16th Edition (reference only) – NOW superseded by the 17th Edition IEE Regulations.

chapter 5
Earthing

chapter 6
Circuits

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

 

 

Return to top of page

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