EP0197227A1 - Electrical cable, especially for aerospatial use, with improved electrical characteristics - Google Patents

Abstract

The cable comprises a central conductor, a first layer of insulation and at least one second layer of insulation around the first, one of these layers being formed by a helically wrapped strip of polyimide synthetic resin and the various layers being bonded together by a thermal treatment. The cable is characterised in that the second layer is formed by an extruded layer of a non-crosslinked perfluoroalkoxy or polyether-ether-ketone thermoplastic resin. <IMAGE>

Description

The present invention relates to an electrical cable, in particular for aerospace use, with improved electrical characteristics, comprising a central conductor, a first layer of insulation, and at least a second layer of insulation around the first, one of these. ci being formed by a strip of polyimide synthetic resin wound helically, and the different layers being joined together by heat treatment. Such a cable is particularly suitable for aeronautical and space applications.

Cables of this type are already known, in which the insulation consists of one or more strips of polyimide synthetic resin, in particular of the quality sold under the brand "Kapton F" by the Société du Pont de Nemours, taped around the conductor central, with the various layers joined together by heating, then coated with a varnish resistant to a high temperature, for example a polyimide varnish.

Cables have also been used, the central conductor of which is surrounded by an extruded thermoplastic insulation, for example made of a copolymer of ethylene and tetrafluoroethylene, crosslinked or not.

Cables whose insulation consists of one or more strips of polyimide resin have insufficient resistance to the path of the electric arc during overvoltages or short-circuits. In addition, they lack flexibility and must be stripped with perfectly calibrated tools.

Those whose insulation consists solely of a crosslinked or non-crosslinked thermoplastic resin are relatively bulky and heavy, and their resistance to thermal overloads and to high electrical overvoltages leaves something to be desired.

The object of the present invention is to provide an electric cable having excellent resistance to arc tracking, a sou satisfactory pliability and strippability, while being compact and light, and resistant to thermal overloads and high electrical overvoltages.

The electric cable according to the invention is characterized in that the second layer is formed by an extruded layer of a non-crosslinked thermoplastic resin perfluoroalkoxy or polyether-ether-ketone.

When the electric cable according to the invention has three superposed insulation layers, it is characterized in that the first insulation layer is formed by an extruded layer of a non-crosslinked thermoplastic resin perfluoroalkoxy or polyether-ether-ketone, in that the second layer of insulation is formed by a strip of synthetic polyimide resin wound helically around the first, and in that the third layer is formed by a varnish hardened by heat treatment of class resistant to a temperature of at least minus 150 ° C.

The thickness of the varnish is preferably from 10 to 100 microns.

The varnish is preferably a polyurethane varnish.

When the electrical cable has only two layers, the first insulation layer is formed by a strip of polyimide resin wound helically around the conductor, and its second insulation layer is formed by an extruded layer of a non-crosslinked thermoplastic resin perfluoroalkoxy or polyether-ether-ketone, devoid of protective varnish.

The invention further extends to an electric cable comprising a central conductor, a first layer of insulation, and at least a second layer of insulation around the first, characterized in that one of these layers is formed. by a strip of polyazole or polyparabane synthetic resin wound helically, and the other is formed by an extruded layer of a non-crosslinked thermoplastic resin perfluoroalkoxy or polyether-ether-ketone.

For a diameter of the central conductor ranging from 0.3 mm to 10 mm, the thickness of each of the cable insulation layers of the invention is preferably between 0.04 mm and 0.35 mm.

It is described below, by way of examples and with reference to the figures of the appended drawing, structures of electric cable for aeronautical use according to the invention and a comparative test of resistance to the path of the electric arc of a cable known and a cable according to the invention.

• FIG. 1 represents in cross section a cable comprising a layer of insulating material in thermoplastic resin with a softening point at least equal to 250 ° C. between the central conductor and a layer of insulating tape in polyimide resin, coated on the outside with a varnish .
• FIG. 2 represents in cross section a cable comprising a layer of insulation in thermoplastic resin around the layer formed by a strip of polyimide resin banded around the central conductor.
• FIG. 3 schematically represents in perspective an apparatus for testing resistance to arc tracking.

Although FIGS. 1 and 2 relate to cables comprising a layer of thermoplastic resin in addition to the layer of polyimide resin, better results are already obtained than with known cables by providing a layer of a thermosetting synthetic resin, in particular polytetrafluoroethylene, either between the conductor and the polyimide resin tape, or above it.

In FIG. 1, the cable comprises a central conductor 1, a layer 2 of thermoplastic resin such as a perfluoroalkoxy resin, a polyether-ether-ketone resin, or a copolymer resin of ethylene and tetrafluoroethylene, which may or may not be crosslinked , for example by irradiation, a layer 3 formed by a ribbon band of polyimide resin, in particular that sold by the company of Pont de Nemours under the brand "Kapton F", and finally a layer 4 of varnish resistant to at least 150 ° C, for example a fluorocarbon varnish, a polyimide varnish, a polyamidimide varnish or a polyurethane varnish, etc.

The central conductor 1 can be made of copper or copper alloy, protected by a coating of tin, silver or nickel, or else of aluminum or aluminum alloy, protected or not by a metallic coating, in particular of tin or nickel. It generally consists of several twisted strands. Its diameter is 1 mm.

The layer of thermoplastic resin 2, with a softening point at least equal to 250 ° C, and preferably at least equal to 300 ° C, obtained by extrusion around the central conductor, has a radial thickness of 0.07 mm. Layer 3 formed by the strip of polyimide resin tape "Kapton F" has a thickness of 0.06 mm. In order to allow the layers of the strip to be bonded together, it is provided with a thermoplastic coating of copolymer of ethylene and fluorinated propylene. This bonding is achieved by heating to at least 275 ° C for 15 seconds to 3 minutes. The varnish 4 for coating the layer 3 has a thickness of 0.02 mm. It is for example a polyurethane varnish, formed of several successive layers applied by dipping, each coating pass being followed by a drying and baking operation at at least 250 ° C for 15 seconds to 3 minutes.

In FIG. 2, the cable comprises a central conductor 11, a layer 13 formed by a polyimide strip "KAPTON F" with layers joined together by heat treatment, and a layer 12 of a thermoplastic resin, deposited by extrusion, without coating of a varnish.

The layer 13 of polyimide resin has the same thickness as that described with reference to FIG. 1, and the bonding of its layers is ensured in the same way. The conductor, when it is made up of several tinned copper strands, will preferably be manufactured by the process which was the subject of patent application FR-A-2472253 of the applicant, so as to avoid soldering of its strands between them, while allowing the layers of the polyimide resin strip to be bonded together.

The thermoplastic resin of layer 12 is for example a copolymer of ethylene and fluorinated propylene, a copolymer of ethylene and tetrafluoroethylene, a polyether-ether-ketone or a perfluoroalkoxy resin. Its thickness is the same as that of the layer of thermoplastic resin in FIG. 1.

The fluorinated ethylene-propylene copolymer acting as an adhesive for the "Kapton F" polyimide resin can optionally be replaced by another adhesive. Some of these, in particular those based on polyesters or on silicones, are compatible with the use as thermoplastic resin of the first layer of the cable shown in FIG. 1 of another polymer such as a copolymer of ethylene and of fluorinated propylene, a copolymer of ethylene and uncrosslinked tetrafluoroethylene, or polyvinylidene fluoride.

In the two cable structures which have just been described, the insulation layer other than that of polyimide resin ensures better resistance to the path of the electric arc, preventing any arcing with the adjacent cables of a bundle. , than the insulation of a known cable. This improvement is all the more significant as this other layer of insulation is thicker, and it is more marked when the resin of this layer is a thermoplastic resin.

The polyimide resin layer provides excellent thermal and mechanical resistance and good resistance to large overvoltages.

The combination of the two insulation layers makes it possible to give the cable a thickness, and consequently a weight, which is lower than for a cable with thermoplastic resin insulation, and also ensures a more smoke emission rate in the event of fire. low.

The arc path resistance test device represented in FIG. 3 comprises a cable element to be tested 21, with a length of 20 cm, notched in the middle by a slot 22 from 0.13 to 0.25 mm wide reaching the central conductor. This cable element is placed between two other adjacent cable elements 23, 24 which are identical, but not notched, arranged like it 0.25 mm above a flat aluminum plate 25, cleaned and pickled for remove all impurities and traces of oxide, and earth at 26. The two conductors of the elements 23, 24 are connected at their ends by conductors 27, 28, and connected on one side to the aluminum plate 25 and by it to the earth.

The cable element 21 to be tested is arranged in a circuit comprising a power supply 29, which can be either continuously at 28 volts or alternatively 400 Hz at 115 volts. The circuit further comprises a circuit breaker 30 tripping at 7.5 amps, a load resistor 31 and an ammeter 32.

Furthermore, a burette 33 disposed vertically from the slot 22 of the element to be tested is filled with a solution of sodium chloride at 3% by weight. Its tap 34 is adjusted so as to let the solution drop drop by drop on the slot.

The arc path resistance test, which is more severe than that defined in standard ASTM D 3638-77, and intended to correspond to the conditions of use of cables on board aircraft, is as follows.

One circulates in the circuit comprising the cable element to be tested, a current of one ampere, either under 28 volts continuous, or under 115 volts in alternating 400 Hz, by dropping the saline solution on its slit at the rate 2 drops per minute. The test is continued for 24 hours, even when the driver breaks, except in the event of circuit breaker operation. The phenomena and the appearance of the cable elements are observed at the end of the test. The surface propagation of the arc on the central cable element must not exceed 10 mm.

• - autour du même conducteur central de diamètre 1 mm, une première couche de résine perfluoroalcoxy d'épaisseur 0,07 mm,
• - autour de celle-ci, une seconde couche de bande de polyimide "Kapton F" rubanée à recouvrement et à couches collées, d'épaisseur 0,06 mm,
• - une couche de vernis polyuréthane d'épaisseur 0,02 mm. L'épaisseur totale d'isolant est donc également de 0,15 mm.

A comparative test was carried out on, on the one hand, a known cable element with a tinned copper central conductor of diameter 1 mm, and with insulation by strip of polyimide tape and covered with a varnish, with total insulation thickness 0.15 mm, on the other hand a cable element according to the invention, comprising

• - around the same central conductor with a diameter of 1 mm, a first layer of 0.07 mm thick perfluoroalkoxy resin,
• - around it, a second layer of "Kapton F" polyimide tape covered with overlap and with bonded layers, 0.06 mm thick,
• - a layer of polyurethane varnish 0.02 mm thick. The total thickness of the insulation is therefore also 0.15 mm.

The control cable and the cable of the invention are first subjected to the DC test at 28 volts.

For the control cable, the central conductor breaks after approximately 10 minutes. The deterioration of the insulation spreads to the adjacent cables. The test is stopped after 3 h by tripping of the circuit breaker. There is arcing with the other conductors, and propagation of the arc over approximately 50 mm.

For the cable of the invention, the central conductor breaks after approximately 20 hours. After 24 h, no propagation of the arc is observed, nor visible damage on the adjacent cables.

The control cable and the cable of the invention are then subjected to the test in alternating current 115 volts, 400 Hz.

For the control cable, the central conductor breaks after 5 minutes. The arc quickly propagates to the adjacent cables and trips the circuit breaker.

For the cable of the invention, the central conductor breaks after 5 h. After 24 hours, the arc has spread over 3 mm on the central cable. There is no propagation of the arc to the adjacent cables.

The cable according to the invention is more particularly suitable for uses in aviation and on spacecraft, but it is also advantageous in all applications where it is desired to have a small footprint, excellent mechanical and thermal resistance and great security against with regard to the possibility of fire due to short circuits or loss of insulation.

Claims (7)

1 / Electric cable, in particular for aerospace use, with improved electrical characteristics, comprising a central conductor, a first layer of insulation, and at least a second layer of insulation around the first, one of these being formed by a strip of polyimide synthetic resin (3, 13) helically wound, and the different layers being joined together by heat treatment, characterized in that the second layer is formed by an extruded layer (2, 12) of a non-crosslinked thermoplastic resin perfluoroalkoxy or polyether-ether-ketone. 2 / Electric cable, in particular for aerospace use, with improved electrical characteristics, comprising a central conductor and three superimposed layers of insulation, characterized in that the first layer of insulation is formed by an extruded layer of a non-crosslinked thermoplastic resin perfluoroalkoxy or polyether-ether-ketone, in that the second layer of insulation is formed by a strip of synthetic polyimide resin wound helically around the first, and in that the third layer is formed by a varnish hardened by heat treatment of class resistant to a temperature of at least 150 ° C. 3 / Electric cable according to claim 2, characterized in that the thickness of the varnish is between 10 and 100 microns. 4 / Electric cable according to claims 2 or 3, characterized in that the varnish is a polyurethane varnish. 5 / electrical cable according to claim 1, characterized in that its first insulation layer is formed by a strip of polyimide resin helically wound around the conductor, and its second insulation layer is formed by an extruded layer of resin non-crosslinked thermoplastic perfluoroalkoxy or polyether-ether-ketone, devoid of protective varnish. 6 / Electric cable, in particular for aerospace use, with improved electrical characteristics, comprising a central conductor, a first layer of insulation, and at least a second layer of insulation around the first, characterized in that one of these layers is formed by a strip of polyazole or polyparabanic synthetic resin wound helically, and in that the other is formed by a layer extruded from a non-crosslinked thermoplastic resin perfluoroalkoxy or polyether-ether-ketone. 7 / Electric cable according to one of claims 1 to 6, wherein the diameter of the central conductor ranges from 0.3 mm to 10 mm, characterized in that the thickness of each of the insulation layers ranges from 0.04 at 0.35 mm.

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