16th Edition (reference only) – NOW superseded by the 17th Edition IEE Regulations.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

5.6.4 - Special requirements PME-fed installations

An installation connected to a protective multiple earth supply is subject to special requirements concerning the size of earthing and bonding leads, which are generally larger in cross-section than those for installations fed by supplies with other types of earthing. Full discussions with the Electricity Supply Company are necessary before commencing such an installation to ensure that their needs will be satisfied. The cross-sectional area of the equipotential bonding conductor is related to that of the neutral conductor as shown in {Table 5.9}

Table 5.9 - Minimum cross-sectional area of main equipotential
----------------- bonding conductor for PME-fed installations
Neutral conductor c.s.a. (mm²)
Main equipotential bonding conductor c.s.a (mm²)
35 or less
Over 35 and up to 50
Over 50 and up to 95

Danger can arise when the non-current carrying metalwork of an installation is connected to the neutral, as is the case with a PME-fed system. The earth system is effectively in parallel with the neutral, and will thus share the normal neutral current. This current will not only be that drawn by the installation itself, but may also be part of the neutral current of neighbouring installations.

It follows that the earth system for an installation may carry significant current (of the order of tens of amperes) even when the main supply to that installation is switched off. This could clearly cause a hazard if a potentially explosive part of an installation, such as a petrol storage tank, were the effective earth electrode for part of the neutral current of a number of installations. For this reason, the Health and Safety Executive has banned the use of PME in supplies for petrol filling stations. Such installations must be fed from TN-S supply systems (RSE booklet RS(C)41
-           'Petrol Filling Stations: Construction and Operation').


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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