16th Edition (reference only) – NOW superseded by the 17th Edition IEE Regulations.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

5.6.2 - Increased fire risk

As with other systems of earth-fault protection, PME does not prevent a fault occurring, but will ensure that the fault protection device operates quickly when that fault appears. For example, if a fault of 2 Ohms resistance occurs in a 240 V circuit protected by a 20 A semi-enclosed fuse in a system with an earth-fault loop impedance of 6 Ohms, the fault current will be 240/(2 + 6) A = 240/8 A = 30 A. The fuse would not blow unless the circuit were already loaded, when load current would add to fault current. If the circuit were fully loaded with a load current of 20 A, total current would be 50 A and the fuse would blow after about 18 s. During this time, the power produced in the fault would be:

P = I²R = 30²x6 = 5400W or 5.4kW

This could easily start a fire. If, however, the earth-fault loop impedance were I Ohm, current would be 80 A and the fuse would blow in about 1.6 s and limit the energy in the fault circuit.


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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