Power cable shielding

Claim 1

A power cable of the type comprising an inner conductive core for carrying electrical energy; insulating means, around the core and of sufficient thickness to insulate the same, subject to thermal expansion upon passage of current through the core and thermal contraction upon lessening current passage therethrough; metallic shield means comprising a multiplicity of helically wound elongate metallic elements providing a generally flat surface in contact with the insulating means providing a low resistance return path; the insulating means being of sufficient thickness to impart a permanent stretch to the metallic elements when the core is operated through a predetermined temperature range of at least 40* C; the improvement wherein the shield means comprises means for accommodating thermal expansion of the insulating means above the predetermined temperature range to maintain the stress in the metallic elements below the elastic limit thereof, the accommodating means comprising the length of the lay of the metallic elements, the length of the lay being at least square root A2 - B2 - C2/C2 - 1 where A pi (dexp + t); B pi (dsh + t); C Shield Length Expanded/Shield Length Starting and is equal to or less than nsh (Tsh - To + pel/E + 1); dexp diameter under the elements at load temperature; t thickness of the metallic elements; dsh diameter under the elements at ambient temperature; nsh linear coefficient of thermal expansion of the elements; Tsh shield temperature during load; To ambient temperature; pel proportional elastic limit; E Young''s modulus.