GB2035666A - Electric cable resistant to high temperatures - Google Patents

Abstract

An electric cable resistant to high temperatures comprises one or more conductors insulated individually by a tape of ceramic material comprising alumina-silica, having high resistance to heat, the tapes being applied helically or longitudinally over the conductor, in one or more layers, after which the insulated conductor or conductors are wrapped with a metal tape of copper, aluminium, copper alloy or aluminium alloy, duly welded to form an airtight tube, which may subsequently be corrugated. The cable may include an external armouring of steel wires or tapes, preferably galvanized, for protection against mechanical damage.

Description

SPECIFICATION Electric cable resistant to high temperatures This invention relates to an electric cable with one or more conductors having insulation chosen to render the cable resistant to high temperatures.

As is generally known, wires and conductors commonly used for the conduction of electricity are normally insulated with organic material such as natural or synthetic rubber, thermoplastic compounds, paper, and a series of other materials, all of them however losing their insulating characteristics at relatively high temperatures of the order of some hundreds of degrees centigrade, consequently enabling the formation of short-circuits between adjacent conductors or between a conductor and earth.

In such case and precisely with such types of insulation an accidental electrical overload on a conductor may cause fire by the ignition of combustible material adjacent the cable. On the other hand, in the case of a fire of external origin the electrical system becomes inoperative once that fire reaches the cable or cables, owing to the composition of the insulation.

Electric cables are known which employ as insulation mineral substances which by possessing high softening and melting temperatures avoid the disadvantages indicated above. Such cables consist basically of an outer tube of copper or aluminium, through which pass a conductor or conductors also of copper or aluminium, insulated from one another and from the outer tube by means of a mineral substance. This substance being resistant to high temperatures, the cable operates reasonably well at those high temperatures.

Having consequently such characteristics, cables insulated with minerals find application in special situations, for example: places where the ambient temperature is high, such as boilers, furnaces, power stations; places where there exists a risk of fire, such as oil refineries, petrochemical plants, tanker ships; enclosures filled with people, such as theatres, cinemas, underground railways, clubs, airports; and fire alarm and safety systems in hospitals, day nurseries, and schools for example.

Mineral insulated cables are subdivided into two distinct groups.

In the first group, the individual conductors are covered helically or longitudinally with asbestos or glass fibre, in relatively slow operations limited by the handling difficulties of those materials. Subse quentlythe individually insulated conductors are combined together and collectively receive a further layer of asbestos or glass fibre and possibly an outer armouring of steel wires or tapes.

In the second group a start is made from a tube of copper, steel or aluminium, having a length varying from 10 to 12 meters, an outer diameter from 30 to 50 millimetres and a wall thickness of 3 to 5 millimetres, into which tube are put the conductors, inserted through holes in discs of mineral insulation, generally magnesium oxide, which also may be put in place in the form of powder separating the conductors from one another and from the tube.

Then the assembly comprising the tube, insulating material and conductors is drawn through dies in single-passes until reaching predetermined lengths, intermediate annealing being applied between draw passes. If the diameter of the cable is to be still smaller the drawing down is done in monobloc units.

The cables insulated with minerals and belonging to either of these groups, although not suffering the disadvantages of cables which are insulated with organic material, nevertheless suffer their own disadvantages arising out of the mineral insulating materials.

Thus, the cables of the first group, insulated with asbestos and glass fibre, and even though used for not very demanding thermal conditions, exhibit instability of behaviour at temperatures of the order of 400-500 C. Mineral chrysotiles which compose asbestos exhibit progressive embrittlement at tem- peratures of the order of 400-500"C. On the other hand glass fibres around 700-800 C exhibit a viscous phase, losing consistency as an insulation.

The cables of the second group, employing magnesium oxide as insulation, are already employed under more severe thermal conditions, but have against them a semi-skilled process of manufacture, requiring special manufacture and handling of the original tubes, besides the fact that the magnesium oxide must not absort moisture otherwise it loses its insulating characteristics, and must be kept at temperatures of the order of 1 50"C. Besides this, very sharp folding can bring about contact between the conductors or between the conductor and the tube, causing a short-circuit.

In accordance with this invention, there is provided an electric cable resistant to high temperatures, comprising one or more conductors insulated individually by a tape of ceramic material comprising alumina-silica. Electric cables in accordance with this invention are able to withstand high temperatures without any deterioration of the insulation.

Thus, in accordance with this invention, there is applied over each conductor a ceramic mixture of alumina and silica in the form of a tape, the resistance of which to heat is excellent, exhibiting a melting point of the order of 2,200 C and a point of initial softening of the order of 1,600 C.

By virtue of their mechanical characteristics such tapes may be applied to the conductor both in the longitudinal direction or helically and the tapes may be applied as one or several layers, in accordance with the design requirements of the cable. The application of these tapes is effected in machines conventionally employed in the electric cable industry in general.

After application of the tape over the conductor, a longitudinal metal tape is applied, which by means of a suitable device is closed around the conductor, its adjacent edges being welded by a continuouswelding equipment, thus forming an airtight tube.

The welding may be effected by electric arc in an inert atmosphere or by electrical resistance, by induction or any other suitable method.

In the case of electric cables containing several conductors, each conductor is insulated individually with a tape or tapes as described above, the insulated conductors, with or without reinforcement material applied helically over them, being subsequently conveniently combined in a continuous process by a longitudinal metal tape which is similarly closed over the conductors through the employment of a suitable device and with complementary welding of the metal tape along its edges in order to from the tube.

The cable thus formed, having one or more conductors insulated individually with tapes of ceramic fibres of alumina-silica and securely contained inside a metal tube obtained by welding of the edges of the longitudinal enveloping metal tape, may be corrugated, with the aim of conferring better flexibility, and may also receive an armouring of tapes of steel or other metal for mechanical protection, as is known in the field of electric cables.

Thus the electric cable then obtained has a temperature of operation dependent upon the material employed as the conductor, normally copper or aluminium, as well as dependent upon the material of the enveloping tube, but not limited by the thermal performance of the insulating material which by its inherent characteristics and by the process of manufacture does not compromise the performance of the cable which in turn, due to the process of manufacture, has practically no limit to its length.

One form of tape for use in the invention is made by melting a mixture of alurnina and silica and blowing fine fibres therefrom. The precise chemical constitution of the fibres is not known and some aluminium silicate may be present. The fibres are glass-like and are commercially available. They can be made into a felt web from which tape may be cut, by conventional procedures.

Claims (9)

1. An electric cable resistant to high temperatures, comprising one or more conductors insulated individually by a tape of ceramic material comprising alumina-silica.

2. An electric cable as claimed in claim 1, in which said tape is applied helically in continguous turns over the or each conductor in one or more layers.

3. An electric cable as claimed in claim 1, in which said tape is applied longitudinally over the or each conductor in one or more layers.

4. An electric cable as claimed in any preceding claim, in which the conductor or conductors insulated with said tape are enclosed within an airtight tube formed by a metal tape wrapped around the conductor or conductors and duly welded.

5. An electric cable as claimed in claim 4, in which said metal tape comprises copper or copper alloy.

6. An electric cable as claimed in claim 4, in which said metal tape comprises aluminium or aluminium alloy.

7. An electric cable as claimed in claim 4,5 or 6, in which the airtight tube is corrugated.

8. An electric cable as claimed in any preceding claim, comprising an armouring of steel wire or tape, optionally galvanized, for protection against mechanical damage.

9. An electric cable as claimed in claim 1 and substantially as herein described.