Earth Fault Loop Impedance Measurement

Load, bonding conductor, earthed neutral point of the tranformer, source line winding and live conductor from the source to the load

Last edit: 17/05/2023

Paragraph 18.2 of EN 60204-1 “Verification of conditions for protection by automatic power failure” describes two tests to be performed on the installation:

  • Test 1 – Verification of the continuity of the equipotential protection circuit;
  • Test 2 – Verification of the impedance of the fault loop and suitability of the corresponding overcurrent protection device.

The conditions for protection by automatic power failure must be verified by means of the following elements:

1. verification of the fault loop impedance by:

  • calculation
  • measurement

2) confirmation that the setting and characteristics of the corresponding overcurrent protection device are compatible with the measured/calculated values of the fault loop impedance.

In TN systems, Test 2, if performed by measurement, is always preceded by Test 1.

Fault Loop Impedance Measurements for Motors under inverter

In the event that the motors on which the fault loop impedance measurement is to be performed are under inverter, EN 60204-1 stipulates that the measurement must be performed on the supply terminals of that drive. The circuit to be referred to is shown in the figure opposite.

Where a power drive system (PDS) is used, the disconnection time for fault protection shall meet the relevant requirements of this Annex A at the incoming supply terminals of the basic drive module (BDM) of the PDS.

The reason for this is related to the fact that if the measurement were to be performed directly at the motor terminals, and therefore downstream of the drive, the measured values would be strongly influenced by the high internal impedance of the inverter.

Even if the point at which the measurement is taken changes, the same requirements described in the previous paragraph for motors directly connected to the mains apply, i.e. the impedance measured must be

Zs ≤ 2U0/3Ia

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