16th Edition (reference only) – NOW superseded by the 17th Edition IEE Regulations.

chapter 5

chapter 6

Special Installations
  --1. - Introduction --2. - Bath tubs and shower basins
  --3. - Swimming pools --4. - Sauna rooms
  --5. - Installations on construction sites --6. - Agricultural & horticultural
--7. - Restrictive conductive locations --8. - Earthing for function & protection
--9. - Caravan, motor homes, caravan parks 10. - Highway power & street furniture
11. - Heating appliances & installations 12. - Discharge lighting
13. - Underground & overhead wiring 14. - Outdoor installations & garden buildings
15. - Installations of machines & transformers 16. - Reduced voltage systems
17. - Marinas 18. - Medical locations
19. - Exhibitions, shows and stands

7.16.3 - Functional extra-low voltage (FELV)

If a separated extra-low voltage (SELV) system is earthed, or if the insulation of the supply which feeds it does not meet the necessary requirements, it ceases to be a SELV system and becomes a functional extra-low voltage (FELV) system. This difference is illustrated by {Fig 7.25}.

If a functional system only fails to be classified as a safety system because it is earthed, it must then be protected by enclosures that prevent it being touched or by insulation capable of withstanding a test voltage of 500 V r.m.s. for one minute. In such a case there is no need for the earth-fault loop impedance of the extra-low voltage circuit to be low enough to prevent danger; compliance of the safety source supply is sufficient. There is therefore no need to bond and earth all associated non-current-carrying metalwork.

Such metalwork must, however, be bonded and earthed if the insulation between the low-voltage supply circuit and the functional extra-low voltage circuit does not meet the requirements stated above. In the event of the supply circuit being of the earth-free bonded type {5.8.3} all non-current-carrying metalwork of the FELV system must he connected to the non-earthed protective conductor of the supply circuit. Plugs and sockets in FELV systems must not be interchangeable with those of other Supply systems in use in the same premises.

becomes a functional extra-low voltage system when

Fig 7.25 - Earthing relationship, SELV and FELV systems



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Extracted from The Electricians Guide Fifth Edition
by John Whitfield

Published by EPA Press Click Here to order your Copy.

Click here for list of abbreviations