16th Edition (reference only) – NOW superseded by the 17th Edition IEE Regulations.

chapter 5

chapter 6

  5.1 - The earthing principle 5.6 - Protective multiple earthing (PME)
  5.2 - Earthing Systems 5.7 - Earthed concentric wiring
  5.3 - Earth fault loop impedance 5.8 - Other protection methods
5.4 - Protective conductors 5.9 - Residual current devices (RCDs)
5.5 - Earth electrodes

5.10 - Combined functional and protective

5.6.3 - Broken neutral conductor

The neutral of a supply is often common to a large number of installations. In the (unlikely) event of a broken neutral, all the consumers on the load side of the break could have a combined neutral and earth potential of the same level as the phase system (240 V to earth). This situation could be very dangerous, because all earthed metalwork would be at 240 V above the potential of the general mass of earth.

To prevent such an event, the Electricity Supply Company connects its combined neutral and earth conductor to earth electrodes at frequent intervals along its run. Whilst this does not entirely remove the danger, it is much reduced. For example, the assumed earth resistance values of {Fig 5.18} show that the maximum possible potential to earth in this case would be 96 V. In practice, much lower resistance values for the earth connections will reduce this voltage. The Electricity Supply Company goes to very great lengths to ensure the integrity of its neutral conductor.

Fig 5.18 Danger due to broken neutral in a PME system

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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