DougW has a good point about higher-voltage cables. At 2-35kV, the cable dielectric layer is typcially ethylene-propylene rubber or cross-linked polyethylene. [Typcially, 15kV-class, 133% insulation-level cable has a dielectric thickness just under ¼ inch.] Inside (under) the dielectric layer in imitate contact with the center conductor, and over the outside of the dielectric are are two semiconducting layers to even out electric stresses in the dielectric. Over the outer semicon is helically-wrapped copper tape or copper wires that must be solidly grounded as part of limiting electric stress in the dielectric layer. A generic term for this material is shielded polymeric cable.

In the last ~80 years, cable manufacturers have found that the dielectric material has to be of extremely high purity with antiseptic conditions during the extrusion process. Also, field preparations during installation of cable terminations and splices must be performed by a skilled and experienced person in a clean area to limit premature failure. Before energization of the cable, it is routine to apply DC-overpotential testing to reasonably assure that the circuit will have acceptable reliability over expected life of the installation. Workmanship on installation can be proven by good {or rejected} by poor} overpotential-test results.

[Linked Image from kerite.com]

An MV-cable splice illustration energy.tycoelectronics.com/rrg/raychem_rrg/59.pdf

On the North American continent, the practical upper limit of routinely manufactured extruded-dielectric shielded cable is presently 138kV.

P.S.: Never forget that In all but limited cases, overhead spans are BARE.




[This message has been edited by Bjarney (edited 09-12-2004).]