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When the voltage reaches tens or even hundreds of kV, a metallic screen is necessary, and it is needed to connect it to earth at least at one point along the route.
Its main function is to nullify the electric field outside the cable. It acts as the second electrode of the capacitor formed by the cable.
In addition to the task of electrostatic screening already mentioned, the metallic screen also has to fulfill the following functions:
1. Draining the capacitive current that passes through the insulation.
2. Draining the zero-sequence short-circuit currents, or part of them. This function is used to determine the size of the metallic screen.
3. The circulation of the currents induced by the magnetic fields from other cables in the vicinity.
4. Reduction of the electrical influence on the cable surroundings in the case of an earth fault.
5. Provision of protection against accidental contact.
SCREEN IS USED TO PROTECT DAD SHORTS WHEN EARTH LEAKEGES OCCURS TO CONDUCTORS
Medium and high-voltage power cables, in circuits over 2000 volts, usually have a shield layer of copper or aluminum tape or conducting polymer. If an unshielded insulated cable is in contact with earth or a grounded object, the electrostatic field around the conductor will be concentrated at the contact point, resulting in corona discharge, and eventual destruction of the insulation.
As well, leakage current and capacitive current through the insulation presents a danger of electrical shock. The grounded shield equalizes electrical stress around the conductor, diverts any leakage current to ground. Be sure to apply stress relief/ cones at the shield ends, especially for cables operating at more than 2Kv to earth.
Shields on power cables are connected to earth ground at each shield end and at splices for redundancy to prevent shock even though induced current will flow in the shield. This current will produce losses and heating and will reduce the maximum current rating of the circuit. Tests show that having a bare grounding conductor adjacent to the insulated wires will conduct the fault current to earth quicker. On high current circuits the shields might be connected only at one end.
On very long high-voltage circuits, the shield may be broken into several sections since a long shield run may rise to dangerous voltages during a circuit fault. However, the shock hazard of having only one end of the shield grounded must be evaluated for the risk!
Shielding of an electric power cable is accomplished by surrounding the assembly or insulation with a grounded, conducting medium. This confines the dielectric field to the inside of this shield.
