Power frequency withstand is one of the criteria in the selection of electrical equipment rating. The selection of the appropriate power-frequency withstand level is related to the earth fault factor for the system. IEC 60071-1 [3.15] defines earth-fault factor k to be

... at a given location of a three-phase system and for a given system configuration, the ratio of the highest r.m.s phase-to-earth power-frequency voltage on a healthy phase during a fault to earth affecting one or more phases at any point on the system to the r.m.s value of phase-to-earth power-frequency voltage which would be obtained at the given location in the absence of any such fault ...

The earth-fault factor is used to determine the highest voltage electrical equipment can withstand during overvoltages such as lightning or switching surges.

IEC 60071-1 [3.17]classify voltages and overvoltages according to their shape and duration. Voltages and overvoltages are divided into the following classes:

60071-1 2011_Table_1

The earth fault factor can be calculated from the symmetrical component parameters of the system R0 , X0 , and X1 , where:
R0 is the zero phase sequence resistance
X0 is the zero phase sequence reactance
X1 is the positive phase sequence reactance

  1. For a solidly earthed system for which {R_0/X_1} < 1 and {X_0/X_1} < 3 where the earth fault factor will not exceed 1.4.
  2. For a non-effectively earthed system for which 1 < {R_0/X_1} and 3 < {X_0/X_1} where the earth fault factor will be sqrt{3} or 1.7.
  3. For a system earthed via an arc suppression coil the earth fault factor will be 1.9

The higher the earth fault factor, the higher should be the rated power-frequency one minute withstand voltage. Lightning impulse withstand voltage is tied to power-frequency withstand voltage so that once the latter has been determined the former is automatically fixed.


  1. IEC 60071-2011 Insulation co-ordination
  2. The J & P Transformer Book, 12th Ed.


  • A Filipino Engineer, Registered Professional Engineer of Queensland (RPEQ) - Australia, and Professional Electrical Engineer (PEE 2574 - 1st Place April 1991) - Philippines with extensive experience in concept selection, front-end engineering, HV & LV detail design, construction, and commissioning of Hazardous and Non-Hazardous Area electrical installations in water and wastewater pipeline and pumping facilities, offshore platforms, hydrocarbon process plants and pipelines including related facilities. Hazardous area classification and design certification (UEENEEM015B, UEENEEM016B, UEENEEM017B).


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