Application of heat-shrinkable plastics in MV

Raychem commenced their development of power-cable termination systems about 1961. The first applications were in MV, weather-protected situations, and were soon extended to outdoor MV applications. 

The first weather-protected application concerned house service installations comprising one phase and one neutral core. The termination for this consists of a Y-shaped heat-shrinkable moulding. The house service cable is prepared by stripping off the lead sheath and separating the phase and neutral cores with the insulation intact. These are then threaded up and out of the arms of the Y piece. The Y piece is then shrunk so that it overlaps the lead sheath and the upper arms are covered with tubing. An ordinary propane gas torch is used as a heat source and the total shrinking time is less than 5 min. A suitable earth clip is fitted to the lead sheath to provide consumer earthing facilities. 

The extension of this technique to the distribution board or feeder pillar where four or five core cables are concerned is relatively simple. Traditionally this termination was made using the iron box filled with bitumen. The cable box was replaced by a preformed plastics glove which is pulled down over the prepared cores so that the lower end overlaps the cable sheath and is then shrunk into position. The individual paper-insulated cores are protected by shrinkable tubing and the terminating lug is sweated or compressed on to the end of the conductor. In a conventional termination the cable box itself forms the direct connection between the cable sheath and the neutral earth bar. 

With heat-shrinkable terminations it is necessary to make special arrangements to bond the lead sheath to the neutral earth bar. 

Many utilities have now replaced paper lead cables by plastics cable for low-voltage power distribution. Since each core has a plastics sheath no additional protection is required at the termination. However, a heat-shrinkable glove is still required such that water cannot penetrate into the body of the cable. If this is not done the cable acts as a water pipe allowing water to be transmitted into terminations and joints, which could result in failure. 

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