Earth Fault Protection

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Recommended Settings

Solidly Earthed Systems

From Alstom Network Protection and Automation Guide (2011) Section 9.16 [3]: "Typical [earth fault relay] settings are 30-40% of nominal full-load current or minimum earth fault current for the circuit being protected." Therefore, a pickup setting of 30% - 40% of In is suggested, where In is:

  • For transformer feeders: In = full load current of transformer
  • For cable feeders: In = cable rated current
  • For motor feeders: In = full load current of motor

Fielding [5] suggests higher sensitivity to earth faults, with pickup settings of 20% of In. Ultimately, the decision depends on the trade-off between high sensitivity to earth faults and the probability of spurious and nuisance tripping.

Impedance Earthed Systems

In an impedance earthed system, earth faults are limited to a maximum value by some impedance. An earth fault pickup setting of 10% - 20% of the maximum earth fault current is recommended, to be selected in conjunction with the following minimum pickup settings suggested in [4] to avoid spurious tripping:

  • For CTs in residual connection: >12% of the nominal CT primary current
  • For core balance CTs: >1A for 0.1s

Unearthed Systems

In an unearthed system, core balance CTs are necessary to have the sensitivity to detect earth fault currents (because earth fault currents are purely capacitive). The minmum threshold for core balance CTs should be >1A for >0.1s as suggested by [4] to avoid spurious tripping.


  • [1] IEEE Std 242, “IEEE Recommended Practice for Protection and Coordination – Industrial and Commercial Power Systems (Buff book)”, 2001
  • [2] IEEE Std 142, “ IEEE Recommended Practice for Grounding of Industrial and Commercial Power systems (Green book)”, 2007
  • [3] Alstom Grid, “ Network Protection and Automation Guide”, May 2011
  • [4] Preve, C., “Protection of Electrical Networks”, 2006, ISTE Ltd, London
  • [5] Fielding, G., “Protection of MV Industrial Networks”, 1997, IEE Colloquium, London