Guide to the Wiring Regulations
D 5.2
Circulating currents and eddy currents in single-core installations Regulations 521.5.2 and 523.10 give requirements concerning ferrous enclosures and single-core armoured cables with respect to eddy currents and circulating currents. While 521.5.2 stipulates that steel wire armoured cables shall not be used with a.c. circuits there are other pitfalls in selecting alternative cables. Single-conductor non-ferrous metal sheathed or armoured cables need careful consideration due to the problems of sheath currents and eddy currents. Circulating (sheath) currents Circulating or sheath currents flow in the metal sheaths or armour of single-core cables. All single-core metallic-sheathed cables have a sheath current induced by the a.c. magnetic field surrounding each conductor. The metallic sheaths and armoured cables are often earth-connected to equipment at both ends, providing a closed circuit path through metal gland plates, enclosures and other metallic paths where sheath currents are permitted to flow, as shown in Figure D 5.2. Induced sheath currents cause a heating effect and temperature rise in metal sheaths or armour of single-conductor cables. This is transferred to the cable insulation and,
Sheath / Circulating Current
Figure D 5.2  Circulating sheath current.
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