Medium Voltage Power Cables Catalogue - AEC Online
Medium Voltage Power Cables Catalogue - AEC Online
Medium Voltage Power Cables Catalogue - AEC Online
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Construction of <strong>Medium</strong> <strong>Voltage</strong> <strong>Cables</strong><br />
Metallic Screen<br />
When the voltage reaches tens or even hundreds of kV, a metallic screen is necessary. Its main function is to nullify the<br />
electric field outside the cable. In addition to the task of electrostatic screening already mentioned, the metallic screening<br />
also has to fulfill the following functions:<br />
Return of the capacitive charging current under operating conditions.<br />
Conduction of the earth fault current in case of a fault until the system is switched off.<br />
Reduction of the electrical influence on the cable surroundings in case of an earth fault.<br />
Provision of protection against accidental contact.<br />
The metallic screen in medium voltage cables shall consist of one or more tapes, or a concentric layer of wires or a combination<br />
of wires and tape(s). When choosing the material of the screen - which is normally of copper – special consideration<br />
shall be given to the possibility of corrosion, not only for mechanical safety but also for electrical safety.<br />
Armour<br />
The armour protects the cable against mechanical stresses. For multi-core cables, galvanized steel is used as an armour<br />
material in one of the following forms:<br />
1. Wires: Single layer applied concentrically over the bedding. Wire armour is recommended for cables which will be<br />
subjected to a horizontal mechanical stresses.<br />
2. Tapes: Two tapes applied helically over extruded bedding in two layers so that the outer tape is approximately central<br />
over the gap of the inner tape. The gap between the adjacent turns of each tape shall not exceed 50 % of the width of<br />
the tape. Tape armour is recommended for cables which will be subjected to a vertical mechanical stresses.<br />
Single core cables in single or three-phase AC (alternating current) systems are not armoured as a rule, in order to avoid<br />
additional losses. However, armour of non-magnetic material (aluminum) has to be provided wherever mechanical<br />
damage or higher tensile stresses are to be expected during or after laying the cable.<br />
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