GB1571113A

GB1571113A - Electric cables and the manufacture thereof

Info

Publication number: GB1571113A
Application number: GB4339076A
Authority: GB
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1976-10-19
Filing date: 1976-10-19
Publication date: 1980-07-09
Also published as: AU2975177A

Description

(54) ELECTRIC CABLES AND THE MANUFACTURE THEREOF (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 electric cables and their manufacture.It has been proposed, for example in the specification of Dutch Patent Application 7508151 (which, or the corresponding British Specification 1465515 published more recenlty, should be consulted for full details), that localised temperature changes, such as those resulting from leaks in tanks of liquified gases, can be detected by means of a cable having two conductors spaced by insulating material loaded with a ferrite or other ferromagnetic material having a suitably chosen Curie point; in the method proposed electrical pulses are launched into the cable from at least one of its ends and any localised cold (or hot) spot is detected by observing a returning pulse reflected from the electromagnetic discontinuity where the temperature is locally below (or above) the Curie point of the electromagnetic material.

Hitherto the preferred method of manufacture for such cables has been by extrusion, but it has proved difficult to extrude material with the high ferrite loading required for this purpose so as to obtain a uniform layer with a smooth surface and this has limited extrusion speed. Attempts have been made to coat wire with a ferrite-loaded rubber/solvent adhesive, but these sticky materials are troublesome to handle and residual solvent may cause defects that would lead to spurious reflections.

In accordance with the invention, a method of producing a wire coated with insulating material loaded with ferro-magnetic powder comprises dipping the wire in an aqueous dispersion comprising the ferromagnetic powder and an electricallyinsulating polymer to form a wet film on the wire, and afterwards drying the film on the wire. Subsequently a second conductor can be added to complete the basic requirement of the cable; the second conductor may be coaxial with the first, lapped round it, twisted with it, or laid parallel to it. Especially where the conductors are parallel, a protective sheath will usually be desirable.

The second conductor may be bare, insulated, or itself coated with insulating material loaded with ferromagnetic powder, preferably by the method described.

Each of the conductors may be a solid or a stranded wire.

Any polymer capable of forming an adequately stable dispersion can be used, but in view of their ready availability, good electrical and film-forming properties and ability to be incorporated into ferrite powders without major difficulty polyvinyl chloride (PVC) and natural rubber are preferred.

Coating can be accomplished simply by running the wire through a bath of the loaded dispersion, but to ensure uniformity a wiping die is preferably used, broadly as is done in wire enamelling and tinning. Preferably the die 'floats' in the sense that it is free to move in any direction transverse to the wire, and preferably within limits along the wire also. It may also float in the sense of being supported by buoyancy on the surface of the dispersion.

In most cases it will be desirable to heat the coated wire, in a baking oven or otherwise, to coagulate the polymeric material, cure it if applicable, and eliminate the water from the coating. Use of chemical coagulants is possible but not recommended. Because neither the ferromagnetic material nor the dispersed polymer has a high affinity for water, its substantially complete elimination is readily obtained.

After solidification of the coating, a further layer or layers can be applied in the same way if a greater coating thickness is required.

To modify the electrical and/or mechanical properties of the coated wire, a concentric layer of insulating material without ferromagnetic loading can be added. For example an extruded layer of polyethylene or other conventional insulating material can be used.

The invention includes coated wires and cables made by the method described.

Example 1 A solid copper wire 1.25 mm in diameter was coated at a speed of 0.04 m/s by dipping in a bath comprising: parts by weight Ferrite (Curie point 55"C) (particle size analysis as shown in the table below) 93 Breon* vinyl latex 576 20 Water 57 Dispersol* (dispersion agent) I Ammonia solution (0.88 specific gravity) 3 *trademark Ferrite Particle Size Analysis Number of Particles Particle diameter above this size Cumulative weight% (,um) (cumulative) above this size 12.7 1 0.3 10.1 16 2.3 8.00 100 8.4 6.35 401 19.4 5.04 1129 33.7 4.00 2828 48.5 3.17 5912 ' 62.6 2.52 11217 74.6 2.00 18869 83.3 1.59 29842 89.4 1.26 ' 44649 93.7 1.00 62935 96.4 0.79 84731 97.8 0.63 110221 98.7 (particles below 0.63 ym not analysed) On emerging vertically upwards from the bath surface the wire passed through a wiping die made of nylon with a bore of 1.80 mm and a mass of 0.9 g, supported in a holder above the bath surface, and free to move, within limits, in all directions and then through an oven 3 m. high maintained at a mean temperature of about 210 C. A uniform smooth coating about 0.05 mm radial thickness and containing 0.55 g. of the ferrite per metre length of coated wire was obtained. The coated conductor was then covered with an extruded layer of polyethylene with a radial thickness of 1.1 mm. A coaxial conductor braided from 120 ends of 0.15 mm copper wire and an outer sheath of a conventional cable sheathing PVC compound, 0.7 mm thick, complete the cable.

Example 2 A solid copper wire 1.25 mm in diameter was coated at a speed of 0.7 m/s by dipping in a bath comprising: parts by weight Ferrite (Curie point 55"C) (particle size size analysis as Example 1) 95 Breon* vinyl latex 576 20 Water 17+ Dispersol* (dispersion agent) 2 *trademark On emerging vertically upwards from the bath surface the wire passed through a wiping die made of stainless steel with a bore of 1.80 mm, supported in a holder above the surface of the fluid coating medium and free to move, within the constraints of the die holder, and the wire then passed through an oven 3 m high maintained at a mean temperature of about 210 C. The coated wire then passed over a pulley and vertically downwards to pass twice more through the bath containing the fluid coating medium, successive wiping dies being of diameter 2.00 mm and 2.20 mm, these dies being also free to move, within the constraints of the die holder, in all directions. A uniform smooth coating about 0.12 mm radial thickness and containing 1.40 g of the ferrite per metre length of coated wire was obtained. The coated conductor was then insulated with extruded polyethylene of radial thickness 1.0 mm. A coaxial braid and a PVC sheath completed the cable.

WHAT WE CLAIM IS:- 1. A method of producing a wire coated with insulating material loaded with ferromagnetic powder comprising dipping the wire in an aqueous dispersion comprising the ferromagnetic powder and an electrically-insulating polymer to form a wet film on the wire, and afterwards drying the film on the wire.

2. A method as claimed in claim 1 in which the polymer is polyvinyl chloride.

3. A method as claimed in claim 1 in which the polymer is natural rubber.

4. A method as claimed in any one of the preceding claims in which a wiping die is used for controlling the thickness of coating on the wire.

5. A method as claimed in claim 4 in which the wiping die is free to move in any direction transverse to the wire.

6. A method as claimed in claim 4 in which the die is free to move in any direction transverse to the wire and within limits along the wire also.

7. A method as claimed in any one of claims 4--6 in which the wiping die is supported by buoyancy on the surface of the dispersion.

8. A method as claimed in any one of the preceding claims in which the coating is dried by heating, in a baking oven or otherwise.

9. A method as claimed in any one of the preceding claims comprising adding a concentric layer of insulating material without ferromagnetic loading.

10. A coated wire made by the method claimed in any one of the preceding

Claims

claims.

11. A method of coating a wire substantially conforming with Example 1.

12. A method of coating a wire substantially conforming with Example 2.

13. A method of making a temperature-responsive cable comprising coating a wire by the method claimed in any one of claims 1--9 and adding a second conductor coaxial with it, lapped round it, twisted with it, or laid parallel to it.

14. A method of making a temperature-responsive cable comprising coating two wires by the method claimed in any one of claims 1--9 and lapping one of them round the other, or twisting them together, or laying them parallel to one another.

15. A method as claimed in claim 13 or claim 14 comprising the further step of applying a protective sheath.

16. A method of making a temperature-responsive cable substantially conforming with Example 1.

17. A method of making a temperature-responsive cable substantially conforming with Example 2.

18. A temperature-responsive cable made by the method claimed in any one of claims 13-15.

19. A wire or cable made substantially as described in Example 1.

20. A wire or cable made substantially as described in Example 2.