Stock material or miscellaneous articles – Hollow or container type article – Shrinkable or shrunk
Patent
1992-04-15
1994-03-29
Robinson, Ellis P.
Stock material or miscellaneous articles
Hollow or container type article
Shrinkable or shrunk
428 364, 428 3691, 428372, 428379, 428383, 428398, 428399, 174211, 174DIG8, H01B 1732
Patent
active
052983013
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an electrical insulator, and in particular to an insulator formed from polymeric material.
Typically, insulators are formed from an elongate body of electrically insulating material such as porcelain, with or without the addition of an outer polymeric component, or from glass fibre covered by a polymeric component. Metal fittings are mounted at each end for connection to electrical equipment at elevated voltage (typically greater, and often much greater than 1 kV) and (usually) earth respectively. The outer surface may be shedded and/or convoluted, so as to prevent water flowing directly between the end fittings and also so as to extend the creepage path length.
In the case of a solid porcelain insulator, the sheds and/or convolutions can be provided integrally with the porcelain core. Alternatively, a cylindrical porcelain rod of uniform diameter may have a polymeric component of shedded and/or convoluted configuration mounted thereon. Due to the poor electrical and water uptake properties of glass fibre, when an insulator core is provided from such material an outer protective component is necessary, and this can conveniently be provided by a shedded and/or convoluted polymeric component.
Porcelain is a traditional insulator material, and is still preferred in some applications because of its superior resistance to damage by electrical discharges, to weathering, and to chemical attack. However, it is relatively heavy, and is a brittle material which can shatter on impact; in this respect, the convolutions or sheds are particularly vulnerable. Furthermore, porcelain has a high surface free energy, which makes it retentive to dirt. Its manufacturing process requires firing in a kiln, and this is not conducive to the easy manufacture of complex shapes. It is, however, not an expensive material to manufacture into an insulator.
Polymeric insulators in general are suitable for many applications, and are widely and successfully used, especially in view of their low weight, particularly in relation to porcelain or other ceramic materials, and their resistance to pollution, under most severe conditions, for example at higher voltages and in adverse operating conditions, particularly of heavy environmental pollution. Furthermore, polymeric materials will usually maintain their mechanical integrity if subjected to mechanical abuse, and are relatively easy to form into complex shapes.
One example of a polymeric insulator is disclosed in British Patent No. 1292276, and comprises a central support, which may be a glass fibre rod or tube, having a metal fitting at each end and an outer surface layer formed from a heat-shrinkable non-tracking insulating polymeric sleeve that extends the entire length of the support and overlaps each end fitting.
A further advantageous form of electrical insulator is disclosed in EP-B-0125884, which comprises an insulator that is a hybrid between a porcelain insulator and a polymeric insulator. This insulator combines the advantages of the structural strength of porcelain to form the insulator core, on the ends of which metal connection fittings are mounted, with the advantages of lightness, formability and mechanical (especially vandal) resistance of polymeric material to form an outer component. The outer component is spaced apart along the procelain core from the metal end fittings to avoid degradation of the polymer at such locations due to intense local electrical activity.
However, porcelain and hybrid insulators still suffer from the problems associated with the high density, and thus weight, of porcelain, and this disadvantage is also applicable to other ceramics such as glass. Insulator cores of fibreglass on the other hand are vulnerable to ingress of moisture which then, due to the glass fibres extending continuously from one end of the insulator to the other, wicks along the entire length of the insulator, forming a conductive path and destroying its operability. Furthermore, in applications involving telecommunication
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J. Polymer Science, 1983, No. 70 pp. 129-143.
Encyclopedia of Polymer Science and Engineering, vol. 14, Reinfored Plastics.
Midgley John
Thornley David W. M.
Burkard Herbert G.
Raychem Limited
Robinson Ellis P.
Williamson Michael A.
Zavell A. Stephen
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