FIRE-RESISTANT CABLE

Background of the Invention

The present invention relates to fire-resistant cables. In particular it relates to cables (e.g. electrical or telecommunications cables) which are required to function in the event of a fire.

Systems such as fire alarm systems, fire prevention systems and close circuit television systems used in buildings, ships, and tunnels use electrical and telecommunication cables which are required to function in the event of a fire. It may be particularly important that they continue to function in the critical early stages of a fire because they may be vital for initiating or monitoring evacuation, or guiding fire and rescue services.

British Standard BS 7346-6:2005 "Components for smoke and heat control systems Part 6 Specification for cable systems" provides a new generation fire test requirement. Some cables used in certain fire detection and prevention systems are presently required to meet this standard.

The test set out in BS 7346-6 is particularly onerous, requiring applying a flame at 85O ⁰ C combined with direct mechanical impact and a period of application of a water jet to simulate a fire fighter's hose. The cable must maintain integrity for 2 hours to reach the maximum "120 minute" rating which is required for certain fire fighting applications. The complete cable fire test method is given in Annex B of BS 7346- 6:2005.

There are presently moves towards transferring the BS 7346-6 test standard into a new identical 'stand-alone' standard, BS 8491. This new standard is presently going through its "Draft for Public Comment" stage - it has yet to become a published standard.

There are various known cable constructions which met the old standard BS 7846 (i.e. prior to BS 7346-6) for fire-resistant cables. In order to maintain integrity under the old standard tests, many cables included a layer of armour in the cable construction, often consisting of a layer of helically applied steel wire. However,

under the new standard BS 7346-6 it has been found that helically applied steel wire armour is insufficient to achieve the maximum 120 minute rating.

Thus, cables have been developed which perform better in the new test. In these cables, the layer of helically applied steel wire armour is replaced with a layer of interlocking steel tape armour. For example, Prysmian Cable & Systems Limited produces a cable under the trademark FP600® which includes a layer of interlocking steel tape armour and which achieves the maximum 120 minute rating under the BS

7346-6 (BS 8491) test conditions.

However, cables including a layer of interlocking steel tape armour have several disadvantages:

(i) The assembly of conducting cores can move within the steel tape armour, leading to slipping of the cores relative to the outer sheath, which increases the chance of loss of cable integrity and subsequent faulting. This can be particularly problematic when running cables vertically (e.g. up walls), and to prevent slipping it may be necessary to attach the cable in a repeating S-shape (in other words "snaking") up the wall; such snaking configurations are unsightly, and further lead to increased costs due to an increased length of cable being required;

(ii) Steel tape armour has a high impedance (relative, e.g. to steel wire armour); thus steel tape armour cannot sufficiently carry a fault current. As a result, cables including steel tape armour often require an additional conductor for the earth fault path; (iii) Interlocking steel tape armour imposes certain manufacturing limitations, such that cables have a limited range of nominal cross-sectional areas (e.g. 10-120mm ² );

(iv) Interlocking steel tape armour is more expensive than traditional steel wire armour and plain flat steel tape armour, and further is more difficult to apply.

Thus, there is a need for fire-resistant cables which meet the new, more rigorous BS

7346-6 (BS 8491) standard, in particular to the maximum 120 minute level, but which do not have the disadvantages set out above.

Summarv of the Invention

According to the present invention there is provided a cable comprising one or more insulated conductive cores; a layer of armour around the insulated cores; one or more layers of glass and/or mica tape around the layer of armour; and an outer sheath of insulating polymeric material.

It has been found that cables of the present invention produced highly satisfactory results in the aforementioned BS 7346-6 (BS 8491) test. Examples of the present invention may be capable of obtaining the maximum 120-minute level in the BS 7346-6 (BS 8491) test. It is believed that provision of a layer of armour with a surrounding layer (or layers) of glass or mica tape surprisingly gives cables of the present invention extra strength and durability in fire conditions. In particular, the combination of armour and glass or mica tape layers surprisingly provides sufficient structural protection such that examples of the present invention may resist the direct impact and powerful water jets which may be applied during this test.

The layer of glass or mica tape (or one of the layers of glass or mica tape if there is more than one layer of tape) may be located immediately adjacent to the layer of armour. Thus, the glass or mica tape may be in direct contact with the armour and remains in close contact with the armour during a fire. If intervening layers of e.g. plastic were introduced, these could char during a fire and the glass or mica tape could become loose around the armour and less effective as a result.

Preferably the layer of armour is applied helically (e.g. in a manner well known in the art). Preferably, the layer of armour is a layer of steel wire armour, more preferably a layer of galvanised steel wire armour.

It has been found that cables of the present invention exhibit significantly reduced movement of the assembly of conducting cores within the layer of armour; thus, slipping of the cores relative to the outer sheath is minimised. In particular, slipping of the cores is significantly less than in presently available cables which have been designed to pass the BS 7346-6 (B 8491) test. Therefore, it is not necessary to 'snake' cables of the present invention up walls, eliminating the associated disadvantages of doing so. It is believed that this advantage arises on account of the specific cable composition of the present invention; the layer or layers of glass or

- A - mica tape surrounding the armour layer may provide a holding mechanism to restr the armour layer from moving during the test. The use of steel wire armour may increase this advantage.

In cables of the present invention which include armour in the form of steel wire armour, additional advantages have been found over presently available cables designed to pass the BS 7346-6 (BS 8491) test. For example, steel wire armour h; a lower impedance than steel tape armour and thus cables of the present inventioi which include steel wire armour may not require an additional conductor for the ea fault path. Further, steel wire armour does not impose the same manufacturing limitations as interlocking steel tape armour, such that it may be possible to produc cables with a wider range of dimensions (e.g. nominal cross-sectional areas). Fina steel wire armour may be more cost-effective and easier to apply than interlocking steel tape armour. For example, steel wire armour can be applied in a longer pitch helix than interlocking steel tape armour, and does not need to be interlocked (i.e. doesn't require the turns of the wire to overlap one another).

In addition It has been found that steel wire armour, and in particular galvanised st wire armour, provides especially good mechanical protecting characteristics, allow cables of the present invention comprising steel wire armour, especially galvanisec steel wire armour, to withstand the more rigorous tests. Steel wire, or galvanised steel wire, may, for example, perform better than other substances such as aluminium or copper.

Cables of the present invention may include a further layer or layers of glass or mil tape around the layer of armour. In particular, the cable may contain two layers of glass or mica tape (or a layer of glass tape and a layer of mica tape) around the Ia of armour; in this case, the inner layer of glass or mica tape may be around and immediately adjacent to the layer of armour and the outer layer of glass or mica ta may be around and immediately adjacent to the inner layer of tape.

The layer(s) of glass or mica tape may be applied helically in a manner well knowr the art. The layer(s) of glass or mica tape may, for example, be wound helically around the armour so that the turns of the tape overlap one another, such that the

gaps, even when the cable is bent. Alternatively, the layer(s) of glass or mica tape may be applied helically and butted around the armour.

In cables which contain more than one layer of glass or mica tape around the layer of armour, the first (i.e. innermost) layer may be applied helically and butted (i.e. so the turns do not overlap each other), and a second layer may then be applied helically, butted and with a 50% registration relative to the first layer of tape. Thus, the width of the butt of the first tape lies below the centre of the second tape, such that the layer of armour is completely covered without spaces or gaps, even when the cable is bent. Alternatively, the layers of tape may be interwoven in a manner known in the art; each layer of tape may interlock with the subsequently applied tape.

The outer sheath of cables of the present invention may be of an insulating polymeric material. The insulating material may be of a halogen-free material. The insulating material may be applied by extrusion in a manner well known in the art. The outer sheath may include a nanocomposite material.

Cables of the present invention may further include a layer of bedding material for the armour layer. The armour layer may surround the bedding layer, and may be immediately adjacent thereto. The layer of bedding material may be an extruded layer of halogen-free material, as is well known in the art. The layer of bedding material may also include a nanocomposite material. The bedding layer may act in combination with other layers of the cable (e.g. a layer of aluminium/polymer laminate foil) as a water barrier. When the cable is subjected to fire conditions, the bedding layer may provide a stable char which may help to repel the subsequently applied water jets.

Cables of the present invention may further include a layer of aluminium/polymeric laminated foil, which may be radially inside the layer of armour (e.g. the layer of aluminium/polymeric laminated foil may be radially inside the layer of bedding for the armour layer). The layer of aluminium/polymeric laminated foil may be applied v longitudinally in a manner well known in the art. Aluminium/polymeric laminated foil is well known in the art, and it will be appreciated by the skilled person that alternative materials could be used The material may, for example, be a laminate of aluminium, polyester and polyethylene. The polyethylene component of the laminated foil may,

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component may bond with the bedding layer in fire conditions; thus, the bonded laminated foil and bedding layer may provide additional stability, and may further act as a moisture barrier.

Cables of the present invention may also further include one or more layers of glass or mica tape radially inside the layer of armour (e.g. the layer of armour may surround the further layer of layers of glass or mica tape). The further layer or layers of glass or mica tape inside the layer of armour may surround and be immediately adjacent to the insulated conductive cores; they may be wound helically around the insulated conductive cores so that the turns of the tape overlap one another, for example such that the insulated conductive cores are completely covered even when the cable is bent.

Cables of the present invention include one or more insulated conductive cores. The cables may include, for example, 2, 3, 4 or more insulated conductive cores. Each insulated conductive core may comprises a copper conductor, or other conductive material well known in the art (e.g. aluminium, gold, etc.), individually lapped with e.g. two or more mica tapes, surrounded by a layer of extruded halogen free insulation that may be cross linked. The insulated conductive cores may be laid and twisted in a manner well known in the art.

The term "layer of armour " as used in the present invention means any material well known in the field for use as armour (e.g. a physically protecting layer) for a cable. The International Electrotechnical Vocabulary, for example, describes armour (in the context of electrical cables) as "covering consisting of a metal tape(s) or wires, generally used to protect the cable from external mechanical effects". Armour (or armouring) is applied to energy and data cables to provide mechanical protection from damage (e.g. during installation and subsequent service). Armour components may comprise, for example, steel wires (optionally galvanised), galvanised steel strips, steel tapes or interlocking steel tapes. Steel wire is often used as an armour layer, as is e.g. steel tape armour. The skilled man will appreciate that steel wire or tape could be substituted by wire or tape made from another suitable metal material (e.g. aluminium or copper), or other materials well known in the art. The skilled man will appreciate that armour made of such less strong materials (e.g. aluminium or copper) would need still need to provide sufficient mechanical protection (e.g. be of sufficient thickness, or be protected against melting during the test).

Cables of the present invention which have steel wire armour as the layer of armour have been found to perform very well in the test set out in BS 7346-6 (BS 8491); thus, the inventors presently prefer the use of steel wire, although it will be appreciated that other materials used in conjunction with the present invention which perform satisfactorily in the required tests could also be used. Steel wire armour is well known in the art, and can e.g. be comprised of many individual strands of steel, laid up and twisted to form steel wire.

In certain types of cable (e.g. co-axial cables), other metallic layers are applied to provide electrical shielding (screening). The lnternation Electrotechnical Vocabulary describes a shield (of a cable) as a "surrounding earthed metallic layer which serves to confine the electric field within the cable and/or protect the cable from external electrical influence"; it also notes that metallic sheaths, foils, braids, armours and earthed concentric conductors may also serve as shields. Thus, whilst an armour layer can also act as an electrical shield, only a robust metal (e.g. steel wire, e.g. of a sufficient thickness) is capable of being used for armour.

Typical metallic layers used for screening or shielding include copper tapes, copper wires, copper braids, foils or a combination of these. Such layers do not constitute armour as they do not provide adequate mechanical protection, for example because the material is not strong and/or thick enough.

The term "co-axial" is strictly applicable only to a single insulated conducter with a screen, where the screen and the conductor both share the same centre (axis) - i.e. they are co-axial. Co-axial screens are also used on medium and high voltage cables, for example where three cores are present each having its own screen, which is co-axial with the conductor. These screens limit the electric fields that are generated. Again, such co-axial screens do not constitute armour since they do not provide adequate mechanical protection.

In one aspect of the present invention, the cables are not co-axial cables. In this aspect, cables do not have a co-axial electrical shielding (screening) layer around the core(s). In a further aspect, the armour is not a co-axial screening layer. It will be appreciated by the skilled person that steel armour should not be applied around a single core (i.e. co-axially) due to unwanted induction heating of the armour. For such

single core cables, hardened aluminium may be used as armour, although such armour may not perform as well as steel wire armour in the BS 7346-6 tests. Thus, in one aspect of the present invention, the cable is not a single core cable (i.e. it is a multi-core cable).

The term "glass tape" is intended to include materials such as e.g. high temperature resistant close weave tape made from S-Glass. However, other types of glass tape of lower quality could be used in cables of the present invention to satisfactory results. Glass tape may include, for example, glass cloth tape (comprising e.g. fibreglass backing, polyethylene liner and rubber adhesive), or closely woven glass fabric with high temperature silicone adhesive. Glass tape is well known in the field of cable manufacturing, usually included as a binder and positioned immediately adjacent to the insulated cores; such glass tapes are suitable for use with the invention.

Mica tape is well known in the art. Mica tape is e.g. made of mica paper (e.g. fluorophlogopite, phlogopite or calcined muscovite mica paper) and glass cloth (or polyethylene film) with e.g. a small-amount of high heat resistance silicone adhesive.

It will be appreciated by the skilled person that materials with similar properties to glass or mica tape could be used in the present invention.

Cables of the present invention including one or more layers of glass tape surrounding the armour layer have been shown to perform very well in the test set out in BS 7346-6 (BS8491). Glass tape may withstand the water jets which may be applied at the end of the tests better than mica tape; thus, the inventors presently use glass tape in preference to mica tape. However, cables of the present invention including one or more layers of mica tape surrounding the armour layer may also provide satisfactory results in the required tests.

The outer sheath and bedding layer of the present invention may be an insulating polymeric material. It may be a material which is halogen-free, and which does not give out substantial amounts of smoke or fumes on combustion. Such materials are very well known in the art, and are described, e.g. in International Patent Application WO2004/044927 of the present applicant.

Nanocomposite material is a composite material which comprises sub-micronic particles dispersed in an organic matrix. Preferred nanocomposite fillers are those sold under the Trade Marks NANOFIL, NANOCOR or CLOISITE ₁ which are well known names in the art. Another preferred nanocomposite filler is that sold under the Trade Mark BENTONE (from Elementis). Incorporation of nanocomposite materials into polymeric insulation is well understood and is described, e.g. in International Patent Application WO2004/044927 of the present applicant.

It will be appreciated that cables according to the invention may also incorporate other components known in themselves which are required by the use to which it is intended to put the cable. For example, an electrostatic sheath may be provided (e.g. inside the layer of armour) or a number of hard drawn copper wires or strips may be included with the armour layer to improve the electrical conductivity.

Brief Description of the Drawings

FIGURE 1 shows a cross-section through a cable embodying the invention.

Detailed Description of the Drawings

The cable shown in Fig. 1 has three sector shaped insulated conductive cores 1, 2, each core of nominal conducting cross sectional area 50mm ² , each comprising stranded copper wire 1 surrounded by four layers of mica tape (as is well known in the art, and not shown for clarity), each of thickness 0.11mm, and further surrounded by an insulating cross linking zero halogen material covering 2 of thickness 1.0mm. The insulated cores 1, 2 are laid and twisted (not shown) in a manner well known in the art. The skilled person would appreciate that cables of the present invention could comprise more or less conductive cores, for example, from 2 to 7 conductive cores, preferably from 2 to 4. For certain applications (e.g. multi-core auxiliary cables), the skilled person would appreciate that cables of the present invention could comprise up to, for example, 48 cores. The skilled person would further understand that cables of the present invention could have cores having a range of nominal conducting cross sectional areas, for example up to 120mm ² or up to 400mm ² . It is also possible to make cables of the present invention having cores with smaller nominal conducting cross sectional areas, for examDle down to 10mm ² . or down to 2.5mm ² . The choice of the number of cores

and the nominal conducting cross sectional area of each core could be made by the skilled person depending on the desired application of the cable.

Around the insulated conductor cores 1 , 2 are arranged two layers 3a, 3b, each of porous siliceous material in the form of a glass fibre tape of radial thickness 0.2mm. Each layer of tape is wound helically around the insulated conductor cores 1 , 2 with each turn of the helix overlapping the next sufficiently so that the insulated cores 1 , 2 and earth are completely covered with the glass fibre tape, even when the cable is bent. The glass fibre of the tape layer 3a is immediately adjacent the cross linking zero halogen material covering 2 of the cores 1. The cross linking zero halogen material is cured before the glass tape is applied. The layer of glass tape 3b surrounds and is directly adjacent to layer 3a. .

A layer of aluminium/polymeric laminated foil 4 of thickness 0.154mm surrounds the glass fibre tape layer 3. The aluminium/polymeric laminated foil layer 4 is applied longitudinally in a manner well known in the art. The aluminium/polymeric laminated foil is a laminate of aluminium, polyester and polyethylene; the individual components of the laminated foil are not shown for clarity.

Around the aluminium/polymeric laminated foil layer 4, is a layer 5 of extruded halogen-free bedding of thickness 0.9 - 1.0mm, which is applied by extrusion in a manner well known in the art. Although not present in the illustrated embodiment, it is possible to include a nanocomposite filler in the material used for the bedding layer, in a manner well known in the art.

A layer 6 of steel wire armour of radial thickness 1.6mm surrounds the bedding layer 5. The steel wire armour layer is made up of 44 steel wires of thickness 1.6mm which are laid and twisted (and applied to the cable) in a manner which is well known in the art.

A layer 7 of glass tape of radial thickness 0.2mm surrounds the layer 6 of steel wire armour; the layer 7 of tape surrounds and is immediately adjacent to the steel wire armour layer. The layer of glass tape 7 is wound helically around the

the leading edge of one turn is positioned against (i.e. touching but not overlapping) the trailing edge of the subsequent turn.

In the illustrated embodiment, a second layer 8 of glass tape of radial thickness 0.2mm surrounds the layer 7 of glass tape. Layer 8 is applied helically in the same way as described for layer 7 (i.e. butted), but is applied with 50% registration relative to the layer 7, such that the width of the butt of the first tape lies directly below the centre of the second tape (i.e. so the butted edges of the first tape lie directly below the centre of the second tape). In this way, the steel wire armour is completely covered without gaps even when the cable is bent.

In an alternative embodiment (not shown), only one layer of glass tape is applied, and each turn of the helix overlaps the next (i.e. lapped) sufficiently so that the layer of steel wire armour is completely covered with the layer of glass tape, even when the cable is bent.

An outer sheath 9 of thickness 1.6 - 1.8mm surrounds the layer 8 of glass tape. The outer sheath 9 is applied by extrusion in a manner well known in the art, and is made up of material sold under the mark OHLS (RTM) ₁ which comprises hydrated alumina in a polyethylene and ethylene vinyl acetate co-polymer composition. In this embodiment of the invention, the material used to make the outer sheath 9 also includes a nanocomposite filler, as described for example in International Patent Application No. WO2004/044927 of the present applicant.

The cable above was tested in accordance with the procedures outlined in the BS 7346-6 (BS 8491) test. The cable passed the 2-hour rating, resisting the powerful water jets at the end of the test.

Table 1 on page 10 illustrates two further examples of the present invention; the headings in the table use the same reference numerals used in Fig 2.

The cables illustrated in Table 1 were tested in accordance with the procedures of the BS 7346-6 (BS 8491) test and each achieved the maximum 2-hour rating.

It will be appreciated by the skilled person that mica tape may be used in place of the glass tape in any of the layers 3a, 3b, 7 or 8.

Table 1

M