to the 16th 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


7.16.4 - Reduced voltage

In situations where the power requirements are high, extra-low voltage systems would need to deliver very high currents. If an increased voltage can still lead to a safe system, the current required will be reduced. We are therefore considering a voltage higher than extra-low but lower than low voltage.

The highest voltage permitted for such systems is 110 V between conductors for both single and three-phase systems, with voltages to earth of 55 V for single-phase and 65 V for three-phase supplies {Fig 7.26}.

The reduced voltage supply can be taken from a double wound isolating transformer, or from a suitable motor generator set, provided it has the unusual feature of a centre-tapped winding. The system must be insulated and protected against direct contact as for a low voltage installation. Earth-fault loop impedance must allow automatic disconnection in a maximum of 5 s (see {Table 7.9}), or an RCD with an operating current of no more than 30 mA must be provided. The result of multiplying earth-fault loop impedance by the RCD operating current (A) must not exceed 50 (V). Plugs, sockets and cable couplers in the reduced voltage system must all be provided with protective conductor contacts (earth pins) and must not be interchangeable with those of any other system in use at the same location.

Fig 7.26 - Reduced voltage Systems

Table 7.9 - Maximum earth-fault loop impedance for 5 s
----------------- disconnection time in reduced voltage systems
Device rating (A) MCB type 1 MCB type 2 MCB types C & 3 BS 88 fuse
- 55 V 63.5 V 55 V 63.5 V 55 V 63.5 V 55 V 63.5 V
6 2.30 2.65 1.32 1.52 0.92 1.07 3.20 3.70
10 1.38 1.59 0.79 0.91 0.55 0.64 1.77 2.05
16 0.86 0.99 0.49 0.57 0.34 0.40 1.00 1.15
20 0.69 0.80 0.40 0.46 0.28 0.32 0.69 0.80
25 0.55 0.64 0.32 0.36 0.22 0.26 0.55 0.63
32 0.43 0.50 0.25 0.28 0.17 0.20 0.44 0.51
50 0.28 0.32 0.16 0.18 0.11 0.13 0.25 0.29
Note : 63.5 V is the voltage to earth (to the star point) of a 110 V
---------- three-phase supply.

 

 

 

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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