GB2106701A - Mineral insulated electric cable - Google Patents

Abstract

In a mineral insulated electric cable there is dispersed within the powdered insulating material a proportion of powdered hydrophobic material, for example methyl polysiloxane, sufficient to reduce moisture penetration. The cable is formed by bending a copper strip 1a into tubular form, welding the edges together at 4, cooling the sheath at 9, and introducing the powder mixture through a hopper 8 and delivery tube 7, the tube having an outlet downstream of the cooling means. <IMAGE>

Description

SPECIFICATION Mineral insulated electric cables This invention relates to mineral insulated electric cables, that is to say cables of the type comprising one or more electrical conductor wires enclosed within a tubular metal sheath, commonly of copper, and insulated from the sheath by a filling of compacted powdered insulating material.

It will be understood that the term "mineral insulated electric cables" is to be understood to include, in addition to wiring cables for the conduction of electric current for general purposes, cables of the construction described above and employed for other purposes, for example heating cables and thermocouple cables. The invention also includes within its scope the manufacture of such cables.

The powdered insulating material which is most commonly used is magnesium oxide either fused or calcined or as sea-washed magnesia, and although magnesium oxide has a high electrical breakdown strength when dry the presence of even a small amount of moisture can reduce this significantly.

Consequently the ingress of moisture is a major problem, particularly in the case of high voltage cables, and it is therefore recommended that the ends of lengths of cable in store be provided with temporary seals to reduce moisture penetration.

Nevertheless prior to forming a termination in a length of stored cable required for use it is invariably necessary to cut off an appreciable length, in some cases as much as eighteen inches, from the ends of which the cable in order to ensure that any damp powder, which would effect the insulating properties of the cable, is removed. This is clearly wasteful.

Moreover the formed termination needs to provide an effective seal to prevent any subsequent moisture penetration in use.

According, therefore, to the present invention a mineral insulated electric cable has dispersed within the powdered insulating material a proportion of a powdered hydrophobic material sufficient to reduce moisture penetration.

The hydrophobic material should, of course, also have electrically insulating properties such that it does not have a deleterious effect on the dry powder filling.

We have found powdered methyl polysiloxane to be a highly efficient hydrophobic material suitable for the purpose, the amount introduced preferably lying within the range 0.1 to 5.0% by weight of the powdered insulating material. Thus a cable having a filling comprising powdered magnesium oxide incorporating only 1% by weight of methyl polysiloxane does not exhibit any significant degree of moisture penetration even after many weeks storage, without the need for any separate end seals.

Accordingly a cable in accordance with the invention can be stored without temporary end seals, and when required to be used the end does not require to be cut back further than is necessary to physically form the required terminations, as there is no damp powder which needs to be removed.

Moreover not only does the incorporation of methyl polysiloxane within the filling have no detrimental effect on the electrical insulating properties of the filling, it has in fact, been found to improve the electrical breakdown strength of the filling, particularly at high voitages, due, it is believed, to the exclusion of free moisture throughout the length of the cable.

Preferably the methyl polysiloxane powder has a grain size of between 20 and 200 microns, and lies mainly in the range 30 to 70 microns.

If methyl polysiloxane is incorporated in the filling material care should be taken that the temperature of the filling material does not rise above 65"C as it is being introduced into the sheath, as it becomes tacky or liquid at higher temperatures, which could adversely affect the filling process. Where the sheath is continuously formed from a ductile metal strip by bending the strip into tubular form and welding the edges together, whilst simultaneously introducing the filling material and the conductor wire or wires into the sheath tube so formed, means will accordingly need to be provided for cooling the sheath tube immediately after the weld to prevent an undue amount of heat being transferred from the sheath tube to the filling material as the latter is being fed into the tube.Following the introduction of the conductor or conductors and the filling material the diameter of the sheath tube will usually be reduced by passing it through a series of reduction rollers or dies and annealing furnaces in known manner.

Conveniently the powder filling is introduced into the sheath tube through a delivery tube having its outlet downstream of the weld, and in such case means may also be provided for cooling the delivery tube, particularly in the region of the weld, and possibly also the conductor wire or wires before the introduction of the latter into the sheath tube.

Conveniently a guide tube for the, or one of the, conductor wires extends adjacent to the path of travel of the meeting edges of the bent metal strip, and at the welding position the guide tube is formed with an aperture, so that some of the heat from the weld is transferred to the conductor and is carried away from the powder delivery tube by the moving conductor.

The use of methyl polysiloxane has the unexpected advantage, however, that its conversion to liquid on being heated, during subsequent reducing and annealing processes, causes it to act as a lubricant, and this results in a pronounced reduction in the degree of abrasion of the conductor wire or wires and of the inner surface of the sheath.

Consequently the pronounced adherence of the filling powder to the conductor wire or wires, as is commonly experienced with mineral insulated electric cables as manufactured hitherto, is virtually avoided, and any loose powder on the surfaces of the wire or wires or on the inner surface of the sheath tube can be removed without difficulty when forming a termination.

However other aryl or alkylpolysiloxanes or mixtures thereof or any other suitable hydrophlic, electrically insulating material might alternatively be used as an additive to the filling powder in a mineral insulated electric cable in accordance with the invention.

One mineral insulated electric cable and the manufacture thereof will now be described by way of example with reference to Figures 1 to 4 of the accompanying schematic drawings, in which Figure 1 represents a transverse section through the cable, Figure2 illustrates diagrammatically a sectional elevation of part of one form of apparatus for use in manufacturing the cable, and Figures 3 and 4 represent on an enlarged scale an elevation and a plan section of a portion of the apparatus illustrated in Figure 2.

Referring first to Figure 1 the cable comprises an outer sheath 1 formed from a copper strip bent into tubular form and argon arc welded along the abutting edges. The sheath contains a plurality of conductor wires 2 (in this case two) separated from each other and from the sheath 1 by powdered magnesium oxide 3, the powder being compacted around the conductors, following the introduction of the powder and conductors into the formed sheath, by a series of reduction stages, each followed by an annealing and quenching stage in known manner. In accordance with the invention the magnesium oxide powder contains, dispersed within it, approximately 1% by weight of powdered methyl polysiloxane having a grain size of between thirty and seventy microns.The methyl polysiloxane even in this small proportion has been found to impart a hydrophobic quality to the filling which resists the penetration of moisture, and prevents any significant deterioration of the insulating properties of the filling adjacent severed ends of the cable for long periods without the need to provide additional seals, either during storage or when forming subsequent terminations.

Consequently when forming a termination it is not necessary to cut back the end of the cable further than is necessary to physically form the termination.

The cable may be manufactured by a process generally similar to that described in patent application No. 8003466, and as illustrated in part in Figures 2to4.

In such a process the cable sheath 1 is formed in a continuous mannerfrom a thoroughly degreased copper strip la, by means of a tube forming machine (not shown) which bends the downwardly fed strip into tubularform, and an argon are welding head 4 which welds the abutting edges of the strip. The formed sheath tube 1 is fed vertically downwards to a reduction machine, which reduces the diameter of the tube and compacts the filling powder 3 around the conductor wires 2. The reduced tube is then fed through an annealing furnace, and then through a water quenching tank in which the cable is turned in a catenary curve to continue travelling horizontally through further reduction machines, annealing furnaces and quenching tanks. The reduction machines and associated equipment have, however, been omitted from the drawing for simplicity.

The conductor wires 2, which are also throughly degreased, are continuously fed into the sheath tube 1 as it is being formed through a pair of guide tubes 5, 6 rigidly located in desired positions within a powder delivery tube 7 through which the filling powder, consisting of magnesium oxide with the methyl polysiloxane additive dispersed within it are introduced into the sheath tube 1. The powder filling is introduced into the delivery tu be 7 from a hopper 8 kept replenished from a vibratory conveyor supplied, in turn, from a powder reservoir.

The lower ends of the powder delivery tube 7 and of the guide tubes 5, 6 terminate below the weld position so that the filling powder is effectively introduced into the formed and already welded tube, and is thereby prevented from contaminating the weld. Surrounding the sheath 1, just below the weld position and above the lower end of the powder delivery tube 7, is a jacket 9 through which a cooling fluid, for example water, is passed to reduce the amount of heat which is transmitted to the powder filling by the sheath. In addition the wall of the powder delivery tube 7 adjacent the weld position is formed with a slot 10 as is the adjacent guide tube 5.

The slots 10 and 11 in the delivery tube 7 and guide tube 5 respectively are in alignment, and their surrounding edges are soldered together to prevent escape of the powdered filling at that position. The one of the conductor wires 2 which is within the guide tube 5 is thus exposed to the weld, and accordingly acts as a heat sink to carry away heat from the weld region, as described in more detail in co-pending Patent Application No. 8003466.

Cooling is necessary when forming a cable by the process described as it is essential to maintain the temperature of the powder filling below 65"C until it leaves the delivery tube 7, because above this temperature the methyl polysiloxane becomes tacky and eventually liquefies as its temperature is further increased, and this could interfere with the delivery of the powder or even clog the outlet from the delivery tube completely.

For this reason the filling powder is preferably maintained at a suitably low temperature, for example less than 50"C, priorto being introduced into the delivery tube 7, and in addition the conductor wires 2 are also cooled, for example by argon at low temperature, before their introduction into the respective guide tubes 5, 6. The powder delivery tube 7 may, if necessary, also be provided, as shown, with spaced double walls 12, 13 the space between which is separated, by longitudinal partitions 14 into two flow paths 15, 16 communicating at the lower end of the tube 7. In operation a cooling fluid, possibly water or freon or possibly argon at a suitably low temperature is passed down one flow path, e.g. 15 and returns via the otherflow path 16.

After delivery into the formed sheath tube 7, heating of the powder filling will not give rise to any difficulties, and it has, in fact, been found that the liquefaction of the methyl polysiloxane during the subsequent reduction and annealing stages has a beneficial, rather than a detrimental, effect. It is believed that the liquid methyl polysiloxane, which reverts again to its solid state on cooling below 65"C, acts as a lubricant which prevents abrasion of the surfaces of the conductor wires 2 and the internal surface of the sheath tube 1 during the reduction stages, as inspection of a compieted cable shows that these surfaces remain smooth, and adhesion of the powder to the surfaces, as is commonly experienced with mineral insulated electric cables formed by conventional processes, is virtually absent, thus facilitating the formation of subsequent termina- tions.

Claims (10)

1. A mineral insulated electric cable having dispersed within the powdered insulating material a proportion of a powdered hydrophobic material sufficient to reduce moisture penetration.

2. A mineral insulated electric cable according to Claim 1 wherein the hydrophobic material comprises powdered methyl polysiloxane.

3. A mineral insulated electric cable according to Claim 2 wherein the powdered insulating material consists of powdered magnesium oxide, and has dispersed within it between 0.1 and 5.0% by weight of powdered methyl polysiloxane.

4. A mineral insulated electric cable according to Claim 3 wherein the methyl polysiloxane powder has a grain size of between 20 and 200 microns, and lies mainly in the range 30 to 70 microns.

5. The manufacture of a mineral insulated electric cable according to Claim 1 in which the sheath is formed continuously from a ductile metal strip by bending the strip into tubular form and welding the edges together, whilst simultaneously introducing the conductor wire orwires and a filling comprising powdered magnesium oxide with a proportion of powdered methyl polysiloxane dispersed within it, into the sheath so formed, wherein the powdered filling is introduced into the sheath tube through a delivery tube having its outlet downstream of the weld, and wherein the delivery tube and sheath tube are cooled in the region of and immediately following the weld respectively to maintain the powdered filling within the delivery tube at a temperature of not more than 65"C.

6. The manufacture of a mineral insulated electric cable according to Claim 5 wherein a guide tube for the, or one of the, conductor wires extends adjacent to the path of travel of the meeting edges of the bent metal strip, and at the welding position the guide tube is formed with an aperture, so that some of the heat from the weld is transferred to the conductor and is carried away from the powder delivery tube by the moving conductor.

7. Apparatus for manufacturing mineral insulated electric cable according to Claim 1 incorporating means for feeding a ductile metal strip continuously in the direction of its length, means for bending it into tubular form and means for welding the edges together to form the sheath, means for continuously introducing the conductor wire or wires into the formed sheath, and a delivery tube having an outlet downstream of the weld position by which the powdered filling can be introduced into the formed sheath, the apparatus including also means for cooling the sheath between the weld position and the delivery tube outlet.

8. Apparatus according to Claim 7 wherein a guide tube for the, or one of the, conductor wires extends adjacent to the path of travel of the meeting edges of the bent metal strip, and at the welding position the guide tube is formed with an aperture, so that some of the heat from the weld is transferred to the conductor and is carried away from the powder delivery tube by the moving conductor.

9. Apparatus for manufacturing mineral insulated electric cable substantially as shown in and as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.

10. A method of manufacturing mineral insulated electrical cable carried out substantially as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.