EoW November 2012

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Technical article
November 2012 72
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
▼ ▼ Figure 2: Metallic armoured 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
1 Micromodule units
2 Ceramifiable inner tube
3 WS separator tape
4 Metallic armour
5 Longitudinal strength members
6 Outer LSZH jacket