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Fig. 19.6 600/1000V Waveconal cable (Pirelli Cables Ltd). Mains Cables 515 are then covered with an extrusion of rubber-like material which acts as a bedding for the concentric neutral conductor, and the same material also completely covers the wires of the neutral conductor. The aluminium wires forming the neutral conductor (Fig. 19.6) are applied in an oscillatory manner instead of in a helical form and this enables the cores to be exposed when making tee-joints under live conditions without the need to cut the neutral wires, and thus break the return path of the combined protective and neutral conductor of a TN-C-S system. The aluminium neutral wires are carefully segregated so that they are individually surrounded by the rubber-like filling material in order to prevent moisture transference around the neutral in the event of mechanical damage. The cable is finally sheathed with PVC to provide mechanical protection to the rubber-like covering around the neutral wires.
516 Handbook of Electrical Installation Practice A similar construction of cable (copper waveform) is available which uses copper wires forming the neutral conductor and is particularly used where there are concerns of possible corrosion of aluminium wires. This design has largely superseded the Waveconal type. Cables with synthetic thermoplastic insulation (PVC) for l.v. applications Many low-voltage cables in and around buildings have PVC insulation, and BS 6346, PVC-insulated cables for electricity supply, fully specifies a large range of cables having both copper and aluminium conductors, with armouring. For vertical risers and some submains, single-core unarmoured designs would be used but in general it is more normal to employ wire armoured or aluminium strip armoured cables for the mains supply. All the cables in BS 6346 have their PVC insulation applied to the copper or aluminium conductors by extrusion; multicore cables with shaped conductors have their insulation applied in sectoral shape so that the complete cable is compact and thus overall dimensions are kept to a minimum. Figure 19.7 shows examples of cross-sections of typical solid sector-shaped aluminium conductor cables. Cables have a bedding for the armour of either an extruded covering of PVC or a lapping of two or more plastic tapes. Armouring on copper conductor and some aluminium conductor cables consists of a conventional layer of galvanised steel wires, although single-core cables, requiring armouring, have wires of non-magnetic material if the cables are to be used in a.c. circuits. Aluminium conductor cables can also have aluminium strip armouring, consisting of a single layer of aluminium strips. This form of armouring is useful where low armour resistances are required; aluminium strip armouring has a much lower resistance than galvanised steel wire armouring. For smaller cables the strip armouring resistance is approximately half that of steel wire while for larger sizes the aluminium strip has only one quarter of the resistance. All BS 6346 cables have an overall extruded sheathing of PVC. Cables to the above specification are also available for operation on 3.3kV circuits. Such cables are similar to those for use at l.v. but the thickness of core insulation is somewhat greater. At 3.3kV there is no necessity for conductor or core screening as is required for higher voltage cables. Concentric neutral-and-earth service cables are made in a variety of combinations having copper or aluminium phase conductors and concentric neutral conductors of aluminium or copper wires. These cables are generally not used in sizes greater than 35mm2 and can have one or three phase conductors. Where a concentric cable is required with separate neutral and earth-continuity conductors, there is a design available in accordance with BS 4553 or BS 7870.These cables, in sizes up to and including 35mm2 , have the central circular stranded copper phase conductor insulated. The neutral conductor is made from a number of single copper wires, each covered by an extruded layer to a diameter the same as the wires forming the earth-continuity conductor. For new installations, cables with thermosetting insulation to BS 5467 or BS 6724 has largely replaced the BS 6346 types.
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HANDBOOK OF ELECTRICAL INSTALLATION
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© 2003 by Blackwell Science Ltd, a
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vi Contents System operation 79 Ide
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viii Contents Training and systems
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x Contents 18 Lighting 475 h.r. kin
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Preface My first task as editor is
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10 Handbook of Electrical Installat
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INTRODUCTION CHAPTER 4 Cable Manage
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CHAPTER 5 Electricity on Constructi
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CHAPTER 6 Standby Power Supplies D.
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INTRODUCTION CHAPTER 7 Ground Earth
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INTRODUCTION CHAPTER 8 Cathodic Pro
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CHAPTER 10 Special Installations or
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CHAPTER 12 Standards, Specification
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CHAPTER 13 Distribution Transformer
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DEFINITIONS CHAPTER 14 Switchgear A
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Table 14.1 (contd) 374 Handbook of
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APPENDIX I Main Authoritative Docum
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670 Index oil, 364, 366 oil filling
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672 Index earthing, 234 earth termi
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674 Index swimming pools, 252 switc
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