DESCRIPTION

The present invention relates to a fire resistant medium and high voltage electric cable.

Electric cables used in industrial facilities, like, for example, onshore and offshore plants, or in public infrastructures, like, for example, underground lines, require high fire resistance properties to maintain operation of the facilities or infrastructures for a certain period of time to allow evacuation of the people before the fire breaks out.

Due to the increasing amount of equipment running on medium and high voltage in the abovementioned industrial facilities or public infrastructures, there is the need of having fire resistant medium and high voltage electric cables.

Typically, medium (MV) and high voltage (HV) electric cables comprise a core and an outer sheath arranged in a radially outer position with respect to the core. The core comprises a conductor, an insulating system arranged in a radially outer position with respect to the conductor and a conductive screen arranged in a radially outer position with respect to the insulating system. The insulating system typically includes a first semiconductive layer arranged in contact with the conductor, an insulating layer arranged in a radially outer position with respect to the first semiconductive layer, a second semiconductive layer arranged in a radially outer position with respect to the insulating layer.

In specific cases, such as, for example, in off-shore plants, the electric cables also include a metallic armour radially interposed between the conductive screen and the outer sheath.

In order to provide a medium and high voltage electric cable with fire resistant properties, a fire resistant layer is typically provided in the cable to create a barrier to the fire propagation within the cable.

EP 2413331 relates to a medium voltage fire resistant electric cable comprising a first layer made of an extruded elastomeric ceramics silicone material, and an optional outer layer made of a like material, thereby providing a thermal protective barrier for the innermost layers of the cable comprising copper conductors, a semiconductor material and an insulating material sheath. In particular, the insulating material sheath is coated by an extruded silicone and ceramizing elastomeric layer. In cooperation with the ceramizing elastomeric layer, a second semiconductor material layer is further co-extruded. The cable is further provided with a screen or shield constituted by a copper strip and a thermal insulating layer including a plurality of glass fibre strips covering in turn the copper shield. The layer made of an extruded ceramizing silicone elastomeric material provides both an electrical insulation and a thermal insulation. Another glass yarn strip operates to further increase the cable fire thermal resistance.

JP 05-182532 relates to a fire resistance cable suitable for the high voltage use of 6600V class. The cable core includes an inner semiconductive layer, an insulating layer, an outer semiconductive layer, a metal shield layer and an external sheath. The cable core is surrounded by a layer of mica and cellulose pulp adhering onto a reinforcement layer made of, e.g. glass fibre. This layer is surrounded, in turn, by a foaming fire preventing layer which is foamed to be carbonized by heat. The outer sheath is made of PVC.

WO 2014/081096 relates to a fire resistant cable for medium or high voltage comprising a conductor, a first semiconducting layer formed outside of the conductor, a first insulation layer formed outside of the first semiconducting layer, a second semiconducting layer formed outside of the first insulation layer, a shield layer formed outside of the second semiconducting layer and a fire resistant layer provided between the first insulating layer and the second semiconducting layer. The fire resistant layer is formed by winding a mica tape two or more times.

The Applicant observed that providing a fire resistant layer into the insulating system of a MV or HV cable is not advisable because any modification to this system, like the insertion of the fire resistant layer, can alter the sought electrical properties of the cable. In addition, the insertion of a fire resistant layer into the insulating system could require a significant change in the manufacturing process of the cable.

The Applicant also observed that the use of mica tapes can be disadvantageous.

Mica tape is a tape manufactured with mica flakes glued, for example with a silicone based glue, onto a substrate to enable ease of handling, the substrate being typically made of paper, polymer or glass fibre. The Applicant observed that, because of its flaky structure, mica tape can create voids within the cable, possibly generating undesired partial discharges. In addition, the flaky structure can cause easy detachment of mica from the support tape. Furthermore, winding mica tapes requires special taping machines and, during winding, mica tapes can be damaged, giving places here again to voids and/or partial discharges. Moreover, the Applicant observed that mica could flake off in a relatively short operation time (e.g. few hours) at about 90°C, thus mica tapes could be used only in MV and HV cables configured to operate for a limited time, e.g. emergency cables. Therefore, mica tapes cannot be relied upon for a continuous use at the operation conditions of MV and HV cables.

Accordingly, Applicant believes that for MV and HV cables it is preferred to make a coextruded insulating system in which all of the layers are in direct contact with the adjacent one.

The Applicant realised that an effective and reliable barrier to the propagation of the fire towards the innermost layers of the cable can be obtained by arranging a rubberised glass fibre tape in a radially inner position with respect to the outer sheath and with the rubberised side directly contacting with the outer sheath. The rubberised side of the glass fibre tape and the outer sheath, being both made of a polymeric material, bind one to the other creating an integral assembly which, in case of fire, allows the ashes of the burnt outer sheath to remain in place and to act as a barrier to the fire propagation inside the cable. In case of fire, thanks to the presence of glass fibres in the rubberised tape, the outer sheath stay in place, carbonizes in a substantially uniform matter and creates a heat barrier preventing the temperature inside the cable to quickly and highly rise and the melted material of the insulation system to move, thus avoiding any risk of short between conductor and conductive screen.

The Applicant realised that a further fire resistant barrier can be effectively and reliably provided by arranging a heat block layer made of halogen-free flame retardant or fire resistant polymeric material in a radial outer position with respect to the cable core and in a radial inner position with respect to rubberized glass fibre tape. This layer allows delaying both propagation of the fire within the core of the cable and heat build-up in the insulation system, thus maintaining the insulation properties and ensuring operation of the cable for the desired time.

Accordingly, the present invention relates to a fire resistant medium and high voltage electric cable, comprising: - a conductor;

- a first semiconductive layer arranged in a radially outer position with respect to the conductor;

- an insulating layer arranged in a radially outer position with respect to the first semiconductive layer and directly contacting the first semiconductive layer;

- a second semiconductive layer arranged in a radially outer position with respect to the insulating layer and directly contacting the insulating layer;

- a conductive screen arranged in a radially outer position with respect to the second semiconductive layer;

- a heat block layer arranged in a radially outer position with respect to the conductive screen, the heat block layer comprising a layer made of a fire resistant or a flame retardant halogen-free material;

- a rubberised glass fibre tape arranged in a radially outer position with respect to the heat block layer and having a rubberised surface outward facing;

- an outer sheath arranged in a radially outer position with respect to the rubberised glass fibre tape and directly contacting the rubberised surface of the rubberised glass fibre tape.

Advantageously, in the cable of the invention the contact between the rubberised surface of the rubberised glass fibre tape and outer sheath forms, in case of fire, an effective barrier against propagation of the fire inside the cable. Such a barrier prevents the insertion of fire resistant layer/s into the insulating system.

Preferably, the rubberised glass fibre tape is bonded to the outer sheath, so as to acts as a single continuous layer. The bonding between the rubberised surface of the rubberised glass fibre tape and outer sheath can be attained while manufacturing the cable by extruding and, optionally, curing the outer sheath around the already wrapped rubberised glass fibre tape.

Advantageously, the cable of the invention further comprises a glass fibre tape radially interposed between the conductive screen and the rubberised glass fibre tape.

Preferably, a first glass fibre tape can be arranged in a radially outer position with respect to the heat block layer and, more preferably, in direct contact thereto. This glass fibre tape contributes to avoid outflow of the melted insulation, thus maintaining the insulation properties of the cable.

A second glass fibre tape is preferably radially interposed between the conductive screen and the heat block layer, more preferably in direct contact with the latter.

Preferably, the first and second glass fibre tape are made of woven glass fibres, self-supported or supported by a polymer layer, for example a rubberised layer.

When both a first and a second glass fibre tape are provided, the heat block layer is sandwiched between two glass fibre tapes which, in case of fire, aid the heat block layer material to remain in place and work as an thermal insulator.

A metallic armour can be radially interposed between the heat block layer and the rubberized glass fibre tape.

Preferably, the metallic armour is radially interposed between the first glass fibre tape and the rubberised glass fibre tape.

In preferred embodiments of the invention, the outer sheath is made of a flame retardant halogen-free material. This allows the outer sheath not to burn quickly.

The material of the outer sheath is preferably also a mud and/or oil resistant material. In particular, the outer sheath of the cable of the invention is preferably classified as SHF 2 according to IEC 60092-360 (2014).

Preferably, the outer sheath is made of a material which is flame retardant halogen-free and mud or mud/oil resistant.

In the present description and claims, the first semiconductive layer, the insulating layer and the second semiconductive can also be collectively referred to as "insulating system".

In the present description and claims, the conductor, the first semiconductive layer, the insulating layer, the second semiconductive and the conductive screen can also be collectively referred to as "cable core" or "cable conductive core".

In the present description and claims as "semiconducting layer" it is meant a layer made of a material having semiconductive properties, such as a polymeric matrix added with, e.g., carbon black such as to obtain a volumetric resistivity value, at room temperature, of less than 500 Q ^» m, preferably less than 20 Q ^» m. The amount of carbon black can range between 1 and 50% by weight, preferably between 3 and 30% by weight, relative to the weight of the polymer.

In the present description and claims, as "fire resistant" it is meant a material capable of withstanding the fire according to IEC 60331 -21 (1999).

In the present description and claims, as "flame retardant" it is meant material capable of delaying the flame propagation according to IEC 60332 3- 22 (2009-02).

For the purpose of the present description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term "about". Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.

Also, use of the "a" or "an" are employed to describe elements and components of the invention. This is done merely for convenience and to give a general sense of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise.

Further features and advantages of the present invention will appear more clearly from the following detailed description of preferred embodiments thereof, such description being provided merely by way of non-limiting example and being made with reference to the annexed figure 1 , which shows a schematic cross-section view of a fire resistant medium and high voltage electric cable according to an exemplary embodiment of the present invention. Such a cable is indicated with numeral reference 100.

Cable 100 is designed to be used, for example, in off-shore installations. Cable 100 comprises, in the radially innermost portion thereof, a core

105 and, in the radially outer portion thereof, an outer sheath 200.

The core 105 includes at least one conductor 1 10, an insulating system 1 15 arranged in a radially outer position with respect to the conductor 1 10 and a conductive screen 150 arranged in a radially outer position with respect to the insulating system 1 15.

The insulating system 1 15 comprises a first semiconductive layer 120 arranged in a radially outer position with respect to the conductor 1 10 and in direct contact with the conductor 1 10, an insulating layer 130 arranged in a radially outer position with respect to the first semiconductive layer 120 and in direct contact with the first semiconductive layer 120 and a second semiconductive layer 140 arranged in a radially outer position with respect to the insulating layer 130 and in direct contact with the insulating layer 130.

While a single core 105 is shown in figure 1 and described below, the following description applies to multicore cables as well.

The first semiconductive layer 120 mitigates concentration of electric field inside the cable 100 by uniformly distributing charges on the surface of the conductor 1 10. Furthermore, the first semiconductive layer 120 minimizes degradation of the insulating layer 130, which is caused by ionization, by filling the gaps formed between the conductor 1 10 and the insulating layer 130.

The insulating layer 130 insulates the conductor 1 10 from outside by covering and protecting the conductor 1 10 so that current may not flow outside of the cable 100.

The second semiconductive layer 140 uniformly distributes electrical stress inside the insulation system 1 15.

The conductor 100 can be made of a rod or of stranded wires made of an electrically conductive metal, such as copper or aluminium. For example, the conductor 100 comprises tinned stranded and compressed copper wires.

The layers 120, 130 and 140 of the cable insulating system 1 15 can be made of extruded polymeric material. Examples of suitable polymeric materials are polyethylene homopolymers or copolymers, such as cross-linked polyethylene (XLPE), or elastomeric ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, also cross-linked, or thermoplastic materials, for example propylene-based materials as disclosed in WO 02/03398, WO 04/066317, WO 04/066318, WO 07/048422, WO1 1 /092533 and WO 08/058572. The semiconducting layers material further comprises a suitable amount of a conductive filler, for example carbon black.

Layers 120, 130 and 140 forming the insulating system 1 15 are preferably coextruded. The conductive screen 150 can be made of a metallic tape or a metallic braid of copper, aluminium, a copper alloy, an aluminium alloy or a combination thereof.

For example, the conductive screen 150 is made of a tinned annealed copper wire braid. The conductive screen 150 can connect the cable 100 with the ground.

A semiconducting tape (not illustrated) made, for example, of the same material of the second semiconductive layer 140 can be interposed between the conductive screen 150 and the second semiconductive layer 140.

A second glass fibre tape 160 is arranged in a radially outer position with respect to the conductive screen 150.

A polymeric tape (not illustrated) made, for example, of polyethylene terephthalate (PET) can be interposed between the second glass fibre tape 160 and the conductive screen 150. The protecting tape allows easing the stripping of the radially outer layer/s during installation.

The second glass fibre tape 160 includes glass fibres. The glass fibre tape can comprise a layer made of polymeric material, for example ethylene- propylene diene monomer (EPDM), ethylene-vinyl acetate (EVA), linear low density polyethylene (LLDPE) and mixture thereof, avoiding damaging the edges of the tape. Self-supporting (not supported by a further layer) woven glass fibre tape are also suitable.

A heat block layer 165 is arranged in a radially outer position with respect to the second glass fibre tape 160.

The heat block layer 165 is made of an extruded polymeric material which can be fire resistant or halogen-free flame retardant (LSOH). Examples of fire resistant materials are ceramifying silicone or polymer materials containing ceramifying charges. Examples of halogen-free flame retardant materials are optionally cross-linked ethylene homopolymer or copolymer or mixture thereof charged with a flame retardant filler, such as aluminium or magnesium hydroxide. Preferably, the halogen-free flame retardant material for the heat block layer of the invention is a mixture comprising ethylene vinyl acetate (EVA) and linear low density polyethylene (LLDPE) comprising from 30 wt% to 70 wt% of aluminium or magnesium hydroxide with respect to the total weight of the mixture. A first glass fibre tape 170 is arranged in a radially outer position with respect to the heat block layer 165. The first glass fibre tape 170 is made substantially as said above in connection with the second glass fibre tape 160

When the first glass fibre tape 170 and/or the second glass fibre tape 160 comprise a layer made of polymeric material, the positioning of said layer (whether in contact with the heat block barrier 165 or not) is inconsequential.

In a preferred embodiment, both the first and the second glass fibre tapes 170, 160 are provided in radially inner and outer position with respect to the heat block layer 165 so as their support tapes are in direct contact with the heat block layer 165.

In case of fire, the second glass fibre tape 160 and the first glass fibre tape 170 firmly keep in position the heat block layer 165, so that the glass fibre tape 160, the heat block layer 165 and the glass fibre tape 170 define an assembly 175 which allows delaying both propagation of the fire within the core 1 10 and heat build-up in the insulation system 1 15.

A metallic armour 180 is arranged in a radial outer position with respect to the first glass fibre tape 170.

The metallic armour 180 can be made of a metallic tape or a metallic braid of galvanized steel, copper, aluminium, a copper alloy, an aluminium alloy or a combination thereof. The metallic armour can also perform as electric screen.

For example, the metallic armour 180 is made of a tinned annealed copper wire braid.

In certain cables applications, the metallic armour 180 can be missing. A rubberised glass fibre tape 190 is arranged in a radially outer position with respect to the metallic armour 180.

The rubberised glass fibre tape 190 includes glass fibres, preferably in woven form, arranged on a support tape made of an elastomeric material selected, for example, from the group comprising ethylene-propylene diene monomer (EPDM), ethylene-vinyl acetate (EVA), linear low density polyethylene (LLDPE) and mixture thereof.

The elastomeric material of the rubberised glass fibre tape has the function of bonding the tape to the outer sheath during extrusion and, optionally, curing of the latter, to hold the sheath in place when burning, and of supporting the glass fibres during production to avoid damages while winding.

An outer sheath 200 is arranged in a radially outer position with respect to the rubberised glass fibre tape 190, in direct contact with the elastomeric support thereof.

The outer sheath 200 is preferably made of a halogen free, flame retardant material analogous to that used for the heat block layer. More preferably, the outer sheath 200 is made of a halogen free, flame retardant and mud and/or oil resistant polymeric material. Examples of halogen free, flame retardant and mud and/or oil resistant polymeric (preferably thermosetting) materials are alkylene/alkyl acrylate copolymer or a mixture of alkylene/alkyl acrylate copolymers, preferably having an average content of alkyl acrylate comonomer of at least 40 wt% by weight with respect to the weight of the copolymer/s, said materials being charged with a flame retardant filler, such as aluminium or magnesium hydroxide. For example, the alkylene comonomer of copolymer is an ethylene co-monomer. For example, the alkyl acrylate comonomer is selected from methyl acrylate and butyl acrylate.

The combination of rubberised glass fibre tape 190 and outer sheath 200 provide a barrier to the propagation of the fire towards the innermost layers of the cable 100.

In case of multicore cables, two or more cores are stranded, a filling material is provided into the gaps between the cores and the heat block layer or, if present, the second glass fibre tape is provided to enclose the cores and the filling material. The structure of the multicore cable in the radial outer portion with respect to the heat block layer or, if present, the second glass fibre tape is identical to the one discussed above with respect to the cable 100 of figure 1 .

A sample of a cable having the construction of cable 100 and having the construction as from Table 1 has been subjected to fire tests according to IEC 60331 -21 at 750°C. Table 1

The cable repeatedly resisted for about 2 hours before short circuit between the conductor and the conductive screen being detected.