Insulation fault location: The concept

How does an insulation fault location system work?

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

Functional sequence:

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

Automatic insulation fault location/permanently installed systems

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

Manual insulation fault location/portable systems

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

  1. In addition to permanently installed EDS systems, a portable insulation fault evaluator is used for insulation fault location in secondary outgoing circuits.
  2. If the installation does not have an insulation fault test device, a portable insulation fault test device is used.
  3. If the IT system has been deenergised, a portable insulation 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 insulation 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 insulation fault location
Operating principle of insulation fault location
Figure: Test current pulse cycle on the EDS system
Test current pulse cycle on the EDS system