GB2247340A - Fire resistant electric or optic cable - Google Patents

Abstract

The cable comprises a self-supporting part incorporating electric or optic conductors surrounded by a gas-tight loosely-fitting conduit comprising a metal shield and of limited fire hazard material. In the embodiment, the conduit is made of overlapped corrugated stainless steel tape 10 covered with an extruded layer of halogen-free plastics 11. The other part of the cable comprises a tension member 1, plastics tubes 2 accommodating loose optic fibres 3, polyethylene fillers 4, paper tape 5, a moisture barrier of A1/plastics laminate 6, an identification tape 7, a ripcord 8 and a limited fire hazard sheath 9. The gap between the conduit and the other part is 1 mm. The loose fit allows gas or smoke generated by a fire to escape from the vicinity of the fire. The sheath 9 may be omitted in an all-electric cable. <IMAGE>

Description

IMPROVEMENTS RELATING TO CABLES This invention relates to cables, particularly limited fire hazard cables.

Limited fire hazard cables are, as their name implies, cables which present a reduced hazard to life and property in the presence of fire, both with regard to the fumes emitted and the temperature at which they burn, than conventional PE/PVC - sheathed cables and British Standard 4066 parts 1 and 3 cover these cables.

Conventionally such cables incorporate materials which are designed to reduce the hazard which would otherwise be presented.

The hazard arises in two ways: firstly an electric cable may itself be the source of a fire if overloaded; secondly, a cable be it an electric cable or other form of cable, such as an optical cable, may support combustion in the presence of a fire caused by external means.

In either event the cable must minimise the support of combustion, the production of smoke, and the production of noxious fumes.

It is an object of the present invention to provide a novel solution to the problems raised above.

According to the present invention a limited fire hazard cable comprises in combination a first self-supporting cable part incorporating electrical and/or optical conductors, and a second part loosely fitting around the first part and forming a tubular conduit, said second part being formed of limited fire hazard material including a metallic shield and being gas tight.

Preferably the second part comprises a corrugated stainless steel tape forming the inside of the second part, surrounded by an extruded layer of limited fire hazard plastics material. Conveniently the plastics material is a halogen free, low smoke emitting material.

The metallic screen can be a longitudinally folded corrugated or flat tape, or indeed a helically applied tape.

The first cable part which is housed in the conduit-like part need not itself be constituted of limited fire hazard materials and could in fact be a conventional cable or cable core, the metallic shield of the conduit-like part preventing the inner first cable part: from emitting fumes into the air should it be the source of the fire or, alternatively, supporting combustion in the presence of a fire from another source.

The outer part fits on the inner part and is what can best be described as a loose sliding fit. In the presence of a fire from an external source, and in the event that the inner cable is of non-limited fire hazard materials, then the combination will ensure that any gases emitted by the inner part in the presence of a fire will be constrained by the conduit-like part and these fumes will be conducted away by the conduit from the location of the fire.

According to another aspect of the present invention there is provided a method of making a cable, comprising providing a first cable part incorporating optical and/or electrical conductors, and progressively forming a metal tube around the first part to form a conduit-like part which is a loose sliding fit on the first cable part and such as to provide a limited fire hazard cable.

In order that the invention can be more clearly understood reference will now be made to the accompanying drawings:- Fig 1 shows in cross section an optical fibre cable according to an embodiment of the present invention, and Fig 2 shows a diagram illustrating the different performance of a cable of Fig 1 compared to an ordinary cable in the presence of fire.

Referring to Fig 1 of the drawings the cable is in two parts, a first inner part surrounded by a second outer part or conduit-like part which is a loose sliding fit on the inner part.

In the case of the copper cable there may only be one sheath i.e. a loose configuration around the core.

The inner part comprises a sheathed central strain member 1, surrounded by a plurality of polymer tubes 2 each accommodating loose optical fibres such as 3. These are helically wound around the strain member 1 but not all the positions are filled by optical fibres and as can be seen there are some polyethylene rod-like filler members 4.

The loose tube fibres and filler members are surrounded by a paper tape 5, in turn surrounded by a longitudinally applied moisture barrier tape 6 (which is optional) comprising an aluminium/plastics laminate. An identification tape 7 and ripcord 8 are incorporated.

Around the moisture barrier tape is extruded a limited fire hazard sheath 9.

Thus far, the first part of the cable has been described and comprises a self-supporting optical fibre cable which is satisfactory in itself in many applications but may not reach certain fire hazard requirements.

In order to enhance this aspect and provide a limited fire hazard cable, a second part is formed around the first part.

The second part comprises a longitudinally applied stainless steel tape 10 which has overlapped longitudinal edges and is corrugated, the degree of overlap in this embodiment is about 25% of the circumference in cross section shown in the drawing, and indicated by the reference numeral 10A.

Around this stainless steel tape 10 is extruded a sheath 11 of halagen-free low-smoke-emitting plastics.

The inner cable part up to and including sheath 9 is a loose sliding fit within the stainless steel tape 10 so that air or other gas can pass longitudinally relatively easily between the external surface of sheath 9 and the internal corrugated surface of tape 10.

In this way even if the inner cable part within the sheath 9 presents a fire hazard, any smoke or fumes generated in the presence of a fire will be constrained by the combination of tape 10 and its surrounding sheath 11 so that these fumes will not escape in the vicinity of the fire. The sheath 11 is of limited fire hazard material and therefore does not support combustion within the terms of British Standard 4066.

It may be possible to utilise a metal other than stainless steel but stainless steel is preferred because it is rust proof, it being understood that any severe corrosion of the metal tape might present a block or severe restriction to the passage of gases longitudinally between the outer surface of sheath 9 and tape 10. It may also cause the cable to fail in a different manner e.g. rodents. In the embodiment described the cable diameter is about 22mm and has a minimum bend diameter of 330mm. The tensile strength is about 1000N.

The inner diameter of the corrugated tape 10 is approximately 16.5mum whereas the outer diameter of sheath 9 of the inner cable part is about 15mm, giving a clearance gap of around 1.5mm. A minimum gap would be about lmm average for a circular sheathed core.

Although an all-optical cable has been described the invention is also applicable to copper cables and to a combination cable comprising both optical and electrical conductors. Thus the polyethylene fillers 4 in the drawing could alternatively be PE or PVC insulated copper conductors (combination cable) and in an all-electric version all the optical conductors 3 could likewise be replaced with insulated electrical conductors. The all-electric cable may not require the sheath 9, leaving just a loose configuration of conductors as a core within the outer part.

As mentioned, the inner cable core need not be of limited fire hazard type, the elegance of the present invention enabling for example small volumes of relatively flammable substances to be constrained within the core package. For example hydrocarbon-based loose-tube filling compounds in the polymer tubes 2 which house the optical fibres 3.

In manufacturing the cable described, the gap is maintained by manufacturing the stainless tape tube 10 formed by a die rather than by forming the tape onto the inner sheath 9. Stainless steel is not the only suitable material, the major consideration being that it is not coated by a material that will contribute to the fire, or produce smoke or noxious gases during burning or reduce the bond between the metal layer and the outer sheath 11 during burning, i.e. at high temperatures. For example another suitable material would be aluminium although for other reasons it is not so good as stainless steel. For example it does not provide rodent protection.

The metal chosen is preferably uncoated and since the limited fire hazard type materials generally available are less able to resist water permeation (i.e.

they are hygroscopic) than for example polyethylene a corrosion resistant metal is indicated. On these grounds the use for example of mild steel is considered inappropriate on the grounds of likely corrosion.

Although the preferred embodiment shows an outer sheath 11 of limited fire hazard material, it would be feasible to weld the steel tape and leave it unsheathed.

The preferred embodiment described provides a metal barrier which is used to provide a shield which will remain intact during burning and although the cable core will still see high temperatures, the flames themselves are never allowed to play upon the inner cable or cable core. For this requirement it can be seen that any metallic components could be considered, although for the purposes of cable manufacture and specific requirements such as water permeation and rodent protection, the metals considered would include steel, brass and aluminium.

In the preferred embodiment a minimum gap of lmm has been mentioned. The major objective for the gap is to allow the inner cable to expand during heating whilst maintaining a longitudinal path that will allow the volatile materials that are generated to be completely expelled from the area of fire. Without such a gap a build up of pressure would cause the cable eventually to explode, exposing the flammable components to the fire.

The invention is applicable mainly to data and telecommunications cables, these cables not being required to carry power. In such cables there is a minimal danger of ignition from within the cable itself unless there is a major equipment failure. If a fire begins to propagate, started by an external cause - the primary-fire - this constitutes a growing hazard to life and property. A stage will be reached when life is threatened by the primary fire itself. Providing the cable does not add significantly to this hazard before the lethal stage is reached, then the cable can be considered safe.

Fig 2 of the drawings illustrates fire hazard and fire growth.

Fig 2 shows on arbitrary scales the growth of a primary fire in an enclosed space growing over time and constituting an increasing hazard to life. Firstly it will impair the possibility of escape and then it will itself become directly lethal. Also shown are the direct hazards caused by an 'ordinary" cable and a limited fire hazard cable.

The combined effects of the primary fire plus the two types of cable show that an tordinary cable adds an earlier hazard to life, but that a properly designed limited fire hazard cable does not. Although a limited fire hazard cable may eventually constitute a hazard, the sheath designs delay this until after the point when the primary fire itself would have caused fatality directly, or as a practical minimum, its contribution is insignificant up to this point in time.

Claims (7)

1. A limited fire hazard cable comprises in combination a first self-supporting cable part incorporating electrical and/or optical conductors, and a second part loosely fitting around the first part and forming a tubular conduit, said second part being formed of limited fire hazard material including a metallic shield and being gas tight.

2. A cable as claimed in claim 1, wherein the outer part comprises an outer limited fire hazard sheath locked to an inner longitudinal metallic tape folded to form a circular metallic conduit.

3. A cable as claimed in claim 2, wherein the metal tape is stainless steel.

4. A limited fire hazard cable substantially as hereinbefore described with reference to the accompanying drawing.

5. A method of making a cable, comprising providing a first cable part incorporating optical and/or electrical conductors, and progressively forming a metal tube around the first part to form a conduit-like part which is a loose sliding fit on the first cable part and such as to provide a limited fire hazard cable.

6. A method as described in claim 5, wherein the metal tube is formed from a longitudinally-folded steel tape with overlapped edges and transversely corrugated, and a sheath is applied by extruding a limited fire hazard plastics material onto the folded tape.

7. A method of making a cable substantially as hereinbefore described with reference to the accompanying drawings.

7. A method of making a cable substantially as hereinbefore described with reference to the accompanying drawings.

AMENDMENTS TO THE CLAIMS HAVE BEEN FLED AS FOLLOWS 1. A limited fire hazard cable comprises in combination a first cable part incorporating electrical and/or optical conductors, and a second part loosely fitting around the first part and forming an annular duct therebetween, said second part being formed of limited fire hazard material including a metallic shield and being gas tight.

2. A cable as claimed in claim 1, wherein the outer part comprises an outer limited fire hazard sheath secured to an inner longitudinal metallic tape folded to form a circular metallic tube.

3. A cable as claimed in claim 2, wherein the metal tape is stainless steel.

4. A limited fire hazard cable substantially as hereinbefore described with reference to the accompanying drawing.

5. A method of making a cable, comprising providing a first cable part incorporating optical and/or electrical conductors, and progressively forming a metal tube around the first part,the metal tube being a loose sliding fit on the first cable part so as to define an annular duct therebetween whereby to provide a limited fire hazard cable.

6. A method as described in claim 5, wherein the metal tube is formed from a longitudinally-folded steel tape with overlapped edges and transversely corrugated, and a sheath is applied by extruding a limited fire hazard plastics material onto the folded tape.