IE42140B1 - Electric cable - Google Patents

Abstract

1480090 Electric cable PIRELLI GENERAL CABLE WORKS Ltd 26 Nov 1975 [5 Dec 1974] 52717/74 Heading H1A An electric cable comprises at least one individually insulated conductor 10, 11, 12 within a sheath 20 of flame retardant insulating material and an insulating, flame retarding, filling material 18 between the conductors and the sheath, the materials of the conductor insulation, filling material and sheath being such that upon combustion no acidic gases are evolved and little smoke is formed. The conductor insulation may comprise non- sulphur cured ethylene-propylene rubber, crosslinked polyethylene, non-sulphur cured butyl rubber, thermoplastic rubber or thermoplastic polyethylene or flame retardent silicone rubber, flame retardent cross-linked polyethylene or thermoplastic polyethylene containing an inorganic flame retardant. The sheath may be flame retardant silicone rubber, flame retardant cross-linked polyethylene or thermoplastic polyethylene containing an inorganic flame retardant. The filling material comprises ethylenepropylene rubber and/or butyl rubber, a filler and a flame retardant; the filler is preferably ground whiting and hydrated alumina. The insulation, filling material and sheath may all be applied by extrusion. A layer of silicone rubbercoated glass tape may be provided around the sheath.

Description

This 'invention relates to electric cables, more particularly to electric cables having one or more individually insulated conductors for carrying control signals or power.

Flame-retardant cables are known wherein the insulation comprises compositions based on chlorine-containing elastomeric or plastics materials such as polychloroprene and polyvinyl chloride. For example, in some cables the or each conductor is individually insulated with an extruded layer of polyvinyl ohloride and an extruded sheath of the same material encloses the insulated conductor(s). In other cables, the or each container is individually insulated with an extruded rubber composition and the whole enclosed within an extruded sheath of flame-retardant composition based on polychloroprene or chlorosulphonated polyethylene.

It is known to produce good flame retardance by using, for the extruded insulation layers of the individual conductors and/or for the extruded sheath, compositions containing an elastomeric or plastics material (not necessarily chlorinated) as the basic component and including in the composition fire retardants such as antimony trioxide, chloroparaffins or • diphenyl chloride. British Patent Specification No. 1,266,310 proposes an improvement wherein the insulating composition comprises from 6 to 40% magnesium carbonate, at least 20% Of a chlorine-containing compound, an elastomeric or plastics - 2 42140 - 3inatorial which m.iy roiiiprj s;n ai: 1 in pari: I lie chiorinocontaining compound, and from 1 to I *>% of antimony irioxide (the percentage.'; being by weight).

However, when these known cables are subject to combustion, they do emit considerable quantities of dense black smoke which contains toxic hydrogen chloride. In tunnels and other enclosed spaces, for example tunnels for underground trains, there is an increasing demand for cables which evolve only minimal quantities of smoke under combustion, so as to reduce the risk of obscuring vision, to reduce the risk to human life from toxic and irritant gases such as hydrogen chloride, and to prevent costly damage to adjacent materials and equipment.

We have now devised an electric cable which is particularly (though not exclusively) useful in tunnels and other enclosed spaces, which does not produce dense smoke or gaseous acids when subjected to combustion.

The invention provides an electric cable which comprises one or more individually insulated conductors within a sheath of flame-retardant insulating material, and an insulating filling material within the sheath and surrounding the individually insulated conductor (s), wherein the sheath, the insulation on each individual conductor , and the filling material aro of compositions such that, upon combustion of the cable, no toxic or irritant acidic gases are evolved and no dense smoke is formed, and wherein the filling material comprises ethylene-propylene rubber and/or butyl rubber, a filler as the main component and a flame retardant, the filler forming an inorganic ash residue around the insulated conductor(s) when the cable is subject to combustion.

The amount of smoke evolved on combustion of a cable of the invention is very substantially less than the amount evolved from conventional PVC cables or other cables based on . chlorinated material. Some small amount of grey-white smoke may be evolved but the amount does not significantly obscure vision in, say, an underground railway tunnel.

For the avoidance of doubt, the expression “flame retardant used herein has its well-known meaning, namely a substance having the property of retarding the propagation of flame.

Among the materials which are suitable for use for insulating the individual conductors are rubbers such as nonsulphur cured ethylene-propylene rubbers (hereinafter referred to as EPR), polyethylene which has been cross-linked chemically or by irradiation (hereinafter referred to as XLPE), nonsulphur cured butyl rubbers and thermoplastic rubbers. Thermoplastic polyethylene may be used where, for example, the Outer sheath comprises flame-retardant thermoplastic polyethylene, thereby avoiding the need to cure the sheath material.

Thermoplastic rubbers are known in the art. Such rubbers, which are commercially available, are synthetic and extrude easily. At extrusion temperatures, typically 200°C, they are thermoplastic but at ambient temperature their characteristics are those of a conventional rubber. Between the range -40°C to 100°C, they generally have elastic properties, but above 100°C they start to soften and become thermoplastic.

The suitability of rubber materials such as EPR and XLPE is surprising since these materials are flammable.

However, this problem is overcome by using a filling material around the insulated conductors, which when the cable is subject to combustion, - 5 form.s an .inorganic ash residue as a protective layer around the insulated conductor(s). Apart from comprising a small amount of rubber (to render it extrudab.1 e and coherent), it mainly comprises inorganic ash-forming ingredients. The ΰ filler material, preferably comprises ethylene-propylene rubber and/or butyl rubber, a filler and a flame retardant. It is particularly preferred to include i.n the filling material a combination of hydrated alumina and ground whiting (tne nydrased alumina serving both as a filler and as a flame retardant) since we Ki have found that, surprisingly, such a combination forms a very good protective ash core around the .insulated conductors when subjected to combustion. The filling material must be sufficiently hard to withstand the pressure of the rubber vulcanisation process (where used), and will normally have a very low polymer content, e.g. less than 10% by weight.

If it is desired to use, for the insulation on tht; conductors, a flame-retardant material, then suitable materials include flame-retardant silicone rubber, flame-retardant cross-linked polyethylene and thermoplastic polyethylene containing an inorganic flame retardant.

Preferred materials for the sheath include flame-retardant silicone rubber, flame-retardant cross-linked polyethylene or thermoplastic polyethylene containing an inorganic flame retardant.

It is sometimes advantageous to provide around the sheath, a protective layer of a silicone rubber-coated glass tape. This protects the cable during installation and also, in the event of combustion, forms an envelope of silica ash around the cable.

In order that the invention may be more fully understood. -διό reference is made to the accompanying drawing which is a cross-section of one form of cable according to the invention.

Referring to the drawing, there is shown a power cable comprising three conductors 10, 11., 12 having respective individual, extruded insulating layers 14, 15, 16, the three insulated conductors being helically laid-up together. Each conductor may be round as shown or any suitable shape in cross-section.

An insulating filling composition IS fills the spaces between the adjacent insulated conductors and gives the laid-up conductor construction a circular outer surface. The filling composition is applied by extrusion. A tape 20 may be lapped helically around the conductors after application of the filling medium as shown. A sheath 22 is extruded over this, and the cable may be completed by the application of armouring layers (not shown) around the sheath 22.

By way of example, filling materials may have the following composition: Parts by Weight EPR 100 Butyl Rubber 10 to 20 Polyethylene 30 to 60 Ground Whiting 1000 to 1500 Hydrated Alumina 250 to 500 Mineral Oil 100 to 150 Stearic Acid 10 to 20 Typical oxygen index values for this composition are in the range 33 to 40, depending on the relative proportions of the various constituents. In this example, the butyl rubber is included to assist the ease with which the composition 'r\C 7 <:.ui bo oxi ι uiloil, Ijitl l in· composition could bo based upon bulyl rubber instead of upon til’ll.

The .-,heath '22 is roc/nirod to bo flame retardant nnd Lire material from which it is made can be selected from flame retardant, silicone rubber, suoh a;; for example Lhe commerci al I γ available Midland ftil.icone M£>Ll>O3, flame retardant cross-linked polyethylene such as, for example, Lhe commercially available Union Carbide IlFDC 4770, and flame retardant thermoplastic polyethylene, (providing in the latter that the flame retardant constituents are inorganic compounds only). In addition to the flame retardant properties of these materials they produce only low quantities of smoke which is free of hydrochloric and other acids. The sheath lends mechanical support to the cable to withstand the stresses to which the cable is subjected during handling and installation.

Although a power cable has been described, the invention is applicable to a cable for carrying control signals (for example signalling currents in an underground train system) and to telecommunications cables. The cable will then include a multiplicity of individually insulated conductors enclosed with an extruded sheath and with the spaces within the sheath filled with the filling composition.

Claims (7)

1. CLAIMS:- '

1. An electjric cable which comprises one or more individually insulated conductors within a sheath of flame retardant insulating material, and an insulating filing material within the sheath and surrounding the individually 5 insulated conductor(s), wherein the sheath, the insulation on each individual conductor, and the filing material are of compositions such that, upon combustion of tne cable, no toxic or irritant acidic gases are evolved and no dense smoke is formed, and wherein the filling material comprises ethylenepropylene rubber and/or butyl rubber, a filler as the main component and a 10 flame retardant the filling material forming an inorganic ash residue as a protective layer around the insulated conductor(s) when the cable is subject to combustion.

2. An electric cable according to claim 1, wherein the or each conductor is individually insulated with a layer of non-sulphur cured ethylene15 propylene rubber, cross-linked polyethylene, non-sulphur cured butyl rubber, thermoplastic rubber or thermoplastic polyethylene, or flame retardant silicone rubber, flame retardant cross-linked polyethylene or thermoplastic polyethylene containing an inorganic flame retardant.

3. An electric cable according to claim 1 or 2, wherein the sheath is 20 composed of flame retardant silicone rubber, flame retardant cross-linked polyethylene or thermoplastic polyethylene containing an inorganic flame retardant.

4. An electric cable according to calim 1,2 or 3, wherein the filler comprises ground whiting and hydrated alumina the hydrated alumina also 25 serving as the flame retardant.

5. An electric cable according to claim 4, wherein the filler material comprises: - 9 Parts by Weight Ethylene-Propylene Rubber 100 Hutyl Rubber 10 to 20 Polyethylene 3() to 60 Ground Whiting 1000 to 1.500 Hydrated Alumina 260 to 500 Mineral Oil 100 to 150 Stearic Acid 10 to 20

6. An electric cable according to any preceding claim, 10 which has been made by forming the sheath, the filling and the insulation on each individual conductor, by extrusion.

7. · An electric cable according to any preceding claim, which also comprises a layer of silicone rubber-coated glass tape around the sheath. 15 S. An electric cable substantially as herein described with reference to the accompanying drawing.