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System earthing and fault levels

The following earthing systems and design fault currents (3-phase symmetrical) are in use throughout the distribution network:

Voltage Type of earthing Fault current Fault level
132kV Solid Earthing 21.9kA 5000MVA
33kV Impedance Earthed 17.5kA 1000MVA
11kV Impedance Earthed 13.1kA 250MVA
6.6kV Impedance Earthed 21.9kA 250MVA
LV Solid Earthed (PME) 27kA 19.4MVA

Where the network is impedance earthed the typical neutral earth resistor will limit the earth fault current to 1000A per transformer, i.e. giving a maximum earth fault level of up to 3kA. Earthing for substations and networks must comply with the following documents:

  • EPD332 - Customer installation earthing
  • EPD333 - Supply system earthing
  • CP332 - LV service connections and application of PME
  • CP333 - Earthing design for high voltage substations and equipment
  • CP335 - Earthing design for 132kV, 33kV and 33/11/6.6kV primary substations and equipment

These documents, in turn, reference a number of industry standards including ENATS 41-24, ENA ER S34, ER S36 and BS7430.

Design earth fault clearance times are:

Voltage Clearance time
132kV 200ms
33kV 500ms
11/6.6kV 1s

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