US1906968A - Single core electric cable - Google Patents

Description

MayZ, 1933. P.v. HUNTER E1- AL A1,906,968

SINGLE CORE ELECTRIC CABLE lFiled July 25, 1931 2 sheets-sheet 1 I May 2,1933. P v. HUNTER HAL 1,906,968

l SINGLE CORE ELECTRIC CABLE Filed July 23, 1931 2 Sheets-Sheet 2 Patented May 2, 1933 UNITED sTAjres :PHILIP VASSAR. HUNTER, 0F LONDON, JQHN FRANCIS WATSON, OF ABBEY WOOD, KENT, AND LESLIE GIDDENS BRAZIER, OF LONJDGN, ENGLAND, ASSIGNORS TO CAL LENDERS CABLE AND CONSTRUCTION COMPANY LMITm, OFLONDON, ENGLAND, A

BRITISH COMPANY SING-LE CORE ELETRIC CABLE Application led July 23, 1931, Serial No. 552,624, and in Great Britain Augustll, 1930 rlhis invention relates primarily to electric cables having insulation formed by a laminated or stratified material (for example, paper lapped on helically in a number of layers) impregnated with liquid or semi-liquid material (for instance oil or compound). It applies particularly to single core cables. In accordance with the invention al single core cable isbuilt up with the layers of insulating material in non-circular form when seen in section at right angles to the axis of the core. be conveniently obtained by the use of a noncircular conductor on to which the layers of insulating material can be directly applied. Alternatively a circular conductor can be used with longitudinally or helically disposed strips of insulating -material spaced apart on its surface so as to provide with it a non-circular base on which the layers of material may he wound. lin general it appears that the use of a non-circular conductor is preferable.

The object of the non-circular stratification of the insulation is the provision of a form of insulation in which increase of the space occupied by the compound between the lay.

ers of insulation and between the innermost layer and thefonductor can take place without imposing a severe tensile stress on the material of the layers, or alternatively or simultaneously the impregnating or enclosed liquid or semiliquid material may be allowed to expand with change of tempera-ture without being forced to make its way through the laminations in a generally radially outward direction. ln the usualcase of circular stratifcation the stressing or penetration of the layers, when the. impregnating and enclosed material expands with increase of temperature, and the corresponding diihculty of obtaining a return to the original conditions,

when the change of temperature is reversed appear to he inherent. To accommodate therri or mitigate their effects it is necessary to use insulating material for the layers which has a certain degree of permeability and to use an impregnating material which has a certain degree of fluidity so that penetration may take place. In the improved arrange- This non-circular stratification mayA ment of the present invention, by choosing an appropriate shape for the stratification, it is provided that a very considerable increase in the volume enclosed by each layer may take place with a comparatively small stretching of the material of the layer. Accordingly the danger of over-stressing does not arise and the necessity for penetration is removed. Apart, therefore, from the physical requirements of the initial impregnation, this permits of the choice of material for the layers and for the impregnation without regard to the obtaining of cyclic penetration under operating conditions. Accordingly a more advantageous choice of the materials can be made than under the conditions at present usual. The paper may, for instance,

'be given a treatment which raises its breakdownl voltage, although at the same time it renders it relatively impermeable and the range of permissible compounds may be extended. even down to' the relatively solid jellies.

llrom the preceding indication of the 0bject in view the choice of non-circular shape to be given to the layers is indicated. 0bviously a large variety of shapes are available. Those mentioned in the following description are only given by Way of example.

An appropriate and advantageous shape is that of an ellipse having a material difference between the major and minor axes. rhis shape is one in which it is a relatively easy matter to produce a core and to apply the layers of insulation. It is also one in which the electric stresses can readily be calculated.

@ther forms which may be employed are triangular or four-sided figures. In general 'the corners will naturally be rounded Od' and it may be advantageous to make the sides somewhat convex.

rllhe conductors will in general be made by stranding with the aid of a die or group of rolls to produce the desired shape. 'llhey can either be given a straight form or a spiral form. ln the latter case the conductor, formed of stranded wires, before being insulated is shaped into the non-circular section desired in such a manner that the non-circular configuration. rotates about the axis of the strand along its length. This characteristic shape is indicated-by Figure 6 of the drawings which shows such a conductor after the application of insulation. Conductors so formed will in this specification be referred to as pre-spiralled conductors or prespiralled strands. Such conductors can be made by known methods, hitherto employed 10 in the manufacture of conductors for multicore cables. a The insulated core will in general be enclosed in a sheath of lead or lead alloy which may be circular or of the same non-circular shape as the core. In the former case suitable filling pieces may be arranged round .the core to bring it up to the circular form.

With the non-circular sheath there are the same advantages of avoiding excessive stress in the sheath in that form as are obtained in the layers of insulatin material. sheath may be made a close t on the core with or without filling pieces as the case may be, or may be of such dimensions orv shape as will allow of a space between the core and the sheath at-some part or parts of its circumference. In the latter case the space is preferably partly filled with compound and partly with gas of an inert nat-ure. The presence of the gas limits the resulting pressure within the sheath due to the temperature variations occurring due to the working conditions.

The space between the/core and sheath is particularly advantageous where pre-spiralled strands are used. The pre-spiralling of the strand limits the forces exerted on and by it due to the longitudinal expansion of the copper as the temperature'rises. The clearance rovided between the core and sheath permits deformation of the core to take place under the iniuence of this longitudinal expansion without distending the sheath or exerting an abnormal pressure upon it. The re-spiralling of the strand causes the wires ormmg the strand to acquire a definite helical form and the effect of the longitudinal forces is to increase the diameter of the helix.

The features referred to in the foregoing paragraphs will be further described by the aid of the accompanying drawings and other features of the invention will be dealt with in the following description.

Figures 1-4 illustrate sectional views of sinlgle' core cables,

' igure 5 is a -sectional view of a detail,

drawn to a larger scale,

Figure 6 is a view in plan of a detail, also drawn to a larger scale, and y Fi `re 7 isa, perspective view of a prespira led conductor.

In Figure 1 the cable 1 has a conductor 2 which is elliptical in cross-section and insulated with impregnated paper 3. The latter The is lapped on to the conductorin the usual way and it naturally assumes the elliptical form of the conductor. A conductive layer 4 is wrapped round the insulation 3 and makes li ht Contact with the inner surface of a lea sheath 6. This sheath is in this case of the usual circular shape in cross-section so that s aces 7 are formed between the sheath and t e core. These spaces preferably are iilled partly with compound and partly with an inert gas as shown for the reasons already stated above. In some cases it may be Idesirable to place the gas' inside flexible containers having expansible walls..

The sheath 6 is covered w-ith suitable impregnated fibrous material 17.

In Figure 2 the cable h as an elliptical conductor and correspondingly shaped insulation as in the previous case but the sheath ts closely around the core and is also elliptical in cross-section.

In Figure 3 the construction is the same as that shown in Figure 1 except that the spaces between the sheath and core are shown filled up with loose paper 8.

In some cases, such as where the conductor is formed of pre-spiralled strands, it may be desirable to provide a sheath of such shape and dimensions that a small radial clearance is left between the sheath and the core. It

is usually necessary to provide that a definite potential be given to the conductive layer 4.

. This is effected conveniently by arranging that the conductive layer be maintained at the same potential as that of the sheath, which usualy will be at earth potential. For this purpose a metal tape having springy projections may be wound over the core. Referring to Fi re 4 of the drawings which illustrates su a construction, the metal tape is shown at 14 and the projections on the tape at 15. Details of one form of construction of the tape are shown in Figures 5 and 6. The clearance between the sheath 6 and the conductive layer 4, and the member 14 is shown drawn to a larger scale in Figure 4 for the sake of clearness. The radial clearance between the core and the sheath will be of the order of 30 thousandths of an inch (.75 millimetres), and the projections 15 will require to be, more than about twice this figure. The tape 14 may be wound over the core in such a manner that the projections 15 Vstretch inwardly towards the conductive layer 4. The projections thus extend across the clearance space and form a metallic connection between the conductive layer and the inner surface of the lead sheath 6. Compression of the rojections 15' by the outward movement o the surface of the core may take place without permanent deformation of the projections. The latter thus .serve as bridging members between the sheath and the conductive layer so that even if there is compoundin the when uncompressed, of noty space beneath the sheath, the conductive layer is maintained at the same potential as that of the sheath. The tape 14 may be 'applied to the core shown in either Figure 1, 2 or' 3. In lthe case of Figures 1 and 3, the radial clearance of thirty thousandths of an inch described above, refers to the radial clearance between the ends of the 'elliptical cores and that portion of the inner surface of the sheath in the neighbourhood of the ends of the cores. In some cases, as for ex- Y ample, the construction shown in Figure 1,

it is possible that the springy projections may have to take care ofa decrease in the length of the major axis of the elliptical core accompanying an increase in the length of the minor axis. Such a decrease, if present, would, however, be small.

The projections 15 may be produced by stamping up the centre part only of the tape 14, and when used as an additional wrapping on the outside of the ordinary conducting 4 layers 4, it can be applied as an open heliX and no attempt need be made to secure it in position over the whole of the surface of the core nor to ensure that it makes good contact at all points where it lies on its surface. It has simply to provide conductive bridges between the surface of the core and the inner surface of the sheath.

The springy projections may be formed in various ways. For example, a metallized fabric or paper tape ma be employed and the projections produced y threading in and out of the fabric a wire or thin strip of metal such as hard-drawn copper or bronzewhich is given a wavy shape with the crests of the waves projecting outwards so as to stand appreciably above thesurface of the tape when this has been tightly wrapped on the core or cores. In the constructions shown the conductors may `or may not be prespiralled.

What we claim as out invention and desire to secure by Letters Patent is 1. A single core electric cable comprising a conductor, llaminated and impregnated insulation around the conductor of noncircular form in dross-section, and a metal sheath enclosing the `core and of the same shape in cross-section as that of the insulation.

2. A single core electric cable comprising a conductor, laminated and impregnated insulation around the conductor of non-circular form in cross-section, a conductive layer around the insulation, a metal sheath enclosing the core and of such dimensions as` to provide a small but definite clearance between the core and sheath, a resilient conconductive layer is maintained at 'the same potential as that of the sheath.

3. A single core electric cable comprising a conductor, laminated and impregnated insulation of non-circular form in cross-section around the conductor and a protective covering for the insulation.

4. A single core electric cable comprising a conductor of non-circular form in crosssection, and laminated and impregnated insulation also of non-circular form in crosssection around the conductor, and a protective covering for the insulation.

5. A single core electric cable comprising a conductor of non-circular form in crosssection and of helical form, laminated and impregnated insulation also of non-circular lform in cross-section and of helical form,

and a protective covering for the insulation.

6. A single core electric cable comprising a conductor of circular crosssection, filling pieces of insulating material spaced around the conductor to produce a non-circular base, laminated. and impregnated insulation covering the said base and of non-circular form in cross-section, and a protective covering for the insulation.

7. A single core electric cable comprising a conductor, laminated and impregnated insulation of non-circular form in cross-section, around the conductor and a metal sheath of such shape and dlmensions as to provide a space between sheath and core to permit deformation of the core to take place without appreciable stretching of the sheath.

8. A single core electric cable comprising a conductor, laminated and impregnated insulation of non-circular form in cross-section, around the conductor, and a metal -sheath of circular cross-section, the space provided between the core and sheath permitting deformation of the core to take place without appreciable stretching of the sheath.

9. A single core electric cable comprising a conductor, laminated and impregnated insulation of non-circular form in cross-sec-l v' tion, around the conductor, the conductor and insulation being both of helical form,- and a metal sheath of such shape and dimensions as to provide a space between sheath and core to permit the insulation to change its shape and the diameter of the helix of the conductor to increase without appreciable stretching of the sheath.

In testimony whereof we atlix ourv signatures.

PHILIP VASSAR HUNTER; JOHN FRANCIS VATSON.'4 LESLIE GIDDENS BRAZIER.

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