Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day, about eddy currents, is answered by SELECT:
Question: I have seen the following:
'When single-core cables are used for three-phase power distribution circuits (e.g. off main 400V transformers), both ends shall be earthed and bonded together. Special attention shall be given to de-rating the cable to account for circulating currents owing to induced voltages in the armour.
Where the reduction in current carrying capacity is demonstrably not acceptable and the value of induced voltage is less than 65V, one end shall be insulated.
Under such circumstances:
- A separate protective conductor shall be provided.
- Insulated glands for cable terminations shall be fitted at one end.
- Trefoil formation shall be used for each three phase group of single core cables.
- Contractors shall provide evidence of ability of cleats to withstand the axial forces.
- Protection from contact with the cable armour shall be provided.'
What are these recommendations for?
Answer: These appear to be measures to prevent problems from circulating or induced currents (eddy currents), and I would refer you to BS 7430 Code of practice for earthing and, in particular, Clause 21.7.2.2 Low Voltage Systems.
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