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

Advantages of insulation fault location with the EDS system

Optimised maintenance

  • Insulation fault location without disconnecting the electrical installation
  • Selective fault location by fast and precise localisation of the faulty sub circuit
  • Centralised indication and operation with LC text display
  • Reduced maintenance costs

Increased operating reliability

  • Electrical installations are kept at a high level of availability
  • Increased productivity because interruptions to operation are avoided
  • More comfort for customers (service area)

Improved economic efficiency

  • Time and costs for maintenance are reduced
  • Weak points in the installation are recognised

Flexible and configurable solutions

  • Cost-effective construction by modular system components
  • Measuring current transformers are available in different sizes and shapes (round, rectangular, split core, flexible)


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