Electricity: conductors and insulators – Anti-inductive structures – Conductor transposition
Reexamination Certificate
1999-04-21
2001-06-05
Reichard, Dean A. (Department: 2831)
Electricity: conductors and insulators
Anti-inductive structures
Conductor transposition
C174S1060SC, C174S1200SC
Reexamination Certificate
active
06242692
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to electric power cables of the kind having at least one metallic conductor, usually a conductor screen of semiconductive material, and insulation all enclosed in an earth screen comprising wires or narrow tapes and an external sheath comprising polymeric material which will usually (but not necessarily) form the exposed surface of the cable.
In most environments, it is desirable, if not essential, that the sheath is more effectively watertight than can be achieved with polymeric material alone, and attempts have thus been made to follow the practice used in communications cables of including a metal/plastics laminate foil beneath the main body of the sheath, so as to become bonded to it, normally when the main polymeric body is extruded. It is difficult, with such structures, to design a sheath in which the laminate foil remains intact when the cable is subjected to the impact tests specified by Standards Authorities and customers, as the laminate tends to be driven into the gaps between the wires or tapes of the screen and to split along resulting crease lines.
In our prior application GB9620394.8 which is related to WO98/14960 and U.S. Ser. No. 09/230,565 cable characterised by a sheath construction comprising a first extruded polymeric layer which encloses the earth screen of the cable without filling the gaps between the components thereof (the wires or strips), a second extruded polymeric layer enclosing the first and between them and bonded to both of them a moisture barrier laminate comprising a metal foil layer coated on each of its major faces with a polymeric material compatible with the material of the extruded layer it contacts.
Surprisingly, we have now discovered that it is possible to achieve adequate performance in impact tests in a similar construction with the moisture barrier foil bonded to only one of the two extruded layers. This offers an improvement in flexibility as well as an appreciable cost saving (as the foil needs to be coated on only one face), though there is some diminution of longitudinal water-blocking effect.
SUMMARY OF INVENTION
The cable in accordance with the present invention is thus characterised by a sheath construction comprising a first extruded polymeric layer which encloses the earth screen of the cable without filling the gaps between the components thereof (the wires or strips), a second extruded polymeric layer enclosing the first and between them a moisture barrier laminate comprising a metal foil layer coated on one of its major faces with a polymeric material compatible with the material of the extruded layer it contacts and bonded to that extruded layer only.
Preferably the metal foil layer is coated on only one face, and this is normally sufficient of itself to prevent adhesion to the other extruded layer of the sheath, but as an additional precaution (or instead) a suitable parting agent may be applied to one or both of the surfaces that it is desired should not bond. Graphite applied from aqueous suspension is preferred as parting agent, but talc (“chalk”) or a suitable oil might be used in some cases.
Preferably the foil is bonded to the outer extruded layer of the sheath and prevented from bonding to the inner layer.
Preferably the extruded polymeric layer to which the foil is to be bonded is of the same polymeric material as the coating on the foil; the other may but need not be of a substantially different composition. Options include the usual cable sheathing materials, namely polyethylene, polyvinyl chloride, “LSF” compositions based on ethylenevinyl acetate copolymer and ethylene-propylene rubbers (in each case with conventional additives).
The laminate is preferably longitudinally applied, but could be helically lapped if desired.
Preferably the insulation of the cable is of a polymeric material, such as crosslinked polyethylene or ethylenepropylene rubber, and in this case the conductor screen is preferably of carbon-loaded polymeric material and the earth screen preferably includes an inner layer of carbon-loaded conductive polymeric material, in addition to the wires or strips of metal.
Preferably the gaps between the wires or strips of the earth screen are waterblocked by applying water-swellable tapes under and/or over them, or in other known ways. Preferably at least one semiconductive water-swellable tape is applied under the wires or tapes and at least one insulating water-swellable tape over the top of them. A waterblocking tape applied over the wires or tapes may facilitate ensuring that the first extruded layer of the sheath does not fill the gaps between them. Other suitable tapes could be used for this purpose if desired.
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Blank Rome Comisky & McCauley
Nguyen Chau N.
Pirelli Cables (2000) Limited
Reichard Dean A.
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