EoW November 2012

Technical article

Fire resistant optical cable By L Caimi, D Ceschiat and M Maritano, Prysmian SpA, Milan, Italy, and E Consonni, Prysmian Cavi e Sistemi Italia Srl, Milan, Italy

Abstract A new family of optical fire resistant cables has been developed, used to provide the necessary levels of safety in critical environments as public buildings, subways and also in industrial areas. These new cables maintain their optical transmission characteristics with very limited change in attenuation for a long time in compliance with international standards. This innovative cable design, using special ceramifiable compound and appropriate flame shields allow control of heat release and to guarantee the right level of mechanical protection for the optical fibres during the burning phase, limiting in this way any variation in attenuations and avoiding transmission interruptions. 1 Introduction Low-Smoke-Zero-Halogen (LSZH) flame- retardant and fire-resistant cables, both copper and optical, are widely used to provide the necessary levels of safety in critical environments such as public buildings, ie hospitals, nursing and rest-homes, cinemas and theatres, subways, railway tunnels, and also in industrial areas. With regard to optical cables, when a fire occurs, the additional attenuation of the optical fibre directly affects the signal transmission performance and hence the effectiveness of the security systems where uninterrupted transmission is needed for operation of emergency devices such as phones, closed-circuit television, automatic doors, building management systems and fire alarms. The functionality of the optical cable must be maintained during the fire occurrence and usually also for a predetermined period of time. Starting from this scenario a fire resistant cable family has been developed with a high fibre count in a compact design, all dielectric or metal armoured constructions.

2 Fire resistant optical cable: new cable family solution The conventional fire resistant cables are not able to completely avoid the attenuation increase of the optical signal during a fire exposure; but, what is worse, the optical performances fully vanish when the fire is off and some mechanical breaks of the brittle glass fibres occur. In fact in the transition areas between the cable portions directly exposed to flame and the contiguous parts not burned, especially

during the cooling phase, the materials still surrounding the fibres cool down and shrink causing local pressure on the fibres that, without the coating protection, can break or can increase dramatically the signal attenuation. 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.

▼ ▼ Figure 1 : All dielectric cable design

1 Micromodule units 2 Ceramifiable inner tube 3 WS separator tape 4 Longitudinal strength members

5 Inner LSZH jacket 6 Fire resistant tape 7 Outer LSZH jacket

▼ ▼ Figure 2 : Metallic armoured cable design

1 Micromodule units

2 Ceramifiable inner tube

3 WS separator tape

4 Metallic armour

5 Longitudinal strength members

6 Outer LSZH jacket

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November 2012