Ground-fault location: The concept

How does a ground-fault location system work?

A ground-fault location system comprises a ground-fault test device and a ground-fault evaluator connected to one or more measuring current transformers. The ISOMETER® of the iso685 series features a built-in ground-fault test device.

Functional sequence:

  • Start ground-fault location by activating the ground-fault test device ISOMETER®. 
  • The ground-fault test device briefly connects the live conductors to ground.
  • Electronic current limiting, this connection and the ground fault create a closed circuit, in which a test current is determined by the system voltage flows. The test current is limited to a maximum value.
  • The test current pulse flows from the ground-fault test device via the live conductors, the ground fault Rf and the ground wire (PE conductor) and back to the ground-fault test device.
  • The test current signal is detected by all measuring current transformers on the circuit and evaluated by EDS ground-fault evaluators.
  • The fault location can be identified using the assignment between measuring current transformers and circuit.

Automatic ground-fault location/permanently installed systems

The ground-fault monitoring device with built-in ground-fault test device, the ground-fault evaluator and the associated measuring current transformers are components of the installation. Ground-fault location is started automatically by the ground-fault monitoring device.

Manual ground-fault location/portable systems

Manual ground-fault location can be implemented to meet the following requirements:

  1. In addition to permanently installed EDS systems, a portable ground-fault evaluator is used for ground-fault location in secondary outgoing circuits.
  2. If the installation does not have an ground-fault test device, a portable ground-fault test device is used.
  3. If the IT system has been de-energized, a portable ground-fault test device with built-in measuring voltage source is used.

Distinguishing features of main circuit/control circuit

The essential difference in the two applications is in the significantly reduced test current for application in control circuits and/or the response sensitivity.

Characteristics of switching function:

  • All measuring channels are scanned in parallel during ground-fault location.
  • The max. scanning time is ≤ 10 s.
  • At the end of this time the relay contact switches the faulty measuring channel concerned.
  • This switching contact can be used to disconnect the faulty outgoing circuit via a contact element.
Figure: Operating principle of ground-fault location
Operating principle of ground-fault location
Figure: Course of the EDS system test current
Course of the EDS system test current
Figure: Test current pulse cycle on the EDS system
Test current pulse cycle on the EDS system