Electric cables of reduced micro-voids in the extruded insulation

An electric cable for use at medium and high voltages, said cable comprising a conductor surrounded by a first semi-conductive layer which is surrounded by solid insulation which in turn is surrounded by second semi-conductive layer, said solid insulation being an extruded, peroxide cross-linked, plastic material which has been treated subsequent to the cross-linking thereof by subjecting it to heat at a temperature in the range from about 120.degree. C. to 150.degree. C. and an ambient pressure less than 10 mm. Hg. to remove decomposition products and reduce the number of micro-voids formed during the cross-linking, said insulation being characterized by a perforation gradient better than the same insulation which has not been so treated, being substantially free of said decomposition products and having a number of micro-voids less than the same insulation which has not been so treated and less than 10.sup.3 per mm.sup.3 whereby the perforation gradient is improved and the stability of the insulating properties of the insulation remains substantially constant with use over a period of time.