SINGLE-CORE aluminium wire armoured cables are often employed in high current industrial applications.
Such cables are available in sizes up to 1000 mm2, whereas it is difficult to obtain multicore steel wire armoured cables in sizes above 400 mm2. Single-core cables have a smaller bending radius than the equivalent SWA multicore cable and are, in general, easier to handle.
Tables 4D3 and 4E3 in BS 7671 give information on current-carrying capacities and voltage drop for singlecore cables with non-magnetic armour. Note that single-core cables armoured with steel wire or tape must not be used for a.c. circuits. (Regulation 521-02-01 refers).
In any armoured cable system the armour is an exposedconductive-part and has to be connected to earth as required by Regulation 413-02-06 for TN systems or Regulation 413-02-18 for TT systems. The connection with earth has to be made at a minimum of one point, usually one end.
For a single-core armoured cable, carrying an a.c. load current, a voltage will be induced in the armour. Similarly a voltage will be induced in a metallic screen or sheath of a single-core cable. The magnitude of the induced voltage depends on factors which include the load current, the length of the cable, the armour diameter and the cable spacing. The armour is effectively the secondary of a transformer and the conductor is the primary.
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(Originally published in Wiring Matters issue 20)