Electrical Screening is necessary only for cables with phase voltage > 1 kv and fulfills the following functions.

• Potential grading and limiting of electrical fields
• Conduction of charge and discharge currents

To satisfy these functions, the screening normally comprises a combination of conducting layers with METALLIC elements.

The magnitude of electric stress and the degree of sensitivity of the insulation material against partial discharge govern the type of screening of the insulation with conducting layers.

Above certain voltages, as a means of containing the electrical field within the insulation, semiconducting screens are applied over the conductor & insulation. Screens are provided to achieve SYMMETRICAL dielectric fields within the cable STRUCTURE & carry current during short circuit. By this it is possible to eliminate any electrical discharges arising from air gaps adjacent to the insulation.

The coefficient of expansion of polyethylene & EPR is approximately ten times greater than that of either Copper or ALUMINIUM, and when the conductor is at its MAXIMUM operating temperature of 90°C a sufficiently large gap is formed between the insulation and the surface of the conductor to enable electrical discharge to occur. This discharge site & any others which are formed around a conductor when conductor is bent can be eliminated by applying a semiconducting layer over the conductor. Similarly, any discharges arising from air gaps between laid-up cores can be nullified by the use of a screen over the insulation.

During the early 1960s semiconducting tapes were applied the conductor but these have since been superseded by an extruded layer. This has the advantage of providing both a smoother finish and, as it fills the interstices between the wires, a circular envelop around the conductor. By reducing the concentration of the flux lines around the individual wires, the electrical stress around conductor is reduced by between 10% to 15%. The semiconducting layer is compatible with, and bonds to, the insulation and a nominal thickness of 0.7 mm is typical.

Electrical Screening is necessary only for cables with phase voltage > 1 kv and fulfills the following functions.

To satisfy these functions, the screening normally comprises a combination of conducting layers with metallic elements.

The magnitude of electric stress and the degree of sensitivity of the insulation material against partial discharge govern the type of screening of the insulation with conducting layers.

Above certain voltages, as a means of containing the electrical field within the insulation, semiconducting screens are applied over the conductor & insulation. Screens are provided to achieve symmetrical dielectric fields within the cable structure & carry current during short circuit. By this it is possible to eliminate any electrical discharges arising from air gaps adjacent to the insulation.

The coefficient of expansion of polyethylene & EPR is approximately ten times greater than that of either Copper or Aluminium, and when the conductor is at its maximum operating temperature of 90°C a sufficiently large gap is formed between the insulation and the surface of the conductor to enable electrical discharge to occur. This discharge site & any others which are formed around a conductor when conductor is bent can be eliminated by applying a semiconducting layer over the conductor. Similarly, any discharges arising from air gaps between laid-up cores can be nullified by the use of a screen over the insulation.

During the early 1960s semiconducting tapes were applied the conductor but these have since been superseded by an extruded layer. This has the advantage of providing both a smoother finish and, as it fills the interstices between the wires, a circular envelop around the conductor. By reducing the concentration of the flux lines around the individual wires, the electrical stress around conductor is reduced by between 10% to 15%. The semiconducting layer is compatible with, and bonds to, the insulation and a nominal thickness of 0.7 mm is typical.