Those standards also require that this material pass a long-time stability test for resistivity at the emergency operating temperature level to insure that the layer remains conductive and hence provides a long cable life.Ī water-impervious material can be incorporated as part of the conductor shield to prevent radial moisture transmission. Industry standards require that the conductor semiconducting material have a maximum resistivity of 1,000 meter-ohms. Special carbon black is used to make the layer over the conductor semiconducting to provide the necessary conductivity. It is extremely important that there be no voids or extraneous material between those two layers. In extruded cables, this layer is now extruded directly over the conductor and is bonded to the insulation layer that is applied over this stress relief layer. This is done, especially on large conductors, in order to hold the strands together firmly during the application of the extruded semiconducting material that is now required for medium voltage cables.Įxperience with cables that only had a semiconductingtape was not satisfactory, so the industry changed their requirements to call for an extruded layer over the conductor. Present standards still permit such a tape over the conductor. The conductor shieldmg materials were originally made of semiconducting tapes that were helically wrapped over the conductor. Both serve the same function of stress reduction.Ĭonductor shields for paper insulated cables are either carbon black tapes or metallized paper tapes. The materials used are either semiconducting materials or ones that have a high dielectric constant and are known as stress control materials. The purpose of the semiconducting, also called screening, material over the conductor is to provide a smooth cylinder rather than the relatively rough surface of a stranded conductor in order to reduce the stress concentration at the interface with the insulation.Ĭonductor shielding has been used for cables with both laminar and extruded insulations. In cables rated over 2,000 volts, a conductor shield is required by indusby standards. This confines the dielectric field to the inside of this shield. Shielding of an electric power cable is accomplished by surrounding the assembly or insulation with a grounded, conducting medium. However, the shock hazard of having only one end of the shield grounded must be evaluated for the risk! 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. On high current circuits the shields might be connected only at one end. Tests show that having a bare grounding conductor adjacent to the insulated wires will conduct the fault current to earth quicker. This current will produce losses and heating and will reduce the maximum current rating of the circuit. 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. Be sure to apply stress relief/ cones at the shield ends, especially for cables operating at more than 2Kv to earth. The grounded shield equalizes electrical stress around the conductor, diverts any leakage current to ground. As well, leakage current and capacitive current through the insulation presents a danger of electrical shock.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |