Electricity: conductors and insulators – Conduits – cables or conductors – Conductive armor or sheath
Patent
1986-08-06
1988-08-30
Nimmo, Morris H.
Electricity: conductors and insulators
Conduits, cables or conductors
Conductive armor or sheath
156 51, 156 56, 174102SC, 174105SC, 174120SC, 174120SR, H01B 902
Patent
active
047678941
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to laminated constructions comprising extruded layers of polymer--based materials having two adjacent layers which are strippably bonded together. In particular, the invention relates to an insulated electric cable comprising at least three layers of polymer--based materials extruded about an electrical conductor, two adjacent layers of the polymer layers being strippably bonded. The construction of insulated electrical conductors, eg wire and cable, is well known in the art. For medium and high voltage applications, the cable generally comprises a central core conductor of one or more metal strands surrounded coaxially by (in sequential order) a semi-conductive polymeric shielding layer, a polymeric primary insulation layer and an outer semi-conductive polymeric shielding layer overlying the insulation. An outer metallic conductor (eg neutral conductor) overlying or embedded in the outer semi-conductive shielding can also be present, eg in the form of braided wires or metal tape. The cable may also be provided with armoured covering and additional layers to provide for example, weather protection or increased mechanical strength. Preferably, the annular surfaces of the polymeric layers are smooth and substantially concentric. Thus, although it is known to use helically wound tape for one or more layers, the layers are preferably formed by extrusion. Layers formed from tape are also generally more expensive to fabricate than extruded layers.
The inner semi-conductive polymeric shielding layer, the polymeric primary insulation layer and the overlying semi-conductive shielding layer of an electric cable form a coaxial laminated structure and can be applied to the metallic conductor using extrusion coating techniques well known in the art. The layers can be applied sequentially using tandem extrusion techniques, or two or more of the layers may be coextruded simultaneously using coextrusion die heads fed by separate extruders. One or more of the layers in the laminated structure can be crosslinked if desired.
Advantageously, for splicing or terminating cables, the outer semi-conductive shielding layer should be relatively easily stripped from the primary insulation layer leaving little or no conductive residue adhering to the primary insulation and without damaging the surface of the primary insulation. However, the outer semi-conductive shielding layer should be sufficiently bonded to the primary insulation so that the two layers do not separate during installation and conventional use and so that the ingress of contaminants, such as air or water, between the layers is avoided.
Combinations of primary insulating materials and semi-conductive shielding materials having the desired mutual adhesion/stripping characteristics have been developed and are used commercially. However, such laminated combinations of materials as have been developed in the prior art suffer from the disadvantage that they generally require the use of a semi-conductive material having a relatively high cost and/or poor physical, chemical or mechanical properties.
For example, if the semi-conductive shielding layer used is relatively hard, it is often quite difficult to strip it from the primary insulation and a hand tool may have to be used to cut through the semi-conductive shielding layer to the primary insulation in order to facilitate removal. The use of such a tool to cut through the semi-conductive shielding layer may cause damage to the outer surface of the primary insulation. If the semi-conductive shielding layer is relatively soft, it may tend to tear as it is being stripped from the primary insulation.
It is an object of the present invention to provide an improved laminated construction having two adjacent layers which are strippably bonded together A further object of the invention is to provide an improved laminated construction comprising cable insulation having a strippable semi-conductive shielding layer which construction overcomes or at least mitigates the problems of known cable insulatio
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BP Chemicals Limited
Nimmo Morris H.
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