GB2247982A - Mineral insulated electric cables - Google Patents

Abstract

In a mineral insulated electric cable comprising three or more electric conductors at least two of the conductors have a transverse cross-sectional shape bounded by a curved line and at least one straight line. The conductors 1 are so disposed that the curved lines lie on an imaginary circle which is concentric with the sheath 9 and which is common to the conductors. Sector and segment shaped conductors in concentric layers are also disclosed (figures 2 to 4). The overall diameter, and hence the cost, of the cable is substantially less than the overall diameters and cost of comparable cables having conductors of the same cross-sectional area hitherto proposed and used. <IMAGE>

Description

MINERAL INSULATED ELECTRIC CABLES This invention relates to mineral insulated electric cables of the kind comprising at least three elongate electric conductors electrically insulated from one another and from a surrounding sheath of metal or metal alloy by compacted mineral insulating powder, usually but not necessarily magnesium oxide.

It is an object of the present invention to provide an improved mineral insulated electric cable of the aforesaid kind which is substantially less expensive than comparable mineral insulated cables hitherto proposed, sold and used.

In the improved mineral insulated electric cable according to the invention, the sheath is of substantially circular transverse cross-sectional shape and the elongate conductors or at least two of the elongate conductors each has a transverse cross-sectional shape bounded by a smoothly curved line and at least one substantially straight line radially inward of said smoothly curved line and said conductors are so disposed within the compacted mineral insulating powder that, at any position along the length of the cable, the smoothly curved lines in part bounding the cross-sectional shapes of the conductors at said position lie on an imaginary circle which is substantially concentric with the sheath and which is common to the conductors.

Where the improved mineral insulated electric cable has three elongate conductors disposed within the compacted mineral insulating powder, at any position along the length of the cable preferably each conductor has a transverse cross-sectional shape bounded b y said smoothly curved line and two substantially straight lines which extend inwardly from the ends of the smoothly curved line and meet at their inner ends.

Where the improved mineral insulated electric cable has four elongate conductors disposed within the compacted mineral insulating powder, at any position along the length of the cable preferably each conductor has a transverse cross-sectional shape bounded by said smoothly curved line and two substantially straight lines which extend inwardly from the ends of the smoothly curved line and meet at their inner ends; alternatively, one conductor may be of substantially circular transverse cross-section and be disposed co-axially with respect to the circular sheath and each of the other three conductors may be disposed between the circular shaped conductor and the sheath and at any position along the length of the cable may have a transverse cross-sectional shape bounded by two radially spaced smoothly curved lines joined at their ends by substantially straight lines inclined at an acute angle to one another, the smoothly curved lines lying on two imaginary circles which are substantially concentric with the sheath and which are common to said three conductors.

Where the improved mineral insulated electric cable has five or more elongate conductors disposed within the compacted mineral insulating powder, in one preferred embodiment at any position along the length of the cable one conductor is of substantially circular transverse cross-section and is disposed co-axially with respect to the circular sheath each of the other four or more conductors has a transverse cross-sectional shape bounded by two radially spaced smoothly curved lines joined at their ends by substantially straight lines inclined at an acute angle to one another and the other four or more conductors are disposed between the circular shaped conductor and the sheath in one or more than one layer, the smoothly curved lines of the conductors in the or each layer lying on two imaginary circles which are substantially concentric with the sheath and which are common to said conductors; in a modification of said one preferred embodiment, the central conductor of substantially circular cross-section is replaced either by two conductors each of which has at any position along the length of the cable a transverse cross-sectional shape approximating to a segment of a circle, or by at least three conductors each of which has at any position along the length of the cable a transverse cross-sectional shape bounded by a smoothly curved line and two substantially straight lines which extend inwardly from the ends of the smoothly curved line and meet at their inner ends the smoothly curved lines in part bounding the cross-sectional shapes of said conductors at said position lying on an imaginary circle which is substantially concentric with the sheath and which is common to said conductors.

In all cases, at any position along the length of the cable the corners at which lines bounding the cross-sectional shape of each conductor meet preferably are radiused or otherwise smoothly rounded to reduce risk of unacceptable voltage stress in these regions.

The improved multi-conductor mineral insulated electric cable of the present invention has the important advantage that the conductors of the cable are of such transverse cross-sectional shapes and are so disposed relative to one another within the compacted mineral insulating powder that, for any given cross-sectional area of each conductor, the overall diameter of the cable is substantially less than the overall diameters of comparable cables having conductors of the same cross-sectional areas hitherto proposed and used.As a consequence, using conductor rods and a sheath tube of predetermined lengths an improved mineral insulated cable can be manufactured of substantially greater length than the lengths of comparable cables hitherto manufactured from conductor rods and sheath tubes of the same predetermined lengths, which improved cable is substantially less expensive because a greater length of cable can be manufactured without any substantial increase in the quantities of materials employed.

The invention is further illustrated by a description, by way of example, of four preferred forms of mineral insulated electric cable with reference to the accompanying drawings, in which: Figure 1 is a transverse cross-sectional view of a mineral insulated electric cable which has four elongate conductors disposed within the compacted mineral insulating powder; Figure 2 is a transverse cross-sectional view of a mineral insulated electric cable which has five elongate conductors disposed within-the compacted mineral insulating powder; ; Figure 3 is a transverse cross-sectional view of a mineral insulated electric cable which has nine elongate conductors disposed within the compacted mineral insulated powder, and Figure 4 is a transverse cross-sectional view of a mineral insulated electric cable which has twenty-one elongate electric conductors disposed within the compacted mineral insulated powder.

Referring to Figure 1, the mineral insulated electric cable comprises four copper conductors 1 electrically insulated from one another and from a surrounding sheath 9 of copper by compacted magnesium oxide 8. At any position along the length of the cable, each conductor 1 has a transverse cross-sectional shape bounded by a smoothly curved line and two straight lines which extend inwardly from the ends of the smoothly curved line and meet at their inner ends, the smoothly curved lines in part bounding the cross-sectional shapes of the four conductors at said position lying on an imaginary circle which is concentric with the sheath 9 and which is common to the conductors.

At any position along the length of the cable, the corners at which lines bounding the cross-sectional shape of each conductor 1 meet are smoothly rounded to reduce risk of unacceptable voltage stress in these regions.

In the mineral insulated electric cable shown in Figure 2, five copper conductors 11, 13 are electrically insulated from one another and from a surrounding copper sheath 19 by compacted mineral insulated powder 18. One conductor 11 is of circular transverse cross-section and is disposed co-axially with respect to the sheath 19. At any position along the length of the cable, each of the other four conductors 13 has a transverse cross-sectional shape bounded by two radially spaced smoothly curved lines joined at their ends by straight lines inclined at an acute angle to one another and the four conductors are disposed between the circular shaped conductor 11 and the sheath 19 in a layer 12, the smoothly curved lines of the conductors 13 in the layer lying on two imaginary circles which are concentric with the sheath and which are common to the conductors.At any position along the length of the cable, the corners at which lines bounding the cross-sectional shape of each conductor 11, 13 meet are smoothly rounded to reduce risk of unacceptable voltage stress in these regions.

The mineral insulated electric cable shown in Figure 3 comprises nine copper conductors 21, 23 electrically insulated from one another and from a surrounding copper sheath 29 by compacted magnesium oxide 28. At any position along the length of the cable, each of three conductors 21 has a transverse cross-sectional shape bounded by a smoothly curved line and two straight lines which extend inwardly from the ends of the smoothly curved line and meet at their inner ends, the smoothly curved lines in part bounding the cross-sectional shapes of conductors 21 at said position lying on an imaginary circle which is concentric with the sheath 29 and which is common to the conductors.Six conductors 23 are disposed between the conductors 21 and the sheath 29 in a layer 22, at any position along the length of the cable each conductor 23 having a transverse cross-sectional shape bounded by two radially spaced smoothly curved lines joined at their ends by straight lines inclined at an acute angle to one another, the smoothly curved lines of conductors 23 in the layer 22 lying on two imaginary circles which are concentric with the sheath 29 and which are common to the conductors. At any position along the length of the cable, the corners at which lines bounding the cross-sectional shape'of each conductor 21, 23 meet are smoothly rounded to reduce risk of unacceptable voltage stress in these regions.

In the mineral insulated electric cable shown in Figure 4, twenty-one copper conductors 31, 33 and 35 are electrically insulated from one another and from a surrounding copper sheath 39 by compacted magnesium oxide 38. At the centre of the cable, three conductors 31, each of which has at any position along the length of the cable a transverse cross-sectional shape bounded by a smoothly curved line and two substantially straight lines which extend inwardly from the ends of the smoothly curved line and meet at their inner ends, are so disposed that the smoothly curved lines in part bounding the cross-sectional shapes of the conductors at said position lie on an imaginary circle which is concentric with the sheath 39 and which is common to the conductors.Six conductors 33 disposed in a layer 32 surround the conductors 31, each conductor 33 at any position along the length of the cable having a transverse cross-sectional shape bounded by two radially spaced smoothly curved lines joined at their ends by straight lines inclined at an acute angle to one another. The smoothly curved lines of conductors 33 in the layer 32 lie on two imaginary circles which are concentric with the sheath 39 and which are common to the conductors.

Disposed between the layer 32 of conductors 33 and the sheath 39 are twelve conductors 35 mutually spaced in a layer 34, each conductor 35 at any position along the length of the cable having a transverse cross-sectional shape bounded by two radially spaced smoothly curved lines jointed at their ends by straight lines inclined at an acute angle to one another. The smoothly curved lines of conductors 35 in the layer 34 lie on two imaginary circles which are concentric with the sheath 39 and which are common to the conductors. At any position along the length of the cable, the corners at which lines bounding the cross-sectional shape of each conductor 31, 33 and 35 meet are smoothly rounded to reduce risk of unacceptable voltage stress in these regions.

In the case of each of the four cables illustrated in Figures 1 to 4, the overall diameter of the cable is substantially less than the overall diameter of a comparable cable having conductors of the same cross-sectional areas hitherto proposed and used.

Claims (9)

Claims:

1. A mineral insulated electric cable comprising at least three elongate electric conductors electrically insulated from one another and from a surrounding sheath of metal or metal alloy by compacted mineral insulating powder, wherein the sheath is of substantially circular transverse cross-sectional shape and the elongate conductors or at least two of the elongate conductors each has a transverse cross-sectional shape bounded by a smoothly curved line and at least one substantially straight line radially inward of said smoothly curved line and said conductors are so disposed within the compacted mineral insulating powder that, at any position along the length of the cable, the smoothly curved lines in part bounding the cross-sectional shapes of the conductors at said position lie on an imaginary circle which is substantially concentric with the sheath and which is common to the conductors.

2. A mineral insulated electric cable as claimed in Claim 1 which has three elongate conductors disposed within the compacted mineral insulating powder, wherein at any position along the length of the cable each conductor has a transverse cross-sectional shape bounded by said smoothly curved line and two substantially straight lines which extend inwardly from the ends of the curved line and meet at their inner ends.

3. A mineral insulated electric cable as claimed in Claim 1 which has four elongate conductors disposed within the compacted mineral insulating powder1 wherein at any position along the length of the cable each conductor has a transverse cross-sectional shape bounded by said smoothly curved line and two substantially straight lines which extend inwardly from the ends of the smoothly curved line and meet at their inner ends.

4. A mineral insulated electric cable as claimed in Claim 1 which has four elongate conductors disposed within the compacted mineral insulating powder, wherein one conductor is of substantially circular transverse cross-section and is disposed coaxially with respect to the circular sheath and each of the other three conductors is disposed between the circular shaped conductor and the sheath and at any position along the length of the cable has a transverse cross-sectional shape bounded by two radially spaced smoothly curved lines joined at their ends by substantially straight lines inclined at an acute angle to one another, the smoothly curved lines lying on two imaginary circles which are substantially concentric with the sheath and which are common to said three conductors.

5. A mineral insulated electric cable comprising five or more elongate electric conductors electrically insulated from one another and from a surrounding sheath of metal or metal alloy by compacted mineral insulating powder, wherein the sheath is of substantially circular transverse cross-sectional shape and wherein, at any position along the length of the cable, one conductor is of substantially circular transverse cross-section and is disposed co-axially with respect to the circular sheath, each of the other four or more conductors has a transverse cross-sectional shape bounded by two radially spaced smoothly curved lines joined at their ends by substantially straight lines inclined at an acute angle to one another and the other four or more conductors are disposed between the circular shaped conductor and the sheath in one or more than one layer, the smoothly curved lines of the conductors in the or each layer lying on two imaginary circles which are substantially concentric with the sheath and which are common to said conductors.

6. A modification of the mineral insulated electric cable claimed in Claim 5, wherein the central conductor of substantially circular cross-section is replaced by two conductors each of which has at any position along the length of the cable a transverse cross-sectional shape approximating to a segment of a circle, the smoothly curved lines in part bounding said segmental shapes of said two conductors at said position lying on an imaginary circle which is substantially concentric with the sheath and which is common'to said conductors.

7.A modification of the mineral insulated electric cable claimed in Claim 5, wherein the central conductor of substantially circular cross-section is replaced by at least three conductors each of which has at any position along the length of the cable a transverse cross-sectional shape bounded by a smoothly curved line and two substantially straight lines which extend inwardly from the ends of the smoothly curved line and meet at their inner ends, the smoothly curved lines in part bounding the cross-sectional shapes of said at least three conductors at said position lying on an imaginary circle which is substantially concentric with the sheath and which is common to said conductors.

8. A mineral insulated electric cable as claimed in any one of the preceding Claims, wherein at any position along the length of the cable the corners at which lines bounding the cross-sectional shape of each conductor meet are radiused or otherwise smoothly rounded.

9. A mineral insulated electric cable substantially as hereinbefore described with reference to and as shown in any one of Figures 1, 2, 3 and 4 of the accompanying drawings.