TECHNICAL
Termination of armoured cables – ensuring earth continuity An important consideration when using armoured cable is the method used to provide reliable earth continuity and adequate earth fault current withstand capacity where the cable is terminated.
T
he armouring must of course be terminated into a suitable gland appropriate to the size of cable, meeting the requirements of BS 6121 or other applicable British or Harmonized Standard and having an appropriate degree of protection (IP rating). Care should be taken to ensure that all the wires or strips of the armouring are properly clamped by the gland and that the gland nut and locknuts are fully tightened. The use of a gland earth tag washer (‘banjo’) together with brass bolt, nuts, washers and copper protective conductor is generally necessary irrespective of whether the enclosure is non-metallic or metallic. Proprietary alternatives to an earth tag washer, such as components with an earthing terminal for use with cable glands, are not covered in this article. However, they may be used as the means of connecting to the copper protective conductor, provided the electrical installation designer is satisfied that the degree of safety required by BS 7671 (particularly in Section 526, Continuity of electrical connections), in the case of the particular application. Sizing the copper protective conductor The csa of the copper protective
58 Summer 2010 NICEIC Connections
conductor connecting the earth tag washer to the earthing terminal of an enclosure has to be selected in accordance with Table 54.7 in Regulation 543.1.4 of BS 7671 or calculated using the adiabatic equation given in Regulation 543.1.3. Cable armouring is sometimes used as a main protective bonding conductor (as well as being used
as a circuit protective conductor). Where this is the case, the csa of a copper protective conductor must also meet the requirements of Regulation Group 544.1 for a main protective bonding conductor. Termination of the armoured cable at a non-metallic enclosure Where the armoured cable terminates at a non-metallic enclosure, an earth tag washer is usually essential if a connection is required to the armouring, such as where the protective conductor is to continue through the enclosure. The earth tag washer should be clamped between two locknuts, as shown in Fig 1, to avoid the risk of the degradation of the continuity of the connection by creepage of the non-metallic material. Termination of the armoured cable at a metallic enclosure Many metallic enclosures and items of switchgear have a powder, epoxy resin or cathodic coating to improve their appearance. Generally such coatings are non-conducting. Where a metallic enclosure or frame of low voltage switchgear or controlgear is used as a protective
Fig1 - Termination at a non-metallic enclosure
Note: Only the incoming armoured cable termination is shown. The outgoing cable, if armoured, would be terminated as for the incomer
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