16th Edition (reference only) – NOW superseded by the 17th Edition IEE Regulations.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

5.3.5 - Protective conductor impedance

It has been shown in the previous sub-section how a low-impedance protective conductor will provide safety from shock in the event of a fault to earth. This method can only be used where it is certain that the shock victim can never be in contact with conducting material at a different potential from that of the earthed system in the zone he occupies. Thus, all associated exposed or extraneous parts must be within the equipotential zone (see {5.4}). When overcurrent protective devices are used as protection from electric shock, the protective conductor must be in the same wiring system as, or in close proximity to, the live conductors. This is intended to ensure that the protective conductor is unlikely to he damaged in an accident without the live conductors also being cut.

{Figure 5.9} shows a method of measuring the resistance of the protective conductor, using a line conductor as a return and taking into account the different cross-sectional areas of the phase and the protective conductors.

Fig 5.9 - Measurement of protective conductor resistance

Taking the cross-sectional area of the protective conductor as Ap and that of the line (phase or neutral) conductor as Al , then

Rp = resistance reading x      Al   
   Al + Ap


For example, consider a reading of 0.72 Ohms obtained when measuring a circuit in the way described and having 2.5 mm² line conductors and a 1.5 mm² protective conductor. The resistance of the protective conductor is calculated from:

Rp =  R x
= 0.72 x 2.5
Al + Ap
2.5 + 1.5
0.72 x 2.5
Ohms = 0.45 Ohms


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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