Cable comprising an element indicating water infiltration and method using said element

Abstract

A cable includes an indicating element for detecting the infiltration of water into the cable and a method using such indicating element.

Claims

1. A cable comprising at least one cable core comprising an indicating element that irreversibly discolours after being in contact with water for at least 10 minutes.

2. The cable according to claim 1, wherein the cable is an electric cable for power transmission/distribution comprising a cable core comprising an electric conductor surrounded and in contact with a protecting layer.

3. The cable according to claim 1, wherein the cable is a telecommunication cable comprising a cable core comprising at least one optical fibre surrounded by a retaining tube.

4. The cable according to claim 1, wherein the indicating element comprises a water-insoluble marker irreversibly discolouring after being in contact with water for at least 10 minutes.

5. The cable according to claim 4, wherein the marker has a solubility in water of less than 1 g in 100 g of water.

6. The cable according to claim 4, wherein the marker is soluble in organic solvents.

7. The cable according to claim 4, wherein the marker is an acid-base indicator.

8. The cable according to claim 4, wherein the marker is an acid-base indicator able to discolour with pH changing in a range of 3.5 to 8.0.

9. The cable according to claim 4, wherein the marker is heat-resistant at least up to 100 C.

10. The cable according to claim 4, wherein the marker is associated with a supporting material.

11. The cable according to claim 1, wherein the indicating element comprises a supporting material.

12. The cable according to claim 11, wherein the supporting material is chemically/physically inert to water.

13. The cable according to claim 11, wherein the supporting material is heat-resistant at least up to 100 C.

14. A process for producing a cable comprising at least one cable core containing an indicating element that irreversibly discolours after being in contact with water for at least 10 minutes, said indicating element comprising a marker and a supporting material, wherein the marker is associated with the supporting material by dissolving the marker in an organic solvent to provide a solution; impregnating the supporting material with said solution; and evaporating the organic solvent to dry the supporting material and provide the indicating element.

15. The process according to claim 14, comprising dissolving the marker in an organic solvent to provide a saturated solution.

16. A method for detecting absence of contamination by water in a cable, comprising: providing a cable comprising at least one cable core containing an indicating element that irreversibly discolours after being in contact with water for at least 10 minutes; causing the cable to get in contact with water for less than 10 minutes; and verifying the indicating element remained unchanged.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The present invention will be better understood by reading the following detailed description, given by way of example and not of limitation, to be read with the accompanying drawings, wherein:

(2) FIG. 1 shows a perspective view of an electric cable according to an embodiment of the present invention;

(3) FIG. 2 shows a cross section of an electric cable according to another embodiment of the present invention;

(4) FIG. 3 shows a cross section of a telecommunication cable according to a further embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(5) FIG. 1 shows a perspective view of an electric cable 11 according to an embodiment of the present invention.

(6) The electric cable 1 of FIG. 11 comprises a conductor 12, an inner semiconductive layer 13, an insulating layer 14, which constitute the cable core. The cable core is surrounded by an outer semiconductive layer 15, a metal shield 16 and an outer sheath 17.

(7) The conductor 12 generally comprises metal wires, which are preferably made of copper or aluminium, and which are braided together by using conventional technique.

(8) The cross sectional area of the conductor 12 is determined in relationship with the power to be transported at the selected voltage. Preferred cross sectional areas for electric cables according to the present invention range from 16 mm.sup.2 to 1,600 mm.sup.2.

(9) Inner semiconductive layer 13, insulating layer 14 and outer semiconductive layer 15 are made polymeric material.

(10) Polymeric materials suitable for layers 13, 14 and 15 can be selected from the group comprising: polyolefins, copolymers of different olefins, copolymers of an olefin with an ethylenically unsaturated ester, polyesters and mixtures thereof.

(11) Examples of suitable polymers are: polyethylene (PE), in particular low density PE (LDPE), medium density PE (MOPE), high density PE (HDPE), linear low density PE (LLDPE), ultra-low density polyethylene (ULDPE); polypropylene (PP) and copolymers thereof; elastomeric ethylene/propylene copolymers (EPR) or ethylene/propylene/diene terpolymers (EPDM); natural rubber; butyl rubber; ethylene/vinyl ester copolymers, for example ethylene/vinyl acetate (EVA); ethylene/acrylate copolymers, in particular ethylene/methyl acrylate (EMA), ethylene/ethyl acrylate (EEA) and ethylene/butyl acrylate (EBA); ethylene/-olefin thermoplastic copolymers; and copolymers thereof or mechanical mixtures thereof.

(12) In the case of inner semiconductive layer 13 and outer semiconductive layer 15, the above listed polymeric materials are added with an electro-conductive carbon black, for example electro-conductive furnace black or acetylene black, so as to confer semiconductive properties to the polymer material.

(13) The insulating layer 14 can be made of polymeric a thermoplastic material, which comprises a thermoplastic polymer material including a predetermined amount of a dielectric liquid. Example of thermoplastic insulating layers are disclosed in WO 02/03398, WO 02/27731, WO 04/066318, WO 07/048422 e WO 08/058572

(14) Preferably, the metal shield 16 is made of a continuous metal tube or of a metal sheet shaped into a tube and welded or sealed using an adhesive material so as to make it watertight.

(15) In a preferred embodiment, the metal shield 16 is made of a continuous metal sheet, preferably of aluminium or copper, which is shaped as a tube.

(16) The outer sheath 17 preferably is made of polymer material, such as polyvinyl chloride (PVC) or polyethylene (PE).

(17) In the embodiment of FIG. 1, an indicating element 18, in form of a yarn supporting material impregnated with a marker of the invention, is provided within the metal wires of the conductor 12. More than one yarn can be present within the metal wires of the conductor/s.

(18) FIG. 2 shows another embodiment of the invention. FIG. 2 illustrates a cable 21 comprising three cable cores. Each cable core comprises a conductor 22, an inner semiconductive layer 23, an insulating layer 24. Each cable core is surrounded by an outer semiconductive layer 25 and by a metal shield 26 and an outer sheath 17. Conductors 22 are each made of a solid aluminium rod.

(19) The three cable cores are stranded and embedded into filler (or bedding) 29 which, in turn, is surrounded by an outer sheath 27. Outer sheath 27 can be made of the same material already disclosed in connection with outer sheath 17 of FIG. 1.

(20) The materials of inner semiconductive layer 23, insulating layer 24, and outer semiconductive layer 25 can be as those already mentioned in connection with cable 11 of FIG. 1 for analogous cable portions.

(21) In the embodiment of FIG. 2, an indicating element 28, in form of a yarn supporting material impregnated with a marker of the invention, is provided at the interface between conductor 22 and the adjacent protecting layer, in the present case the inner semiconductive layer 23 of at least one cable core.

(22) Indicating element 28 can be provided for each cable core of a multicore cable.

(23) The indicating element 28 can be, alternatively or additionally, a yarn or tape wound around the conductor/s 22.

(24) FIG. 3 shows a cross section of a telecommunication cable 31 according to an embodiment of the invention. A group 32 of six optical fibres are loosely contained in a retaining tube 33 to constitute the cable core. Cable 31 comprises four cable cores contained in a polymeric sheath 34. Embedded in the sheath 34 are two radially opposed strength members 35 made, for example, of fibres glass or Kevlar.

(25) An indicating element 36, in form of a yarn supporting material impregnated with a marker of the invention, is provide within the retaining tube 33.

(26) The yarn 18,28,36 is made of cotton.

(27) The marker supported by the yarn 18,28,36 is alizarin red, an acid-base indicator of formula

(28) ##STR00001##

(29) CAS Registry Number 72-48-0, which is yellow at pH 5.5 and irreversibly turns to red at pH 6.8

(30) Alizarin red is virtually insoluble in water and soluble, for example, in ethanol and acetic acid. The melting point is of about 290 C.

(31) The cable according to the present invention can be manufactured by process known to the skilled in the art. The indicating element can be paid using common process apparatus at a suitable step of the manufacturing process. For example, when the indicating element is to be positioned within the wires of an electric conductor, the indicating element in form of yarn/s is stranded together with the wires. For example, when the indicating element is to be positioned between the electric conductor and the protecting layer (insulating layer or inner semiconducting layer), the indicating element in form of yarn/s or tape is wound around the conductor before extruding said layer. For example, when the indicating element is to be positioned within a retaining tube for housing optical fibres, the indicating element in form of yarn/s is joined to the optical fibre bundle and the polymeric material is extruded around according to known technique.

(32) The following examples are intended to further illustrate the present invention, without however restricting it in any way.

Example 1

(33) Alizarine red (0.0206 g) was dissolved, at room temperature, in n-butyl alcohol (85 ml) to provide a saturated solution.

(34) Two samples of white 100% cotton yarn (510 dtex; weight of 0.23 g/m) were immersed into the resulting solution, kept them until impregnated, then taken off and dried in an oven at 50 C. for 5 minutes. Both the dried samples became yellow cream-coloured. The red alizarine content in the yarn was of about 2.Math.10.sup.4 g/m.

(35) Subsequently, one sample yellow cream-coloured was immersed in tap water, at room temperature, for 20 minutes, while the other was immersed in tap water for 10 days.

(36) Alter about 15 minutes from the immersion in water both the samples became red-purple. The sample left in immersion for 10 days did not loose colour. Both the samples maintained such colour even after complete drying.

(37) The test was repeated by dissolving to saturation red alizarine in acetone and ethyl acetate. Equivalent results were obtained.

Comparative Example 1

(38) A paper tape sample was immersed for 5 minutes in an aqueous solution of methylene blue at 2 wt % at room temperature, until impregnation. The sample was then taken off and dried in an oven at 60 C. for some hours.

(39) The dried sample was immersed in tap water and kept therein for 24 hours without any stirring. No discoloring was observed.

(40) Methylene blue, though soluble in water, was not washed off. Subsequent tests performed also under mild stirring provided ambiguous results, i.e. in some cases the paper tapes resulted somewhat discoloured, but not in an unquestionable way.

(41) The use of a water soluble dye as methylene blue does not provide affordable results and is not suitable for the indicating element according to the invention.

Comparative Example 2

(42) A paper tape sample was dipped for 5 minutes in an aqueous suspension of calcium hydroxide at room temperature. The sample was then dried in an oven at 60 C. for some hours.

(43) The dried sample was immersed in an alcoholic solution of phenolphthalein at 1 wt % and kept therein for 5 minutes, then taken off and dried in an oven at 60 C. for 30 minutes.

(44) The dried sample, having substantially the original colour of the tape, was then immersed into tap water and immediately displayed a vivid pink colour due to phenolphthalein turning. Remaining the sample immersed into water, the pink colour of the sample started to fade and completely disappeared after a couple of hours.

(45) Due to the solubility in water, phenolphthalein cannot be used in an indicating element of the present invention.