The most common form of metalllic sheath used as an alternative to a lead or aluminium sheath is one of corrugated steel. This consists of a long stell strip, shaped into a tube round the cable core, welded by a suitable process (inert-gas arc, mains frequency or high frequency heating) along the longitudinal seam and then corrugated. Outer protection for the steel sheath is provided by means of a special viscous, anti-corrosion compound enclosing one or more plastic tapes and laid so that the troughs of the corrugations are completely filled. An external plastic covering is then extruded over the compound- protected steel to form a smooth outer covering.