Wenglorz - Dipl. Ing. Helmut Wenglorz GmbH

Notes about "Insulation Monitor in IT-network"

1. Insulation Monitors are used in ungrounded IT-networks.


2.The IT-system in composition with the insulation monitoring is the best network confuguration concerning security of energy supplies for the electrical equipment


3. The IT-system is an ungrounded electrical low voltage power for 2-AC, 3-AC and DC

  • All conductive bodies (housings of appliances) not belonging to electric circuit are grounded
  • For description IT-network look at DIN VDE 0100-401

4. The IT-System basically requires the installation of insulation monitors > DIN VDE 0100-410.

  • ISO monitors requirements look at VDE DIN EN 61557-8

5. The fault happens during a first insulation fault (single-pole fault) is safely low, so operation can be continued

  • Single-pole faults are the most frequent faults with a probability over 90%.
  • However, there must be taken arrangements to avoid a second fault at another conductor acting on persons with dangerous touch voltage.
  • The first fault changes an earthed network to a IT-network which is comparable to TN- or TT-network.
  • In case of insulation resistance is fallen to a low value dangerous for persons it is called an insulation fault

6. Corresponding to possible net configurations regarding the type of voltage there are available various types of appliance

  • pure AC-networks > ISO serie 3000, ISO serie 4000, ISO serie 5000
  • pure DC netzworks > ISO serie 6000
  • mixed netzworks > ISO serie 7000, ISO serie 8000


7. Insulation Monitor

  • continously monitoring of the insulation resistance during the operation
  • can activate an audible or visual signal when a predetermined desired value is fallen low

8. The ISO monitor is connected between the active conductor system and earth

  • AC-system > single-pole, alternatively > three-pole
  • DC-system > two-pole


9.ISO monitors with single-pole connection are monitoring all 3 phases in a 3-phase system with equal sensitivity

  • The internal resistances of feeding transformers and generators normally are at least 3 power of ten lower than the insulation resistances to be monitored and also negligible
  • ISO monitors with three-pole connection are equivalent comparable with the devices with single-pole connection


10. Depending on the type of voltage of the monitored network different measuring methods are used to determine a insulation fault; a possible classification is the distinction between active and passive measuring method.

  • ISO 3000, 4000, 5000
  • actice measuring method superposition of a positive DC voltage
  • ISO 6000
  • actice measuring method unbalanced method
  • ISO 7000, 8000
  • actice measuring method superposition of a clocked DC voltage

    Connection AC area > ISO 7000, Connection DC area > ISO 8000


    10.1. Superposition of a positive DC voltage

    • The measuring DC voltage is generated in the ISO monitor and is superposed over the low resistive feed-in between all phases and earth
    • The numerical value of measuring generated by measuring voltage is a measure of the internal resistance with a constant internal resistance of ISO monitor
    • The leakages of capacity of network do not affect the measuring after the transient time; they are charged to the measuring DC


    10.2. Measuring of unbalance

    • The measuring method applies the principles of voltage unbalance
    • Balanced insulation faults cannot be detected


    10.3. Superposition of a clocked DC voltage

    • A measuring DC voltage of alternating polarity is generated in the ISO monitor and is superposed by the low resistive feed-in between both phases and earth
    • look at Pos. 10.1: The numerical value of measuring currently generated by the measuring voltage is a measure of the insulation resistance with a constant internal resistance of ISO monitor.
    • The leakages of capacity of network affect the measuring in contrast to item 10.1; in industrial plants the leakages of capacity change permanently due to aging and/or modification or expansion of the electrical system. Changed leakages of capacity can
    • Leakages of capacity of network can be 100 µF and more in extensive networks; based on practical values 2 versions are available with regard to leakages of capacity at serie 7000 and 8000 : 1.) 1 - 20 µF 2.) 20 - 100 µF


    11. In a galvanically connected IT-system it is allowed to install only one ISO monitor at above mentioned active measuring methods because of mutual interference.


    12. ISO monitors of series 3000, 4000 and 5000 can also be used in mixed IT networks e.g. in three-phase networks with galvanically connected DC loads;


    13. ISO monitors can also be used to monitor disconnected net areas in grounded electrical systems. The ISO monitoring must be disconnected automatically in case the grounded net area is added-on.


    14. By initial testing of low-voltage supply systems the insulation resistance has to be at least 500 kOhm in de-energized status according to DIN VDE 0100-610.


    15. For low-voltage supply systems operating the following values are valid with all connected loads:

  • Power systems
  • 50 Ohm / V DIN VDE 0105-100
  • Auxiliary systems
  • 100 Ohm / V DIN VDE 0100-551


    « back to overview