. For the above reasons a different cable design has been developed to increase the density of fibres and to facilitate the fibre access. Depending on the application sometimes the fire resis- tance requirement is combined with barriers to animal attacks and superior mechanical performance, so a metallic protection is needed; in other cases the problems induced by magnetic or electric interference lead to an all dielectric solution. Both constructions have been developed taking care to meet needs of the diversified installations and markets. Therefore the developed cable family has been designed to match the following requirements: • maintain the optical transmission capability during the fire • avoid breaks of the optical fibres after the fire extinguishing • increase the fibre count in a more compact design • have a metallic protection or a full dielectric design As a consequence of the above requirement the new cables have been designed with a construction based on: • optical fibres organised in bundles in the form of micromodules • a surrounding tubular layer made of a special ceramifiable material • a supplementary flame shielding, metallic or dielectric • a flame retardant LSZH sheath

retardant version with proper mineral additives, fully disappear reduced in brittle ashes. The solution is a material that is able to withstand the flame action without burning or collapsing for a sufficient time to allow the formation of an underlying layer of a ceramifiable material to complete the ceramisation. A special compound has been developed based on a mixture of inorganic fillers characterised by different behaviour in temperature progressively melting and controlling the viscosity and the sintering ability. It is helpful to introduce a second flame shielding layer in the design of the cable, in order to avoid a direct contact of the ceramifiable tube with fire; in fact the shielding layer allows a more homogeneous and progressive compacting process of the ceramifiable special compound, obtaining a final solid tubular element which protects uniformly the optical fibres. In this case the type of shielding can be quite conventional, ie made by mica tape or steel tape. 2.2 Cable design Starting from the idea of a fire resistant cable based on a ceramifiable tube surrounded by an external fire shielding that protects from direct contact with fire, the other elements of the cable design are driven by the mechanical and optical requirements based on the installation and operating conditions.

❍ ❍ Figure 3 : Fire resistant test of all dielectric versions according to IEC 60331-25

Fire resistant test IEC 60331-25 All dielectric micromodules cable 90min flame + 15 min cooling

2.1 Ceramifiable layer as first absolute fire barrier

In order to ensure an absolute protection to the optical fibres during fire, it is important to build around them an impassable barrier. Whilst a metallic tube could repre- sent an obvious solution, different shrinkage behaviours between metal and glass and some manufacturing limitations can make this way not so viable. Moreover plastic materials are not suitable to resist temperature up to 800-1,000°C, and also if in flame

Attentuation [dB/fibre]

Time (minutes)

54

www.read-wca.com

Wire & Cable ASIA – September/October 2007 January/February 2013