GB1583955A

GB1583955A - Electric cables

Info

Publication number: GB1583955A
Application number: GB1801/77A
Authority: GB
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1977-01-17
Filing date: 1977-01-17
Publication date: 1981-02-04
Also published as: ZA78218B; MY8200119A; ZA78121B; HK44181A

Description

(54) ELECTRIC CABLES (71) We, Bicc LIMITED, a British Company, of 21 Bloomsbury Street, London WC1B 3QN, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to insulated electric cables. It is a requirement for certain classes of cable, such as those used in fire alarm and emergency lighting circuits, to remain functional for a significant length of time, preferably several hours, after they have been damaged by a fire. Various cables have been designed to meet this requirement, including some with polymeric-based insulation and sheathing.Usually the insulation has been of a silicone rubber composition, which leaves an electrically insulating residue of silica when burnt. It has however proved difficult to ensure that the integrity of the sheathing is maintained during and after exposure to fire conditions to the extent necessary to prevent loss of the powdery residue of the insulation, without at the same time reducing the flexibility of the cale to an undesirable degree.

The cable in accordance with the invention includes two or more metallic conductors insulated from one another by silicone rubber or other polymeric-based material that leaves an electrically-insulating residue on combustion. Usually the insulation will be in the form of a concentric layer on one or more of the conductors; one conductor will often, but not always, be left bare for use as an earth conductor; if the number of conductors is large more than one bare conductor could be included, but this would be an unusual arrangement. The insulated conductors may be helically laid up, or they could be assembled in other ways, for example with the conductors all straight and parallel.

The insulated conductor assembly is en closed, in accordance with the invention, in a composite sheath comprising an inner bedding layer of a resilient polymeric-based electrically insulating material, and an outer layer of flexible metal. The cable may also include an outermost layer of insulating material of low flamability.

The bedding layer is preferably extruded, but could if desired be formed by a tape applied helically or longitudinally to the insulated conductor assembly.

The bedding layer is preferably of a material that leaves an insulating residue on combustion, such as a silicone rubber.

Alternatively however it may be of other flame-ratardant, low-smoke compositions, for example an alkylene homo- or copolymer with a high proportion of a mineral filler. Two suitable formulations of the latter kind comprise: PARTS BY WEIGHT Formula- Formula tion 1 tion 2 Ethylene-propylenerubber 22 60% Ethylene-40% Vinyl acetate co polymer - 24 Alumina trihydrate (Hydral 310) (Nominal particle size lFm) 62 70 Plasticisers: Polyiso butylene 7-7 Paraffin wax 3 3 Processing aid:Stearic 2 acid 2 Curing agents (optional) & Antioxidants I up to 5 up to 4 The metal layer may be of foil or tape and may be longitudinally or helically applied; especially (but not exclusively) if it is of foil and especially (but not exclusively) if it is helically applied a longitudinal auxiliary wire conductor may be arranged in contact with it, either inside or outside, to ensure that all parts of the metal layer remain securely connected to earth; the layers should be applied tightly enough to ensure continuous effective contact between the metal layer and the auxiliary wire conductor.

When the metal layer is of foil, the insulating outermost layer should in most cases be used, and one very suitable material for this layer is a plasticised PVC (polyvinyl chloride) compound, preferably formulated to enhance its flame-retardant properties.

Hard grades of compound are preferred.

Formulations in accordance with our British Patent No. 1418027 are preferred, but other flame-retardant grades, including the conventional type having calcium carbonate as filler and antimony trioxide as the principal flame-retardant additive, can be used. In some cases the use of basic fillers of very high specific area (small particle size) may be desirable as it reduces the amount of hydrogen chloride gas and/or hydrochloric acid droplets in the smoke produced if the compound should be burnt. When the foil is enclosed in an outermost layer it is preferably pre-coated on its surface with a polymeric material that promotes adhesion of the outermost layer to the foil. Such a coating preferably covers the whole of the outer surface of the foil, except when an auxiliary conductor is located outside it, when an appropriate area or areas must be left bare.

When the metal layer is of tape, the insulating outermost layer will not usually be needed for mechanical protection, but may be desirable to improve resistance to corrosion. In this case adhesion to the metal layer will not necessarily be of importance, and as alternatives to PVC may be used for example fire-retardant ethylene-propylene rubber compositions (such as the one described above), fire-retardant cross-linked polyethylene (such as the one sold under the trademark "Petrothene"), flame-retardant grades of silicone rubber, or chlorosulphonated polyethylene compounds. The metal layer can be seam-eelded if desired.

The metallic conductors are preferably of tinned copper, but bare or nickel-plated copper can be used, as can other metals with sufficiently high conductivity and sufficiently high melting point.

The insulated conductors may be distinguished by any of the conventional ways of marking, e.g. by colouring each differently if the number of conductors is small or by colouring two adjacent conductors in each layer to define an origin and direction for counting the remainder of the layer.

Silicone rubber complying with British Standard 6899-1976 Type E12 is very suitable for the conductor insulation and for the bedding layer of the composite sheath.

If made with suitably-chosen silicone rubber and PVC compounds of the classes indicated, the cable described may comply with the requirements of lEC Publications 331 and 323 (International Electrotechnical Commission), respectively for maintenance of circuit integrity under prescribed fire conditions and for flame retardance.

EXAMPLE Two wires, each of plain copper wire 138 mm in diameter and each insulated with 0 6 mm radial thickness of a conventional silicone rubber formulation, are laid up with a single earth wire of tinned copper also 1 38 mm in diameter. An extruded bedding made of the Type E12 silicone rubber referred to and of 1 mm nominal wall thickness has an external diameter of about 7.4 mm. Over this is applied a metal layer formed by a longitudinally-wrapped aluminium foil, 28 mm wide and 0 15 mm thick, coated on the outside only with a thin layer of PVC; an auxiliary earth conductor of 0 9 mm diameter tinned copper wire is laid under the foil as it is applied.Over the wrapped coated foil is promptly extruded an outermost layer about 0-8 mm thick of a PVC compound containing finely-divided chemically-basic filler.

Attention is drawn to our copending cognate applications 10267/77 (Serial no.

1583956) and 173/78 which claim an electric cable comprising at least one polymericinsulated conductor, a bedding layer surrounding the conductor and made from a composition comprising a curable alkene homopolymer or copolymer, at least 55% of inert mineral filler, a low smoke plasticiser, a curing agent for the polymer and an antioxidant, and an enclosing sheath of low flammability material.

WHAT WE CLAIM IS: 1. An electric cable comprising two or more metallic conductors insulated from one another by a polymeric material that leaves an electrically-insulating residue on combustion and enclosed in a composite sheath comprising an inner bedding layer of a resilient polymeric-based electrically insulating material and an outer layer of flexible metal.

2. An electric cable comprising two or more metallic conductors insulated from one another by a polymeric material that leaves an electrically-insulating residue on combustion and enclosed in a composite sheath comprising an inner bedding layer of a resilient polymeric-based electrically insulating material, an outer layer of flexible metal foil, and an outermost layer of insulating material of low flamability.

3. An electric cable as claimed in Claim 2 in which the foil is pre-coated on its surface with a polymeric material that promotes adhesion of the outermost layer to the foil.

4. An electric cable as claimed in Claim 1 in which the outer layer is of metal tape.

5. An electric cable as claimed in Claim

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

be applied tightly enough to ensure continuous effective contact between the metal layer and the auxiliary wire conductor.

When the metal layer is of foil, the insulating outermost layer should in most cases be used, and one very suitable material for this layer is a plasticised PVC (polyvinyl chloride) compound, preferably formulated to enhance its flame-retardant properties.

Hard grades of compound are preferred.

Formulations in accordance with our British Patent No. 1418027 are preferred, but other flame-retardant grades, including the conventional type having calcium carbonate as filler and antimony trioxide as the principal flame-retardant additive, can be used. In some cases the use of basic fillers of very high specific area (small particle size) may be desirable as it reduces the amount of hydrogen chloride gas and/or hydrochloric acid droplets in the smoke produced if the compound should be burnt. When the foil is enclosed in an outermost layer it is preferably pre-coated on its surface with a polymeric material that promotes adhesion of the outermost layer to the foil. Such a coating preferably covers the whole of the outer surface of the foil, except when an auxiliary conductor is located outside it, when an appropriate area or areas must be left bare.

When the metal layer is of tape, the insulating outermost layer will not usually be needed for mechanical protection, but may be desirable to improve resistance to corrosion. In this case adhesion to the metal layer will not necessarily be of importance, and as alternatives to PVC may be used for example fire-retardant ethylene-propylene rubber compositions (such as the one described above), fire-retardant cross-linked polyethylene (such as the one sold under the trademark "Petrothene"), flame-retardant grades of silicone rubber, or chlorosulphonated polyethylene compounds. The metal layer can be seam-eelded if desired.

The metallic conductors are preferably of tinned copper, but bare or nickel-plated copper can be used, as can other metals with sufficiently high conductivity and sufficiently high melting point.

The insulated conductors may be distinguished by any of the conventional ways of marking, e.g. by colouring each differently if the number of conductors is small or by colouring two adjacent conductors in each layer to define an origin and direction for counting the remainder of the layer.

Silicone rubber complying with British Standard 6899-1976 Type E12 is very suitable for the conductor insulation and for the bedding layer of the composite sheath.

If made with suitably-chosen silicone rubber and PVC compounds of the classes indicated, the cable described may comply with the requirements of lEC Publications 331 and 323 (International Electrotechnical Commission), respectively for maintenance of circuit integrity under prescribed fire conditions and for flame retardance.

EXAMPLE Two wires, each of plain copper wire 138 mm in diameter and each insulated with 0 6 mm radial thickness of a conventional silicone rubber formulation, are laid up with a single earth wire of tinned copper also 1 38 mm in diameter. An extruded bedding made of the Type E12 silicone rubber referred to and of 1 mm nominal wall thickness has an external diameter of about 7.4 mm. Over this is applied a metal layer formed by a longitudinally-wrapped aluminium foil, 28 mm wide and 0 15 mm thick, coated on the outside only with a thin layer of PVC; an auxiliary earth conductor of 0 9 mm diameter tinned copper wire is laid under the foil as it is applied.Over the wrapped coated foil is promptly extruded an outermost layer about 0-8 mm thick of a PVC compound containing finely-divided chemically-basic filler.

Attention is drawn to our copending cognate applications 10267/77 (Serial no.

1583956) and 173/78 which claim an electric cable comprising at least one polymericinsulated conductor, a bedding layer surrounding the conductor and made from a composition comprising a curable alkene homopolymer or copolymer, at least 55% of inert mineral filler, a low smoke plasticiser, a curing agent for the polymer and an antioxidant, and an enclosing sheath of low flammability material.

WHAT WE CLAIM IS: 1. An electric cable comprising two or more metallic conductors insulated from one another by a polymeric material that leaves an electrically-insulating residue on combustion and enclosed in a composite sheath comprising an inner bedding layer of a resilient polymeric-based electrically insulating material and an outer layer of flexible metal.
2. An electric cable comprising two or more metallic conductors insulated from one another by a polymeric material that leaves an electrically-insulating residue on combustion and enclosed in a composite sheath comprising an inner bedding layer of a resilient polymeric-based electrically insulating material, an outer layer of flexible metal foil, and an outermost layer of insulating material of low flamability.
3. An electric cable as claimed in Claim 2 in which the foil is pre-coated on its surface with a polymeric material that promotes adhesion of the outermost layer to the foil.
4. An electric cable as claimed in Claim 1 in which the outer layer is of metal tape.
5. An electric cable as claimed in Claim

4 including an outermost layer of insulating material of low flamability.
6. An electric cable as claimed in any one of the preceding claims in which the bedding layer is of a material that Icaves an insulating residue on combustion.
7. An electric cable as claimed in Claim 6 in which the said material is a silicone rubber.
8. An electric cable as claimed in any one of claims 1-5 in which the bedding layer is of a flame-retardant low-smoke composition comprising an alkylene homo- or co-polymer with a high proportion of a mineral filler.
9. An electric cable as claimed in Claim 8 in which the bedding is substantially of the composition described herein as "formulation 1".
10. An electric cable as claimed in Claim 8 in which the bedding is substantially of the composition described herein as "formulation 2".
11. An electric cable as claimed in any one of claims 1-9 including a longitudinal auxiliary wire conductor in continuous effective contact with the outer metal layer.
12. An electric cable as claimed in Claim 2 or Claim 3 or Claim 4 in which the outermost layer is of a plasticised PVC compound.
13. An electric cable as claimed in Claim 12 in which the PVC compound is a flameretardant formulation.
14. An electric cable as claimed in Claim 13 in which the compound is claimed in any one claim of British Patent No. 1418027.
15. An electric cable as claimed in Claim 13 in which the PVC compound has calcium carbonate as filler and antimony trioxide as flame-retardant additive.
16. An electric cable as claimed in any one of claims 12-15 in which the PVC compound contains a basic filler with a specific area high enough to reduce the amount of hydrogen chloride gas and/or hydrochloric acid droplets in the smoke if the compound should be burnt.
17. An electric cable substantially as described herein as an Example of the invention.