Structural insulator

1. An electrically insulating cross-arm for a tower arranged to carry power conductors spaced from a body of said tower, the cross-arm comprising at least one first insulating structural member having a proximal end adapted for attachment to the tower body and a distal end adapted for attachment to a conductor support means,

wherein the first insulating structural member comprises a solid rigid spine extending along a long axis between the distal and proximal ends and wherein the spine has a cross sectional profile, normal to the long axis, having a centroid and wherein the second moment of area of the cross sectional profile about every axis normal to the long axis and dissecting the centroid has a value of A 2 /2π or more, where A is the area of the cross sectional profile, and

wherein the rigid spine comprises three rib-like protrusions extending along the axis, arranged to form at least two substantially concave faces therebetween and wherein the cross-sectional profile of the rigid spine is substantially T or Y shaped, the rib-like protrusions providing the arms of the T or Y shape.

2. The cross-arm of claim 1 wherein the ribs are arranged to provide two concave faces and one flat or convex face therebetween.

3. The cross-arm of claim 2 adapted for positioning in use such that the first insulating member is positioned with the concave faces directed upwards and outwards, and the flat or convex face directed downwards.

4. The cross-arm of claim 1 wherein the cross-sectional profile of the rigid spine of the first insulating structural member has a curved perimeter such that the curved perimeter has a radius of curvature at all points thereon whose magnitude is greater than 1% of the greatest width of the cross-sectional profile measured along all axes dissecting the centroid.

5. The cross-arm of claim 1 wherein the cross-arm further comprises a second insulating structural member having a proximal end adapted for attachment to said tower body at a base and a distal end adapted for attachment to the conductor support means, the second insulating structural member being adapted to resist at least tensile loading.

6. The cross-arm of claim 5 wherein the cross-arm in use comprises two first insulating structural members located below two second insulating structural members together arranged to form a pyramidal structure extending from a quadrilateral base at the tower body to an apex at the conductor support means.

7. The cross-arm of claim 6 wherein proximal conductive grading rings are provided at the proximal ends of the first and second insulating members.

8. The cross-arm of claim 6 wherein a distal conductive grading ring assembly is provided at the apex of the pyramidal structure.

9. The cross-arm of claim 8 wherein the distal conductive grading ring assembly comprises a distal conductive grading ring arranged to encircle the distal ends of the first and second insulating members and a conductive post in conductive contact with the distal grading ring, located centrally within the distal grading ring and extending into a space formed between the distal ends of the first and second insulating members.

10. The cross-arm of claim 1 wherein the insulating structural members are provided with sheds whereby a creepage distance along each insulating member exceeds its length.

11. The cross-arm of claim 10 wherein the sheds are shaped such that a creepage path length contributed by each shed is substantially the same around its entire perimeter.

12. A tower for high voltage power distribution comprising one or more cross-arms according to claim 1 .

13. An insulating structural member, for use with high voltage power networks, comprising a solid rigid spine extending along a long axis between the distal and proximal ends wherein the spine has a cross sectional profile, normal to the long axis, having a centroid, wherein the second moment of area of the cross sectional profile about every axis normal to the long axis and dissecting the centroid has a value of A 2 /2π or more, where A is the area of the cross sectional profile, and

wherein the rigid spine comprises three rib-like protrusions extending along the axis arranged to form at least two substantially concave faces therebetween and wherein the cross-sectional profile of the rigid spine is substantially T or Y shaped, the rib-like protrusions providing the arms of the T or Y shape.

14. The insulating structural member of claim 13 wherein the ribs are arranged to provide two concave faces and one flat or convex face therebetween.

15. The insulating structural member of claim 13 comprising one or more channels adapted to permit high voltage conductors to extend through the insulating structural member.

16. The insulating structural member of claim 15 wherein at least one of the rib-like protrusions comprises a channel suitable for carrying a high voltage conductor.

17. The insulating structural member of claim 13 wherein the cross-sectional profile of the rigid spine of the insulating structural member has a curved perimeter such that the curved perimeter has a radius of curvature at all points thereon whose magnitude is greater than 1% of the greatest width of the cross-sectional profile measured along all axes dissecting the centroid.

18. The insulating structural member of claim 13 wherein the insulating structural member is provided with sheds whereby a creepage distance along the insulating member exceeds its length, and

wherein the sheds are shaped such that a creepage path length contributed by each shed is substantially the same around its entire perimeter.