GB2138168A - Fire resistant optical fibre cable - Google Patents

Abstract

A fire-proof optical fibre cable comprises a signal carrying, optical fibre (2a-2d), which is embraced by a protecting jacket (4), preferably of organic material. The jacket (4) may be filled with silicone grease. The protecting jacket (4) is surrounded by in inorganic fire retardant material (5) which during a fire provides a pipe substituting the protecting jacket (4). Preferably the protecting jacket (4) is covered by mica tape, possibly an inorganic mass or another material which during fire is subjected to a sintering process and forms a fire protecting pipe. Preferably the protecting jacket (4) and its surrounding mica tape (5), or a group of such assemblies, is surrounded by a layer of glass tape (6), as well as a filler jacket (7), and another glass tape (8), possibly reinforced with steel therearound. Further, the cable may be equipped with an outer jacket (9) of a material which yields a protective ash during fire. <IMAGE>

Description

SPECIFICATION Fire-resistant fibre cable The present invention relates to fire resistant fibre cable comprising a signal carrying, optical fibre which is embraced by a protecting jacket, preferably a tube of organic material.

It is usual that optical fibres consist of highly purified vitreous silica, silica which for the achievement of the necessary variations in deflection index is doped with germanium, phosphorus, boron or fluorine. The optical fibre itself can thus comprise a doped core (of e.g. 50 micrometres diameter having a surrounding layer of glass with a diameter of for example 125 micrometres.

The surface of the fibre must immediately be protected against humidity and mechanical damage by a layer of acrylate, acetate, silicone rubber or similar. This is called the primary layer of the fibre.

The fibres are often further protected before being included in a cable structure, and this protection will as a rule comprise a loose or tight coverage.

With a loose coverage the fibres are surrounded by a hollow pipe, preferably a pipe of organic material, having an inner diameter of for example 1 mm and a wall thickness of 0,2-0,3 mm. The fibre is loosely arranged within this pipe ("loose tube") which further is often filled with grease for further protection against humidity.

A cable can now be constructed in such a way that the mechanical stresses are transferred to the fibre in no substantial degree.

With tight covering, the fibre is surrounded by a relatively thick (typically 100 micrometres) primary layer of for example soft silicone rubber, and directly in contact with this silicone rubber layer there is extruded a layer of stiffer plastics. The total diameter of the protected fibre can with tight coverage be approx. 0, > 1,0 mm.

The protected fibre can then be included in a larger cable structure.

In the development of fibre cable a great emphasis has been given to making the cable as resistant against mechanical damage as possible. However, this development has been for installation areas which have not been classified as especially fire hazardous. Thus conventional fibre cable, for exam ple the type which is produced according to the loose coverage or "loose tube" principle for avoiding micro bending of the fibre and for giving good mechanical protection, will not give the necessary protection during a fire, since the organic material embracing the fibre cable will burn out so that the mechanical protection of the fibre will disappear.

The object of the present invention is a fibre cable which not only has good mechanical protection under normal operation, but also tends to retain operational properties in case of fire.

The present invention provides fire-resistant fibre cable comprising a signal carrying, optical fibre which is surrounded by a protecting jacket in turn surrounded by inorganic fire retardant material which is transformable by fire into a protecting pipe.

Preferably the protecting jacket is covered with mica tape, preferably mica tape on a glass carrier, and preferably in the form of a winding.

During a fire these materials will be subjected to a sintering process and form a new pipe which protects the fibre and resists the formation in the fibre of micro bends due to mechanical or thermal influences. Such micro bends cause loss of light from the fibre and hence detract from the quality of transmission through the optical fibre, and it is therefore important that the mechanical protection be retained.

In a preferred embodiment of the present invention the protective jacket holds between itself and the optical fibre a material which is as passive as possible during a fire (low volume increase, gas development, combustibility etc.), for example a modified silicone grease.

A silicone grease will aside from aiding in the retainment of the loose coverage of the fibre cable, also aid in low supply of oxygen to the fibre during the fire, discouraging oxidization of the optical fibre.

An uncovered optical fibre will during a fire immedi ately oxidize and the mechanical properties of the fibre will be destroyed. The strength of the fibre can then be reduced to less than 10% of the original.

The ash of the silicone grease is silica in the form of tine powder which also will give mechanical support and thus mechanical protection to the fibre.

By using the materials as stated above it is also possible to use fibre having various primary layers available on the market, for example silicone layer, acrylate or some form of varnish, possibly aluminium layer.

In the cable according to the present invention a fire can transform the organic protection of the fibre at normal conditions to a temperature resistant inorganic protection. The use of mica tape in combination with a silicone grease filled organic jacket can - in such a fire - on the one hand entail that the mica tapes will sinter together to an inorganic pipe as a substitute for the organic jacket, and on the other hand entail that the ash from the silicone grease (which constitutes a fine silica powder) will still be held in position by the now organic pipe for protecting the inner optical fibre. The holding of the silica powder in a fixed position means that the fibre receives the necessary support to hold it in a proper functional position.

It is to be understood that the organic jacket does not have to be filled with silicone grease, since any material which has low volume increase, gas development and combustibility during a fire, and which during normal opeation has a small expansion coefficient and small change in viscosity relative to temperature, can be used.

For further support of the protecting jacket which is covered with the mica tape, there can outside one or more elements of the covered jacket be provided a layer of glass tape.

To further protect the cable structure against fire there can be provided an outer filler jacket having good fire retardant properties.

Preferably there is outside such a filler jacket provided a second glass tape, possibly including a steel reinforcement over the tape. Such a further measure will enhance the mechanical properties of the cable before, during and after a fire.

Finally, the cable can be provided with an outer jacket of a preferably halogen-free material with appropriate fire retardant properties.

Cables according to the present invention have been tested in a fire chamber wherein they have been subjected to fire at 930"C for 3 hours; the cables were fully operational during the complete fire test, i.e. transferred the necessary light energy which was transmitted at the one cable end for satisfactorily influencing the receiver element provided at the other end of the cable.

The cable structure has few or no corroding effects on the surroundings during a fire, and will because of its composition of materials comply with stringent fire technical, mechanical and operations requirements both before, during and after a fire.

In the following various embodiments of the invention will be described, by way of example, with reference to the accompanying drawings, in which like reference numerals indicate like items and in which: Figure 1 is a section through a first embodiment of a fire-proof fibre cable according to the present invention; and Figure 2 is a section through another embodiment of a fire-proof fibre cable according to the invention, In Figure 1 reference number 1 designates an optional inner core of steel or fibre reinforced plastics having high mechanical strength and low temperature expansion coefficient. Such a core can favourably be used where the cable is to contain more than one fibre.

Around the core 1 are four optical fibres 2a-2d, which can be covered with a layer of acrylate, acetate or silicone rubber, this layer protecting against humidity and mechanical damage of the optical fibre itself.

Around each of the optical fibres 2a-2d there is provided a jacket 4 of organic material, e.g. an acrylic, and the spaces between the pipes 4 and the individual fibres 2a-2d are tilled up with silicone grease 3, the combined acrylic jacket 4 and silicone grease constituting a so-called loose coverage or "loose tube" mounting of the optical fibres 2a-2d.

Around each jacket 4 of organic material there is wound a layer 5 of mica tape, the mica material preferably being arranged on a glass carrier.

During a fire the material with which the jacket 4 is covered will be subjected to a sintering process so that a new pipe is formed as a substitute for the organic jacket, a fact which aids in the maintenance of the mechanical protection of the optical fibres 2a-2d during and after a fire. At the same time the filler mass 3 in the spaces between the jacket 4 and the individual optical fibres 2a-2d will be of such a constitution that during a fire there produced as little residue as possible, avoiding substantial ozidization, gas development, carbon deposit and pressure increase between the jacket and the fibres 2a-2d.

Aside from reducing the danger of pressure increase during a fire, the silicone grease 3 preferably has a small expansion coefficient and a small change of viscosity with temperature. This favours the least possible displacement of the fibres 2a-2d both during normal operation and during fire conditions, so that the fibres are protected mechanically against the formation of micro bends which cause light transmitted through the optical fibre to be emitted and not reach its destination for the maintenance of communication between the two cable ends. The possibility of breakage in the fibre is reduced correspondingly.

The ash of the silicone grease is silica in the forrr: of fine powder which will give mechanical support to the fibre. At the same time the pipe of sintered mica tape substitutes for the organic pipe and constitutes a support for the silica powder, so that this is not allowed to flow away but remains embracing the fibre.

Around one or a group of organic jackets with fire retardant covers 5 there is provided a layer of glass tape 6, the tape 6 serving to support the structure both before and after a fire and giving extra flame protection.

Outside the glass tape 6 there is provided a filler jacket 7 having good fire retardant properties.

Outside the filler jacket 7 there may be an armour, braiding, covering or wiring of glass, steel or other fire-proof material.

Outside the filler jacket there is in the embodiment illustrated in Figure 1 provided a further glass tape 8, possibly reinforced with steel over the tape. Such a combination serves to keep the filler jacket 7 in position during fire conditions.

On the outside of the glass tape 8 there is provided an outer jacket 9 having good fire retardant properties, the material being of such a constitution that it provides a protecting ash during fire. The glass tape 8 may have an outer jacket 9 which yields protective ash in a fire.

In Figure 2 there is illustrated a section through another embodiment of a cable structure according to the present invention, the basic components included in this cable structure being the same as those of the embodiment illustrated in Figure 1; however, the number of fibre cores is different from the previously discussed embodiments. Thus, Figure 2 illustrates a central element 1 of steel or fibre reinforced plastics, possibly glass having high mechanical strength and small temperature expansion coefficient, and around the core 1 there is provided a ring of optical fibres 2a-2f.

Otherwise the structure of the cable version in Figure 2 is as discussed in connection with Figure 1.

Tests have indicated that a cable structure in accordance with the invention can resist a fire at 930"C for 3 hours without losing its functionality, i.e.

without losing the signal level for light transmission which is necessary for reliable communication between the two ends of the cable. The cable structure according to the invention should thus meet the requirements for being operable for half an hour during a hydrocarbon fire in which temperatures above 900"C often can be present.

Claims (15)

1. Fire-resistant fibre cable comprising a signal carrying, optical fibre which is surrounded by a protecting jacket in turn surrounded by inorganic fire retardant material which is transformable by fire into a protecting pipe.

2. Cable according to claim 1 wherein the inorganic material comprises mica tape.

3. Cable according to claim 2 wherein the mica tape is on a glass carrier.

4. Cable according to claim 1 or 2 or 3 wherein the protecting jacket holds between itself and the optical fibre a filler material which does not favour oxidization of the optical fibre and which generates little excess products during a fire.

5. Cable according to claim 4 wherein the filler material comprises silicone grease.

6. Cable according to claim 5 wherein the silicone grease is one which during a fire forms a fine silica powder which supports the fibre, the mica tape providing a sintered pipe which keeps the silica powder in position.

7. Cable according to any preceding claim comprising one or more of said surrounded jacketed fibres with glass tape therearound.

8. Cable according to claim 7 having a filler jacket of fire retardant properties around the glass tape.

9. Cable according to claim 8 having outside the filler jacket an armour, braiding, covering or wiring of glass, steel or other fire-proof material.

10. Cable according to any preceding claim having an outer jacket of halogen-free material.

11. Cable according to any of claims 1 to 9 having an outer jacket which yields protective ash in afire.

12. Cable according to any preceding claim having a reinforcing core.

13. Cable according to claim 12 wherein the core is of steel, fibre reinforced plastics or glass.

14. Cable according to any preceding claim wherein the protecting jacket is of organic material.

15. Fire-resistant cable substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.