AU2009315516B2

AU2009315516B2 - Fireproof electric cable

Info

Publication number: AU2009315516B2
Application number: AU2009315516A
Authority: AU
Inventors: Alain Clertant; Yannick Goutille; Nicolas Rousselet
Original assignee: Nexans SA
Current assignee: Nexans SA
Priority date: 2008-11-13
Filing date: 2009-11-04
Publication date: 2015-03-19
Anticipated expiration: 2029-11-04
Also published as: FR2938368B1; FR2938368A1; EP2346931A1; EP2346931B1; AU2009315516A1; WO2010055247A1

Abstract

The invention relates to an electric cable comprising a set of insulated electrical conductors each comprising an electrical conductor (1) surrounded by an insulating layer that can be at least superficially transformed in the ceramic state, at high temperatures corresponding to the conditions of a fire, and an outer sheath (3) surrounding the set of insulated electrical conductors, spaces (4) being provided between the sheath (3) and the set of insulated electrical conductors. The invention is characterised in that the insulating layer comprises a first cross-linked layer (2a) surrounding the electrical conductor, and a second cross-linked layer (2b) surrounding the first layer, the first layer (2a) being obtained from a first composition comprising a polymer matrix based on polyorganosiloxane, and the second layer (2b) being obtained from a second composition comprising a polymer matrix based on polyolefin.

Description

C09356 1 Fireproof electric cable Field of the Invention [001] The present invention relates to an electric cable comprising an assembly of insulated electrical conductors, and also to a process for manufacturing said cable. [002] The invention applies typically, but not exclusively, to the field of fireproof safety cables, especially halogen-free safety cables, capable of functioning for a given period of time under fire conditions, without, however, propagating the fire or generating a lot of fumes. These safety cables are in particular power transmission cables or low-frequency transmission cables, such as control or signaling cables. Background of the Invention [003] One of the major challenges of the cable industry is the improvement of the behavior and performance of cables under extreme thermal conditions, especially those encountered in a fire. For essentially safety reasons, it is in fact desirable to maximize the capacities of the cable to retard the propagation of flames, on the one hand, and to withstand fire, on the other hand, in order to ensure continuity of functioning. [004] Significant slowing-down of the progress of flames will gain valuable time for evacuating the premises and/or for implementing appropriate extinction means. In the event of a fire, the cable must be able to withstand the fire in order to function for as long as possible and to limit its degradation. A safety cable must also not be hazardous to its environment, i.e. it must not give off toxic and/or opaque fumes when it is subjected to extreme thermal conditions. [005] Document EP0942439 discloses a halogen-free fireproof safety electric cable comprising an assembly of insulated electrical conductors, said assembly being surrounded by an outer sheath. Each insulated electrical conductor is formed from an electrical conductor surrounded by an insulating layer obtained from a composition comprising a polymer material and at least one ceramic-forming filler, said insulating layer thus being capable of being converted at least superficially into the ceramic state at high temperatures corresponding to fire conditions. The polymer material of this single insulating layer is chosen from a polysiloxane and an ethylene copolymer, or a mixture thereof. [006] However, it has been found that this safety cable of the prior art, although having good fireproof properties, is mechanically fragile. More particularly, the insulated electrical C09356 2 conductors are sensitive to the various mechanical constraints to which these cables are typically subjected during their manufacture, transportation, handling, installation or connection. [007] The aim of the present invention is to overcome the drawbacks of the techniques of the prior art by especially proposing an electric cable that is easy to handle, which limits the risks of mechanical degradation of the insulated electrical conductors of which it is composed, while at the same time maintaining excellent fireproofing properties that satisfy standard NF C 32-070 CR1. [008] Any reference herein to known prior art does not, unless the contrary indication appears, constitute an admission that such prior art is commonly known by those skilled in the art to which the invention relates, at the priority date of this application. Summary of the Invention [009] The present invention provides an electric cable comprising: -an assembly of insulated electrical conductors, each insulated electrical conductor comprising an electrical conductor surrounded with an insulating layer that is capable of being converted at least superficially into the ceramic state at high temperatures corresponding to fire conditions, and -an outer sheath surrounding the assembly of insulated electrical conductors, empty spaces being provided between said sheath and the assembly of insulated electrical conductors, wherein insulating layer comprises a first crosslinked layer surrounding the electrical conductor, and a second crosslinked layer surrounding said first layer, the first layer being obtained from a first composition comprising a polyorganosiloxane-based polymer matrix, and the second layer being obtained from a second composition comprising a polyolefin based polymer matrix. [010] In the text hereinbelow: - the term "polyorganosiloxane-based polymer matrix" denotes a polymer matrix comprising only one or more polymers predominantly formed from polyorganosiloxane, and the term "polyolefin-based polymer matrix" denotes a polymer matrix comprising only one or more polymers formed predominantly from polyolefin. [011] The first composition can also comprise at least one ceramic-forming filler.

C09356 3 [012] The second composition can also comprise at least one flame-retardant mineral filler and/or at least one ceramic-forming filler. [013] The outer sheath can comprise a polyolefin-based polymer matrix and at least one hydrated flame-retardant mineral filler. [014] The empty spaces can occupy at least 10% of the cross section of said cable. [015] The cable may not comprise any halogenated compounds. [016] The thickness of the first layer can be from 0.10 mm to 1.50 mm, and the thickness of the second layer can be from 0.05 mm to 1.50 mm [017] The cross section of the electrical conductor can be 1.5 mm 2 to 4 mm 2 . [018] The first layer and the second layer can be extruded layers. [019] The outer sheath can be a tubular sheath. [020] The first layer can be directly in physical contact with the second layer. [021] The first layer can be directly in physical contact with the electrical conductor. [022] The second layer of at least one insulated electrical conductor can be directly in physical contact with the outer sheath. [023] The second layer of at least one insulated electrical conductor can be directly in physical contact with the empty spaces. [024] The present invention also provides a process for manufacturing an electric cable as mentioned in the paragraphs above, comprising: i. forming the insulating layer of the insulated electrical conductor by extrusion and crosslinking of the first composition and of the second composition around an electrical conductor, ii. assembling at least two insulated electrical conductors as obtained in step i, and iii. extruding an outer sheath in a tubing manner around the assembled insulated electrical conductors of step ii.

C09356 4 [025] By virtue of the invention, the second layer, known as the outer layer, based on polyolefin mechanically protects the first layer, known as the inner layer, based on polyorganosiloxane. This property allows the insulating layer of each insulated electrical conductor to improve the mechanical properties of the cable, especially the hardness, the abrasion strength and the tear strength of the insulated electrical conductors of which it is composed, to facilitate the installation of said cable, and to make the cable more robust during its manufacture, irrespective of whether there is good or poor adhesion between the first layer and the second layer of the insulating layer. [026] Furthermore, the safety electric cable comprising the insulating layer according to the invention satisfies standard NF C 32-070 CR1. The electrical conductors are thus protected against fire, or in other words, the electric cable affords high-quality fire behavior in terms, at least, of ash cohesion. [027] The invention as thus defined also has the advantage of being economical since it makes it possible to significantly reduce the amount of polyorganosiloxane in the insulating layer of the insulated electrical conductors, while at the same time having very good fireproofing properties. [028] The polyorganosiloxane, or polyorganosiloxane polymer, according to the present invention contains not more than 4% and preferably from 0.01% to 3% by weight of vinyl groups. When these polyorganosiloxane polymers have viscosities at 250C of between 50 000 and 1 000 000 mPa.s, they are known as oils, but their viscosity may be higher than 1 000 000 mPa.s and they are then known as gums. [029] In the compositions according to the present invention, the polyorganosiloxane polymers may be oils or gums or mixtures. [030] According to one particular example, the polyorganosiloxane contains not more than 4% by weight of vinyl groups and has a viscosity of at least 1 000 000 mPa.s at 25 0 C and preferably a viscosity of about 20 000 000 mPa.s at 250C. [031] These polyorganosiloxanes are linear polymers, whose diorganopolysiloxane chain is formed essentially from units of formula R 2 SiO. [032] This chain is blocked at each end with a unit of formula R 3 Sio.

5 and/or a radical of formula OR'. In these formulae: C09356 5 -the symbols R, which may be identical or different, represent monovalent hydrocarbon-based radicals such as alkyl radicals, for example methyl, ethyl, propyl, octyl, octadecyl, etc., aryl radicals, for example phenyl, tolyl, xylyl, etc., aralkyl radicals such as benzyl, phenylethyl, etc., cycloalkyl and cycloalkenyl radicals such as cyclohexyl, cycloheptyl, cyclohexenyl, etc. radicals, alkenyl radicals, for example vinyl, allyl, etc. radicals, alkaryl radicals, cyanoalkyl radicals such as a cyanoethyl radical etc., haloalkyl, haloalkenyl and haloaryl radicals such as chloromethyl, 3,3,3-trifluoropropyl, chlorophenyl, dibromophenyl, trifluoromethylphenyl radicals, -the symbol R' represents a hydrogen atom, an alkyl radical containing from 1 to 4 carbon atoms or a -methoxyethyl radical. [033] Preferably, at least 60% of the groups R represent methyl radicals. [034] The presence, along the diorganopolysiloxane chain, of small amounts of units other than R 2 SiO, for example units of formula RSiO 1 5 and/or SiO 2 , is, however, not excluded in a proportion of not more than 2%, these percentages expressing the number of T and/or Q units per 100 silicon atoms. The trifunctional units, of symbol T, make it possible to obtain three-dimensional networks. The tetrafunctional units, of symbol Q, lead to three-dimensional products whose structure is similar to that of silicates. [035] As concrete examples of units of formulae R 2 SiO and R 3 SiOo.

5 that may be mentioned are those of formulae: (CH 3

)

2 SiO, CH 3

(CH

2 =CH)SiO, CH 3

(C

6

H

5 )SiO, (C 6

H

5

)

2 SiO,

CH

3

(C

2

H

5 )SiO, (CH 3

CH

2

CH

2

)CH

3 SiO, CH 3 (n.C 3 Hz)SiO, (CH 3

)

3 SiOo.

5 , (CH 3

)

2

(CH

2 =CH)SiOo.

5 ,

CH

3

(C

6

H

5

)

2 SiOo.

5 , CH 3

(C

6

H

5

)(CH

2 =CH)SiOo.5. [036] As concrete examples of units of radicals of formula OR' that may be mentioned are those of formulae: OH, -OCH 3 , -OC 2

H

5 , -O-n.C 3

H

7 , -O-iso.0C 3

H

7 , -O-n.C 4 H, -OCH 2

CH

2 0CH 3 . [037] These oils and gums are sold by silicone manufacturers or may be manufactured by working according to already-known techniques. [038] The polyolefin of the second composition according to the invention may be chosen from ethylene homopolymers and copolymers, or a mixture thereof. [039] As a preferred example of ethylene homopolymers, mention may be made of low density polyethylene (LDPE). [040] The ethylene copolymers may advantageously be chosen from ethylene/octene copolymers (PEO), ethylene/vinyl acetate copolymers (EVA), ethylene/butyl acrylate C09356 6 copolymers (EBA), ethylene/methyl acrylate copolymers (EMA) and ethylene/ethyl acrylate copolymers (EEA), ethylene/propylene/rubber copolymers (EPR) and ethylene/propylene/diene/monomer copolymers (EPDM), or a mixture thereof. [041] As a preferred example of ethylene copolymers, mention may be made of ethylene/vinyl acetate copolymers (EVA). [042] In one preferred embodiment, the first composition also comprises at least one filler that forms ceramic under the effect of the high temperatures of a fire. [043] In another preferred embodiment, the second composition also comprises at least one flame-retardant mineral filler and/or at least one ceramic-forming filler. [044] The ceramic-forming filler, or ceramizable filler, mentioned in the present description may comprise at least one mineral filler of the meltable ceramic filler and/or refractory filler type, and preferably at least one meltable ceramic filler and at least one refractory filler, so that the first layer affords sufficient insulation when the organic part of the insulating layer, more particularly the second layer, has disappeared as a result of combustion. [045] More particularly, the meltable ceramic filler has a melting point below a high temperature T, and the refractory filler has a melting point above said temperature T. This temperature T is advantageously at least 750 C and may be up to 1100 C. [046] The meltable ceramic filler may be at least one mineral filler chosen from boron oxides (e.g. B 2 0 3 ), anhydrous zinc borates (e.g. 2ZnO 3B 2 0 3 ) or hydrated zinc borates (e.g. 4ZnO B 2 0 3

H

2 0 or 2ZnO 3B 2 0 3 3.5H 2 0) and anhydrous boron phosphates (e.g. BPO 4 ) or hydrated boron phosphates, or a precursor thereof. [047] This meltable ceramic filler typically has a melting point below 5000C and gives rise to a glass when the temperature exceeds 5000C. [048] The refractory filler may be at least one mineral filler chosen from magnesium oxides (e.g. MgO), calcium oxides (e.g. CaO), silicon oxides (e.g. SiO 2 or quartz), aluminum oxides or aluminas (e.g. A1 2 0 3 ), chromium oxides (e.g. Cr 2 0 3 ), zirconium oxides (e.g. ZrO 2 ) and phyllosilicates such as, for example, montmorillonites, sepiolites, illites, attapulgites, talcs, kaolins or micas (e.g. mica muscovite 6 SiO 2 -3 A1 2 0 3

-K

2 0-2H 2 0), or a mixture thereof.

C09356 7 [049] The flame-retardant mineral filler mentioned in the present description may be a hydrated flame-retardant mineral filler, chosen especially from metal hydroxides, for instance magnesium hydroxide or aluminum trihydroxide. [050] According to the present invention, the empty spaces promote the total combustion of the outer sheath subjected to fire, due to the presence of oxygen in these spaces, and the conversion of this sheath into residual ash that detaches from the insulating layer of the electrical conductors. As a result, the empty spaces do not comprise, for example, any filling polymer material (or polymeric filler) between the assembly of insulated electrical conductors and the outer sheath. This outer sheath is typically known as the tubing sheath or tubular sheath. Preferably, the empty spaces of the electric cable occupy at least 10% of the cross section of said cable. [051] In addition, according to a further characteristic, the electric cable according to the invention is free of metallic shield, for instance a metallic strip or a composite strip placed along said cable between the assembly of insulated electrical conductors and the outer sheath. The term "composite strip" means a strip composed of layers of metal and of polymer. Said metallic strip or said metallic layers of the composite strip may be made, for example, of copper or of aluminum. [052] This type of metallic shield limits, or even cancels out, the benefit of said empty spaces and thus significantly disrupts the process of ceramization of the ash of the insulating layers, leading to non-conformity of the fire resistance tests. [053] In one particular embodiment, the outer sheath comprises a polyolefin-based matrix and at least one hydrated flame-retardant mineral filler chosen especially from metal hydroxides, for instance magnesium hydroxide or aluminum trihydroxide. [054] This sheath can thus burn completely locally and be converted into residual ash under the effect of the high temperatures of a fire, without, however, being a fire propagator. The flame-retardant fillers used act mainly via a physical route by decomposing endothermically, which has the consequence of lowering the temperature of the material and limiting the propagation of flames along the cable. [055] In order to provide an HFFR cable "Halogen-Free Flame Retardant cable", the electric cable according to the invention does not comprise any halogenated compounds. These halogenated compounds may be of any nature, for instance fluoropolymers or C09356 8 chloropolymers, for instance polyvinyl chloride (PVC), halogenated plasticizers, halogenated mineral fillers, etc. [056] In one particular embodiment in accordance with the invention, the thickness of the first layer is from 0.10 mm to 1.50 mm and the thickness of the second layer is from 0.05 mm to 1.50 mm, especially when the cross section of the electrical conductor is from 1.5 mm 2 to 4 mm 2 . [057] According to a first variant of this embodiment, when the cross section of the electrical conductor is 1.5 mm 2 , the thickness of the first layer is preferably from 0.30 mm to 0.80 mm and more preferentially 0.45 mm. In this case, the thickness of the second layer is itself preferably from 0.10 mm to 0.50 mm, and more preferentially 0.35 mm. [058] According to a second variant of this embodiment, when the cross section of the electrical conductor is 2.5 mm 2 , the thickness of the first layer is preferably 0.30 mm to 0.90 mm, and more preferentially 0.50 mm. In this case, the thickness of the second layer is, itself, preferably from 0.10 mm to 0.60 mm, and more preferentially 0.40 mm. [059] According to a third variant of this embodiment, when the cross section of the electrical conductor is 4 mm 2 , the thickness of the first layer is preferably 0.35 mm to 1 mm, and more preferentially 0.55 mm. In this case, the thickness of the second layer is, itself, preferably from 0.10 mm to 0.70 mm, and more preferentially 0.45 mm. [060] Another subject according to the invention is a process for manufacturing an electric cable as described above in accordance with the invention, characterized in that it comprises the steps consisting in: i. forming the insulating layer of the insulated electrical conductor by extrusion and crosslinking of the first composition and of the second composition around an electrical conductor, ii. assembling at least two insulated electrical conductors as obtained in step i, and iii. extruding an outer sheath in a tubing manner around the assembled insulated electrical conductors of step ii. [061] According to this manufacturing process, the first and second compositions are, on the one hand, extruded, and, on the other hand, crosslinked via standard techniques that are well known to those skilled in the art so as to obtain an extruded and crosslinked insulating layer.

C09356 9 [062] According to a first variant of the process for manufacturing an electric cable, step i may consist in: i.1 extruding around an electrical conductor the first composition using a first extruder and the second composition using a second extruder, the first and second extruders comprising the same extrusion head, to form at the extruder outlet the insulating layer of the insulated electrical conductor, i.2 crosslinking the extruded insulating layer of step i.1, steps ii and iii of the process as described above remaining unchanged. Step i.1 of extrusion is referred to as coextrusion. [063] According to a second variant of the process for manufacturing an electric cable, step i may consist in: i.1 extruding using a first extruder the first composition and crosslinking at least partially the extruded first composition, i.2 extruding using a second extruder the second composition, to form the insulating layer of the insulated electrical conductor, and crosslinking the extruded insulating layer, steps ii and iii of the process as described above remaining unchanged. [064] Other characteristics and advantages of the present invention will emerge in the light of the examples that follow and of the attached figure, said examples and figure being given as illustrations and are in no way limiting. Brief description of the drawing figures [065] Figure 1 is a schematic view in cross section of an electric cable according to the invention. Detailed description of the embodiment or embodiments and examples [066] For reasons of clarity, the same elements have been denoted with identical references in this figure. Similarly, only the elements that are essential for the understanding of the invention have been schematically represented, and are not drawn to scale. [067] The electric cable shown in Figure 1 comprises three electrical conductors 1, a first insulating layer 2a (inner layer) around each electrical conductor 1, a second insulating layer 2b (outer layer) around each insulating first layer 2a, and an outer sheath 3 surrounding the assembly of the three insulated electrical conductors, the insulated electrical conductors having a substantially circular cross section. This outer sheath 3 creates empty spaces 4 between itself and the assembly of insulated electrical conductors it surrounds. This outer C09356 10 sheath 3 is said to be tubing since it leaves the insulated electrical conductors free inside said sheath. [068] The inner layer 2a is made from a first composition comprising a polyorganosiloxane-based polymer matrix, and the outer layer 2b is made from a second composition comprising a polyolefin-based polymer matrix, these two compositions being extruded and crosslinked. [069] The outer sheath 3 is itself made from a flame-retardant composition comprising a polyolefin-based polymer matrix. Examples Process for manufacturing an electric cable [070] First, the following compositions are prepared: - composition C01 based on polyorganosiloxanes and comprising ceramic-forming fillers, - composition C02 based on EVA and comprising flame-retardant fillers, and - composition C03 being a 50/50 mixture of composition C01 and of composition C02, compositions C01 to C03 also comprising a crosslinking agent of organic peroxide type. [071] To do this, compositions C01 to C03 are mixed, for example in an internal mixer. The mixing temperature of composition C01 is such that it allows the polyorganosiloxane to be softened while at the same time avoiding initiating the decomposition of the organic peroxide. By way of example, the mixing temperature is between 30 C and 50 0C. The mixing temperature of compositions C02 and C03 is, itself, such that it allows the polyolefin to melt (molten state), while at the same time avoiding initiating the decomposition of the organic peroxide. By way of example, the maximum mixing temperature is 115 0C. [072] Compositions C01 to C03 are then extruded (step i) and deposited around an electrical copper wire 1.78 mm in diameter (2.5 mm 2 circular section of the copper) so as to obtain the following layers: - an insulating layer C11 (monolayer) from composition C01, - an insulating layer C12 (monolayer) from composition C02, - an insulating layer C13 (monolayer) from composition C03, and - an insulating layer C14 (bilayer) comprising a first layer C14-1, of identical C09356 11 composition to composition C01, deposited around the electrical wire, and a second layer C14-2, of identical composition to composition C02, deposited around the first layer C14-1. [073] The insulating layers C11 to C14 all have an identical thickness equal to 0.9 mm. [074] More particularly, compositions C01 and C02 are extruded on said electrical wire, respectively, using two extruders comprising the same extrusion head, composition C02 (outer layer) covering composition C01 (inner layer). This is then referred to as coextrusion of compositions C01 and C02. The insulating layer C14 is thus formed from the inner layer 0.5 mm thick and the outer layer 0.4 mm thick. [075] Next, the insulating layers C11 to C14 are rapidly immersed in a salt bath at 2400C so as to crosslink the monolayers C11 to C13 and the two layers of the insulating layer C14 (step ii), under the action of decomposition of peroxide. [076] The insulated electrical wires thus obtained, noted as insulated wires F1 to F4, have a diameter of about 3.58 mm. [077] In an additional step (step iii), the respective insulated electrical wires F1 to F4 are then assembled in threes using an assembler, the assembling process being well known to those skilled in the art. [078] Finally, a flame-retardant composition comprising a polyolefin-based polymer matrix is extruded in a tubing manner around each group of three assembled electrical conducting wires F1 to F4, to form an outer sheath. [079] The electric cables thus obtained are noted cables CE1 to CE4. Tensile strength property The tensile strength mechanical properties are determined according to standard NF EN 60811-1-1. [080] To do this, tubular specimens are prepared from the insulating layers of the insulated wires F1 to F4. The specimens thus prepared, the surface area of which is measured precisely, are then tested on a tensile strength test bed with a traction speed of 250 mm/minute. [081] The tensile strength is thus the maximum tensile stress withstood by the specimen during the tensile test up to the point of failure.

C09356 12 [082] The results of these tests are collated in Table 1 below. Abrasion resistance property [083] The abrasion resistance mechanical property is determined according to standard EN 50305 part 5.2. [084] To do this, samples corresponding to portions 75 cm long of the insulated wires F1 to F4 are placed in an abrasion test device as described in said standard. [085] Each sample undergoes four tests. Each test is terminated when the cutting edge of said device reaches the conducting core of said insulating wires. [086] The abrasion resistance measurement corresponds to the mean value of the number of cycles performed in the four tests. [087] The results of these tests are collated in Table 1 below. Compression strength property [088] The compression strength mechanical property is determined according to standard EN 50305 part 5.6. [089] To do this, a tensile testing machine functioning in compression mode is used, which is equipped with a device for recording the force required to drive a needle cutting edge onto the insulating layers of insulated wires F1 to F4. A low-voltage detection circuit, designed to stop the machine when the edge crosses the insulating envelope, is introduced into the installation. [090] The force applied to the cutting edge that drives it onto the insulating envelope is increased at a constant rate until there is contact with the conducting core. [091] Four tests are performed on the sample, and the force corresponding to the electrical contact is recorded. [092] The results of these tests are collated in Table 1 below. Fire resistance properties [093] Standard NF C 32-070 CR1 concerns the duration of functioning of electric cables burning under defined conditions. The fire resistance is the consequence of the production of ash, which must have a certain cohesion making it possible to maintain sufficient insulation for the functioning of the electrical cables.

C09356 13 [094] In this test, samples of the electric cables CE1 to CE4 are placed in an oven whose temperature reaches 9200C in 50 minutes, and this temperature is then maintained for 15 minutes. During this test, said samples fed with a nominal voltage of 500 V are subjected to regular impacts (every 30 seconds). The test is satisfactory if the sample of cable does not show any electrical failure (breakdown or short circuit) after 65 minutes. [095] The results of these tests are collated in Table 2 below. Insulated wire F1 F2 F3 F4 Tensile strength 6-8 18 6- 88-10 12 (MPa) Abrasion 6-7 / 7-8 210-280 resistance (cycles) Compression 16-17 / 30-40 59-60 resistance (N) Table 1 Electric CE1 CE2 CE3 CE4 cable Pass Fail Fail Pass CR1 (> 65 minutes) (< 30 minutes) (< 40 minutes) (> 65 minutes) Table 2 [096] The insulated electrical wire F4 that constitutes the electric cable CE4 significantly shows mechanical properties, such as the tensile strength, the abrasion resistance and the compression strength, greater than the electrical wire F1 constituting the electric cable CE1. In addition, the electric cable CE4 satisfies the fire resistance standard NF C 32-070 CR1, which is not the case for the electric cables CE2 and CE3. [097] Finally, the amount of polyorganosiloxane is reduced by about half between the insulating layer C14 in accordance with the invention and the insulating layer C11, for an identical total thickness of the insulating layer. [098] Where ever it is used, the word "comprising" is to be understood in its "open" sense, that is, in the sense of "including", and thus not limited to its "closed" sense, that is the sense of "consisting only of". A corresponding meaning is to be attributed to the corresponding words "comprise", "comprised" and "comprises" where they appear.

C09356 14 [099] It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text. All of these different combinations constitute various alternative aspects of the invention. [0100] While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, and all modifications which would be obvious to those skilled in the art are therefore intended to be embraced therein.

Claims

1. An electric cable comprising: an assembly of insulated electrical conductors, each insulated electrical conductor comprising an electrical conductor surrounded with an insulating layer that is capable of being converted at least superficially into the ceramic state at high temperatures corresponding to fire conditions, and an outer sheath surrounding the assembly of insulated electrical conductors, empty spaces being provided between said sheath and the assembly of insulated electrical conductors, wherein said insulating layer comprises a first crosslinked layer surrounding the electrical conductor, and a second crosslinked layer surrounding said first layer, the first layer being obtained from a first composition comprising a polyorganosiloxane-based polymer matrix, and the second layer being obtained from a second composition comprising a polyolefin-based polymer matrix.

2. An electric cable as claimed in claim 1, wherein the first composition also comprises at least one ceramic-forming filler.

3. An electric cable as claimed in claim 1 or 2, wherein the second composition also comprises at least one flame-retardant mineral filler and/or at least one ceramic-forming filler.

4. An electric cable as claimed in any one of the preceding claims, wherein the outer sheath comprises a polyolefin-based polymer matrix and at least one hydrated flame retardant mineral filler.

5. An electric cable as claimed in any one of the preceding claims, wherein said empty spaces occupy at least 10% of the cross section of said Electric cable.

6. An electric cable as claimed in any one of the preceding claims, wherein it does not comprise any halogenated compounds.

7. An electric cable as claimed in any one of the preceding claims, wherein the thickness of the first layer is from 0.10 mm to 1.50 mm, and the thickness of the second layer is from 0.05 mm to 1.50 mm

8. An electric cable as claimed in claim 7, wherein the cross section area of the electrical conductor is 1.5 mm 2 to 4 mm 2 .

9. An electric cable as claimed in any one of the preceding claims, wherein the first layer and the second layer are extruded layers.

10. An electric cable as claimed in any one of the preceding claims, wherein the outer sheath is a tubular sheath.

11. An electric cable as claimed in any one of the preceding claims, wherein the first layer is directly in physical contact with the second layer. C09356 16

12. An electric cable as claimed in any one of the preceding claims, wherein the first layer is directly in physical contact with the electrical conductor.

13. An electric cable as claimed in any one of the preceding claims, wherein the second layer of at least one insulated electrical conductor is directly in physical contact with the outer sheath.

14. An electric cable as claimed in any one of the preceding claims, wherein the second layer of at least one insulated electrical conductor is directly in physical contact with the empty spaces.

15. An electric cable being substantially as herein before described with reference to figure 1 and or examples.

16. A process for manufacturing an electric cable as claimed in any one of claims 1 to 15, comprising: i. forming the insulating layer of the insulated electrical conductor by extrusion and crosslinking of the first composition and of the second composition around an electrical conductor, ii. assembling at least two insulated electrical conductors as obtained in step i, and iii. extruding an outer sheath in a tubing manner around the assembled insulated electrical conductors of step ii.

17. A process for manufacturing an electric cable, being substantially as herein before described with reference to figure 1 and or examples.