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CABLE SHEATHING

The external sheath of power cables and communication cables has to protect the internal elements from mechanical damage and from environmental influences.

Lead cable sheaths have been extruded directly onto cable cores for more than 100 years, since the invention of cable sheathing by Borell in 1879.

Today, approximately 10% of overland and underseas cables are sheathed with metal, usually Lead.

The thickness of cable sheath is standardized and is usually between 1 and 3 mm the sheath has to be completely tight and protect the internal conductor and insulation layers against moisture as well as mechanical, chemical and thermal influences.

Further requirements are:

A narrow wall thickness tolerance is prescribed in the standard. The mean value should be at least equal to the nominal value.

Stop marks (known as bamboo rings) can form when the press is stopped and reloaded.

These marks are undesirable because of the associated wall thickness deviations.

The cable itself should not be damaged by severe temperature effects during the sheathing process.

Cable sheathing with Lead

In Lead cable sheathing, liquid metal is poured into a vertical container. Usually, it is filled from an electrically heated crucible containing several tons of the melt through a floor valve with various flow systems at a maximum temperature of 450 C.

The cast Lead should bond well with the discard from the previous filling on the die head to prevent possible defects from inclusions. Air blisters should not form during filling. The Lead is, therefore, usually filled under vacuum together with a lost head that is sheared after solidification at the top edge of the container.

The metal solidifies in water cooled container under a small load applied by the steam within 5 to 10 minutes after which the extrusion process can begin.

The metal then flows into the bifurcated hollow mandrel from two sides and is pressed into the prechamber. It is then pressed onto the cable and forms the cable mantel as it leaves the die.

The material flow necessities longitudinal weld. Particular attention must be paid to these potential sources of defects. The boundary surface between the extruded and the newly filled metal (transverse weld) extend as a tongue over approximately 20% of an extrusion charge and can be scarcely recognized by metallography.

The wall thickness of the extruded cable sheath can be set and adjusted by axial movement of the die.

A cooling ring located immediately behind the die sprays water onto the extrusion providing rapid cooling and hinders coarse gain formation in the Lead mantle as well as damage of the temperature-sensitive cable insulation. The cooled cable is coiled directly behind the press.

As mentioned previously, the stop mark between two extrusions, during which the extrusion is stationary for several minutes, is a problem. The bamboo ring that forms have a reduced well thickness due to the elastic deformation of the die when the load is reduced and the subsequent reduction in the gap between the die and the hollow mandrel. The strength and structure also changes at these stop marks. The pressure at the die also reduces during extrusion from the start of extrusion to the and, resulting in wall thickness variations.

Careful control of the die setting during extrusion and when stopped are used to compensate for this in order to obtain a constant wall thickness. Built-in thermocouples are used to carefully monitor the tooling temperature. Heating elements are used to control the temperature.

The machines used for cable sheathing extrusion with Lead range from 6 to 30 MN press power with a container capacity of 135-1000 Kg Lead. The container diameters are selected to give a specific pressure of 350-450 N/mm2. mantle diameters up to 110 mm are standard.

A sheath and other layers surrounding the insulation to exclude moisture and protect it from corrosion and mechanical damage during the lifetime of the cable. Lead as a sheathing agent plays an important role due to its excellent proven corrosion resistance when in contact with a wide range of industrial and marine environments, soils and chemicals, Lead was one of the first materials to be used to provide an impervious sheath on electric cables. A Lead Sheath is available on many types of wire and cable insulations. This sheath is completely moisture proof and very durable. Where both mechanical strength and moisture resistance are required, a combination of Lead Sheath and armor can be used. Underground ducts and wet locations are typical areas where Lead-Sheathed cables are used.

Lead has the major advantage that it can be applied to the cable core in unlimited lengths by extrusion at temperatures which do not damage even the most sensitive conductors (optical fibers) or insulating materials (paper or plastics). Lead is pliable and so can withstand the several coiling, uncoiling, handling and bending operations involved during the later manufacturing stages and installation of the cable. A Lead sheath can be readily soldered (again at low temperatures) when cable lengths need to be jointed or new cables installed. With modern screw-type continuous extruders, un jointed lengths of submarine power cables as long as 100 kilometers have been produced.