Electrical stress control

Claim 2

A high voltage cable comprising a conductor (12), electrical insulation (10) enclosing the conductor, and a screen (4, 6, 8) enclosing said insulation, wherein the insulation (10) extends beyond the screen (4, 6, 8), and the conductor (12) extends beyond the insulation (10), thereby to effect jointing or termination of the cable; wherein two layers (16, 18) of electrically semiconductive, stress control material, having different a.c. electrical impedance characteristics from each other, overlap each other and enclose at least part of the insulation (10) of the cable; the cable being characterised in that at least the outer (18) of the two layers (16, 18) has a non-linear impedance characteristic and does not form a continuous electrical path between the conductor (12) and the screen (4, 6, 8) of the cable.

Claim 3

A cable according to claim 1, characterised in that said other layer (16) has a linear a.c. electrical impedance characteristic.

Claim 4

A cable according to claim 1 or 2, characterised in that the inner (16) of said layers has a linear impedance characteristic; in that the inner layer (16) extends in contact with the screen (4, 6, 8) and along at least part of the insulation (10), and in that the outer layer (18) overlaps a part only of the inner layer (16).

Claim 5

A cable according to any preceding claim, characterised in that said layers (16,18) are integral with each other.

Claim 6

A cable according to any preceding claim, characterised in that one of said layers (16,18) is provided as a tubular member and the other of said layers (16, 18) is provided as a coating on the inside of said tubular member (16, 18) or of another tubular member (20).

Claim 7

A cable according to claim 5, characterised in that another tubular member (20) comprises electrically insulating material.

Claim 8

A cable according to any of claims 1 to 4, characterised in that each of said layers (16,18) is provided as a tubular member.

Claim 9

A cable according to any preceding claim, characterised in that said arrangement is recoverable by the application of heat thereto.

Claim 10

A cable according to any preceding claim, characterised in that one (16) of said semiconducting layers (16, 18) comprises an electrically insulating polymeric material that contains carbon black.

Claim 11

A cable according to any preceding claim, characterised in that one of said semiconducting layers (16, 18) comprises iron oxide; or zinc oxide; or silicon carbide; or a polymeric material having dispersed therein at least one particulate filler selected from: compounds having a perovskite type crystal structure; compounds having a spinel crystal structure, other than y-Fe 2 0 3 and spinel itself, selected from compounds of the general formula A II B III 2 O 4 wherein A is Mg, Co, Cu, Zn or Cd and B is Al, Cr, Fe, Mn, Co or V, provided that when A is Mg, B is not Al, when A is Cu, B is not Cr and when A is Zn, B is not Fe, and compounds of the general formula A IV B 2 II O 4 wherein A is Ti or Sn and B is Zn, Co, Ni, Mn, Co or Cd; compounds having an inverse spinel crystal structure; compounds having a mixed spinel crystal structure; MoSe z , MoTe 2 , Mn0 2 and Sn0 2 ; dichalcogenides of transition metals and of tin; Agl, Prussian blue, Rochelle salt and other alkali metal tartrates, compounds of the formula i.e. XH 2 YO 4 , wherein X is K, Rb or Cs and Y is P or As, ammonium fluoroberyllate, thiourea, ammonium sulphate and triglycine sulphate; Si 3 N 4 ; a mixture of MoS 2 and non-stoichiometric or stoichiometric Fe 3 0 4 ; a mixture of stoichiometric or non-stoichiometric Fe 3 O 4 and barium titanate; a mixture of stoichiometric or non-stoichiometric Fe 3 O 4 and at least one metallic powder selected from Fe, Al, Cu, Mn, Cr, Pb, Ni, Zn and Ag; and a mixture of barium titanate and a carbon black; the total weight of said compound(s) being at least 10% based on the weight of the polymeric material and the material having a y-value of at least 1.5 at least at some direct current (DC) electrical stress between 0.01kV/mm and 10kV/mm.

Claim 14

A method according to claim 11, characterised in that at least one of said layers (16,18) comprises a heat-recoverable member and is recovered into position on the cable by the application of heat to said member.