EoW November 2012

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Technical article
November 2012 73
Depending on the application sometimes
the fire resistance 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
important to build around them an
impassable barrier.
Whilst a metallic tube could represent
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 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,
▲ ▲ Picture 1: Metallic and all dielectric cable version
• a supplementary flame shielding,
metallic or dielectric
• a flame retardant LSZH sheath
2.1 Ceramifiable layer as first absolute
fire barrier
In order to ensure an absolute protection
to the optical fibres during fire, it is
▲ ▲ Picture 2: IEC 60331-25 fire test ▲ ▲ Picture 3: EN 50200 fire test
▼
▼ 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
Attentuation [dB/fibre]
Time (minutes)