Electrical power cable for underground use is typically formed by coating a metallic conductor with one or more layers of thermosetting materials. If multiple coating layers are placed over the conductor, one or more of these layers may be of a semi-conducting material, with the remaining layers being of insulating material. The layers are coated or extruded onto the conductor in an uncured, molten state. After coating, the cable is heated to a termperature sufficient to cure the insulating layer or layers, followed by a cooling under pressure of the now-cured cable. Some additional curing may occur during the cooling cycle.
The conductor may be composed of a plurality of strands of a conducting metal, such as aluminum or copper, or could be in the form of a single conductor rod or strand.
In the past, the curing operation was often carried out in a tubular conduit which was heated and pressurized by steam.
More recently, the steam heating tubes have increasingly been replaced by radiant heat curing, with the curing taking place in a tubular conduit under inert gas pressure. Other curing alternatives include circulating heated gases, liquids salts and irradiation of the cable.
In any case, and particularly, but not entirely, when a single conductor cable is formed, the insulation has a tendency to "shrink back" after cooling, resulting in exposure of a length of bare conductor at either end of the completed cable.
The shrink back tendency may not be observable during cable production. The problem occurs most often when cable is subjected to alternating heating and cooling cycles, and may become apparent when cable is subjected to electrical current passing through it for a time period, followed by a time span with no current passing through the cable, sometimes referred to as a load cycle. Purchasers require that electrical cable not shrink back more than a specified amount in a load cycle test.
It is desirable, therefore, to reduce the conductor exposure by substantially reducing insulation shrink back.