16th Edition (reference only) – NOW superseded by the 17th Edition IEE Regulations.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

7.16.2 - Separated extra-low voltage (SELV)

The safety of this system stems from its low voltage level, which should never exceed 50 V ac or 120 V dc, and is too low to cause enough current to flow to provide a lethal electric shock. The reason for the difference between ac and dc levels is shown in {Figs 3.9 & 3.10}.

It is not intended that people should make contact with conductors at this voltage; where live parts are not insulated or otherwise protected, they must be fed at the lower voltage level of 25 V ac or 60 V ripple-free dc although insulation may sometimes he necessary, for example to prevent short-circuits on high power batteries. To qualify as a separated extra-low voltage (SELV) system, an installation must comply with conditions which include:

1. - it must be impossible for the extra-low voltage source to come into contact with a low voltage system. It can be obtained from a safety isolating transformer, a suitable motor generator set, a battery, or an electronic power supply unit which is protected against the appearance of low voltage at its terminals,

2. - there must be no connection whatever between the live parts of the SELV system and earth or the protective system of low voltage circuits. The danger here is that the earthed metalwork of another system may rise to a high potential under fault conditions and be imported into the SELV system.

3. - there must be physical separation from the conductors of other systems, the segregation being the same as that required for circuits of different types {6.6},

4. - plugs and sockets must not be interchangeable with those of other systems; this requirement will prevent a SELV device being accidentally connected to a low voltage system,

5. - plugs and sockets must not have a protective connection (earth pin). This will prevent the mixing of SELV and FELV devices. Where the Electricity at Work Regulations 1989 apply, sockets must have an earth connection, so in this case appliances must be double insulated to class II so that they are fed by a two-core connection and no earth is required,

6. - luminaire support couplers with earthing provision must not be used.


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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