GB2195816A

GB2195816A - Manufacture of a mineral insulated cable

Info

Publication number: GB2195816A
Application number: GB08622527A
Authority: GB
Inventors: Murray Bolser
Original assignee: Haiste Mechanical & Electrical
Current assignee: Haiste Mechanical & Electrical
Priority date: 1986-09-18
Filing date: 1986-09-18
Publication date: 1988-04-13
Also published as: GB8622527D0

Abstract

A mineral insulated sheathed cable is made by assembling a number of cores within a sheath and introducing an insulating material in between the cores, and coating at least one of the cores with a ductile metal or metal alloy, for example, copper if the core is of aluminium. The assembly is then heated and drawn out into a cable, the heating causing the coating to oxidise which results in a colour change, thus rendering the coated core or cores readily identifiable. The coating may be formed by electroplating or hot dipping, and may be of Sn, Al, Zn, Ni or Cr.

Description

SPECIFICATION Improvements in and relating to cable manufacture The present invention relates to an improved method of manufacturing a cable, and particularly, though not exclusively, to an improved method of manufacturing a mineral insulated sheathed cable.

Mineral insulated sheathed cables are conventionally manufactured by drawing down, through rollers and dies, a billet consisting of a copper tube containing mineral insulation and copper rods. The billet, which is originally about 3 metres long and 90 millimetres in diameter, is progressively drawn down through annealing ovens to a minimum of 5 millimetres diameter by about 1000 metres long. This represents a drawing down ratio of approximately 300 to 1. The sheath and/or the cores may alternatively be made of aluminium.

Recent advances in the manufacture of mineral insulated sheathed cables have resulted in a method involving a much lower drawing down ratio, comprising rolling a 70 millimetre wide copper strip through dies to form it into a tube which is closed by continuous argon arc welding joining the strip edges together.

At the same time as the tube is formed, a number of copper cable cores are fed into the tube through guide tubes, and a magnesium oxide powered insulant is vibrated into place.

The composite cable starts off at approximatetly 22 millimetres diameter and is drawn down down to about 5.1 millimetres through a series of dies and various annealing oyens which are heated to temperatures in excess of 600 degrees C. This represents a drawing down ratio of approximately 20 to 1.

One of the problems with mineral insulated sheathed cable manufacture is that no satisfactory method exists of marking one or more of the cable cores whereby the marked core may be identified when the finished cable is being installed. In practice, it is only necessary to mark one of the cores, as the other cores always follow round in the same sequence.

According to the present invention there is provided a method of manufactuing a mineral insulated sheathed cable, the method including assembling a plurality of cores within a sheath and introducing an insulating material between said cores, and subsequently heating the assembly and forming it into a cable so that its diameter is reduced and its length is increased, the method further including coating at least one of said cores, prior to heating and forming, with a ductile material which will, in the finished cable, render said coated core identifiable.

Both the cores and the sheath may be made of copper, or alternatively the cores and/or the sheath may be made of aluminium.

Preferably, the ductile material should not contaminate the insulating material which is typically a metal oxide.

Preferably, a ductile material will be able to withstand the mechanical stresses during formation of the cable.

Preferably, the ductile material forms a bond with the core.

Preferably, the ductile material is a metal.

An embodiment of the present invention will now be described, by way of example only, in order that the present invention may be better understood.

A mineral insulated copper (or aluminium) sheathed cable is manufactured by asssembling together a number of copper or aluminium cable cores within a sheath of copper, or aluminium, an insulating material such as magnesium oxide powder separating the cores within the sheath. One of the cores is coated with a ductile metal or metal alloy, examples of which are tin, aluminium, zinc, nickel, or chromium. If the cores are made of aluminium, then copper may be used as a coating. This coating may be achieved either by electroplating or hot dipping, either of which leaves a thin film of the material on the core.

The assembly is then drawn down through a series of annealing ovens heated to temperatures in excess of 600 degrees C, and is formed into a cable by means of rollers and dies.

The coated core will, in the finished cable, be easily identified as a result of its appearance which is different to that of the other cores. The difference may be in the colour, and certain of the metals mentioned above oxidise at temperatures about 600 degrees C, the resulting darkening making identification easier.

It should be appreciated that any suitable material may be used to coat the core, provided that it renders the coated core distinguishable from the remaining uncoated cores in the final cable.

1. A method of manufacturing a mineral insulated sheathed cable, the method including the steps of: assembling a plurality of cores within a sheath and introducing an insulating material between said cores; heating the assembly and forming it into a cable whereby its diameter is reduced and its length is increased; and coating at least one of said cores, prior to said heating and forming, with a ductile material which will, in the finished cable, render the or each coated core identifiable.

2. A method according to Claim 1 in which the or each core is coated with a ductile metal.

3. A method according to Claim 1 or Claim 2 in which the or each core is coated with a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in and relating to cable manufacture The present invention relates to an improved method of manufacturing a cable, and particularly, though not exclusively, to an improved method of manufacturing a mineral insulated sheathed cable. Mineral insulated sheathed cables are conventionally manufactured by drawing down, through rollers and dies, a billet consisting of a copper tube containing mineral insulation and copper rods. The billet, which is originally about 3 metres long and 90 millimetres in diameter, is progressively drawn down through annealing ovens to a minimum of 5 millimetres diameter by about 1000 metres long. This represents a drawing down ratio of approximately 300 to 1. The sheath and/or the cores may alternatively be made of aluminium. Recent advances in the manufacture of mineral insulated sheathed cables have resulted in a method involving a much lower drawing down ratio, comprising rolling a 70 millimetre wide copper strip through dies to form it into a tube which is closed by continuous argon arc welding joining the strip edges together. At the same time as the tube is formed, a number of copper cable cores are fed into the tube through guide tubes, and a magnesium oxide powered insulant is vibrated into place. The composite cable starts off at approximatetly 22 millimetres diameter and is drawn down down to about 5.1 millimetres through a series of dies and various annealing oyens which are heated to temperatures in excess of 600 degrees C. This represents a drawing down ratio of approximately 20 to 1. One of the problems with mineral insulated sheathed cable manufacture is that no satisfactory method exists of marking one or more of the cable cores whereby the marked core may be identified when the finished cable is being installed. In practice, it is only necessary to mark one of the cores, as the other cores always follow round in the same sequence. According to the present invention there is provided a method of manufactuing a mineral insulated sheathed cable, the method including assembling a plurality of cores within a sheath and introducing an insulating material between said cores, and subsequently heating the assembly and forming it into a cable so that its diameter is reduced and its length is increased, the method further including coating at least one of said cores, prior to heating and forming, with a ductile material which will, in the finished cable, render said coated core identifiable. Both the cores and the sheath may be made of copper, or alternatively the cores and/or the sheath may be made of aluminium. Preferably, the ductile material should not contaminate the insulating material which is typically a metal oxide. Preferably, a ductile material will be able to withstand the mechanical stresses during formation of the cable. Preferably, the ductile material forms a bond with the core. Preferably, the ductile material is a metal. An embodiment of the present invention will now be described, by way of example only, in order that the present invention may be better understood. A mineral insulated copper (or aluminium) sheathed cable is manufactured by asssembling together a number of copper or aluminium cable cores within a sheath of copper, or aluminium, an insulating material such as magnesium oxide powder separating the cores within the sheath. One of the cores is coated with a ductile metal or metal alloy, examples of which are tin, aluminium, zinc, nickel, or chromium. If the cores are made of aluminium, then copper may be used as a coating. This coating may be achieved either by electroplating or hot dipping, either of which leaves a thin film of the material on the core. The assembly is then drawn down through a series of annealing ovens heated to temperatures in excess of 600 degrees C, and is formed into a cable by means of rollers and dies. The coated core will, in the finished cable, be easily identified as a result of its appearance which is different to that of the other cores. The difference may be in the colour, and certain of the metals mentioned above oxidise at temperatures about 600 degrees C, the resulting darkening making identification easier. It should be appreciated that any suitable material may be used to coat the core, provided that it renders the coated core distinguishable from the remaining uncoated cores in the final cable. CLAIMS

3. A method according to Claim 1 or Claim 2 in which the or each core is coated with a ductile metal alloy.

4. A method according to any of the preceding Claims in which the ductile material used to coat the or each core forms a bond with the core or cores.

5. A method according to any of the preceding Claims in which the coating is achieved by electroplating.

6. A method according to any of Claims 1 to 4 in which the coating is achieved by hot dipping.

7. A method according to any of the preceding Claims in which the coating is oxidised during the heating, said oxidation resulting in a colour change which renders the coated core or cores identifiable.

8. A mineral insulated sheathed cable formed from an assembly of cores assembled within a sheath and separated by an insulating material, in which at least one of the cores is coated, prior to said forming, with a ductile material which renders said coated core or cores identifiable in the finished cable.

9. A method according to Claim 1 and substantially as herein described.

10. A mineral insulated sheathed cable according to Claim 8 and substantially as herein described.