Wenglorz - Dipl. Ing. Helmut Wenglorz GmbH

Isolationsüberwachung nach VDE-DIN-EN-IEC

The fundamental requirements at insulation monitors for the application in IT-networks for AC up to 1.000 V and DC up to 1.500 V regarding the electrical and mechanical design and regarding the test and documentation are defined in DIN EN IEC 61557-2 and DIN EN IEC 61557-8 valid starting from 11.03.1997

In VDE 0100 part 300/IEC 364-3 are fixed 3 main configurations of electrical systems.

IT-Net The live lines are not directly connected to system ground; the bodies of the electrical system are grounded.
IN-Net The neutral point of the live lines is connected to system ground; the bodies of the electrical system are connected via the protection conductor to system ground.
TT-Net The neutral point of the live lines is connected to system ground; the bodies of the electrical system are grounded, however separately the system ground.


The AC IT-Netz is different from the TN and IT network substantially in the electrical treatment of the neutral point in connection with the bodies of electrical system. By definition insulation monitors may be used only in IT-nets. The insulation monitor has been designed for a continous insulation monitoring in the IT-network. The essential features of an error-free IT network are:


  • No point of the live system is connected to the protective conductor (system ground) directly.
  • All bodies of the electrical system are grounded.


The insulation monitor is connected (AC one-phase or three-phase, DC two-phase) between the live line system and ground. Continually the ISO monitors the electrical system regarding insulation resistance RF, that means any change in the insulation levels is recorded as soon as possible in the birth and is evaluated by a fail-safe electronics. So the ISO is an important tool for preventive maintenance.


Advantages of IT-network
Thus also a creeping failure is determined by aging, humidity etc. continuously. The user can specify via a message with an acustic or visual signal, in which insulation resistance the network is to be repaired. This should be initiated as soon as possible. As long as only a one-phase insulation fault is present, an operating can be continued as fuse and over current equipment upstream to the consumer do not trigger because of the low measuring current (fault current).


DIN VDE 108 Part 1: Among to the protective measures

  • a) protective insulation,
  • b) protective low voltage
  • c) functional low voltage
  • d) protective electrical separation


the protection with insulation monitors in the IT network to DIN VDE 0100 part 410 is preferably applied. Electric power systems can be distinguished and classified by various criteria; in connection with insulation monitors it is obvious to take as a basis the montored networks as a distinguishing feature, because it is necessary tu use different measuring methods in different networks.

Depending on the monitored network and the connected loads the following basic methods of measurement are preferred, which often must be modified to accommodate specific network characteristics.


AC-System Active measuring method superimposed DC
DC-System a) Passive measuring method unbalanced voltages
b) Active measuring method superimposed clocked DC
AC/DC-System Active measuring method superimposed clocked DC
connection possible to AC or DC


AC-Systems: Superposition of DC on monitored network
Apllication Pure AC systems and AC systems with electrically connected DC consumers
Connection One-phase or three-phase to AC-systems
Funktion A DC measuring voltage is superimposed on monitored electrical system, the size of measuring current is a measure of the insulation resistance of network: The larger the measuring current the smaller the insulation resistance (internal resistance of ISO Monitor = const.).
One-phase The insulation resistance of error-free, ungrounded electrical systems including all resistances of
Connection electrical equipment against ground is in the 3-digit range kOhm or in the range MOhm usually,
dependent on nominal voltage. In contrast the internal resistance of transformers and generators are lower approximately 3 powers of ten and more so that these internal resistances are negligibly small compared with the insulation resistances to ground of an IT-network.Thus, non-connected lines are monitored with the same
accuracy as the connected live line considering specified conditions and the allowed tolerances
DC Consumer Insulation faults at DC-consumers, for example galvanicly connected to the AC circuit across a bridge
circuit, distort the measuring current in the measuring arrangement. In a first approximation, it is
generally accepted that insulation faults are reported on the DC side with increased sensitivity.
The difference to values as specified in the data sheets , which are valid only for pure AC systems is
dependent on the quality of rectification.
Thus it is possible, that different response values occur in the same measuring arrangement,
dependent on wether the rectifier is a simple bridge circuit or as high-quality smoothed rectification.
The same is valid if the DC consumer is a resistive or inductive load.
DC-Systems: Unbalanced voltages circuit = Bridge circuit
Application Pure DC networks
Connection two-phase to L+ and L-
Passive measuring method A one-sided insulation fault causes unbalance voltages between the live lines and ground which are evaluated by the ISO and reported.
Note Exactly symmetrical faults to earth are not reported.
AC-, DC- and mixed AC/DC-Systems: Clock synchronous
Application AC networks with downstream, galvanicly connected DC loads, connection to AC side; DC networks with upstream, galvanicly connected AC networks, connection to DC side.
Connection 1) DC network 2-phase 2) AC network 1- or 3-phase
Function A square-wave AC measuring voltage is superimposed on the monitored network. The insulation resistance is alternately recognized between L+ / PE and between L- / PE across the measuring current and analyzed. The capacities of the network must be considered in determining of the frequency of test voltage, because they affect the measuring time.
Connection to AC side as above, but with changed input electronics
Note Insulation faults are reported in both applications on the AC and DC side with the same accuracy.


Used terms in the data sheets

ISO-Monitor Test Resistance RP System Leakage Capitance CE
Devices for continuous monitoring of
insulation resistance in IT-systems.
Development and production:
Dipl. Ing. Helmut Wenglorz GmbH
Test of ISO-electronic:
By operating the test button
a test resistor is connected
between UN and ground.
Max permissible total capacity of the network
up to the ISO monitor works as intended.
Extraneous D.C. Voltage UFG Internal Impedance Zi Response Time TAN
DC in AC networks between live lines
and ground caused by DC components.
Total impedance of ISO monitor
between UN and ground at 50 Hz
Time the ISO monitor needs to respond
under defined conditions.
Insulation Resistance RF Internal D. C. Resistance Ri Relative Percentage Uncertainty
Total impedance of ISO monitor
between UN and ground at 50 Hz
Reactance of ISO-monitor between
UN and ground at 50 Hz
A (%)      = RA - RAN   x  100 %
Specified Sponse Value RAN Measuring Voltage UM General
Set or adjustable value of insulation resistance RF at ISO, which is monitored on below.
Voltage, which is during the measurement between the measuring connections.
(Excerpt from DIN EN IEC 61557-8) Insulation monitors must be able to detect symmetrical and asymmetrical faults to earth. Monitoring devices, which evaluate as measuring
criterion only occurring in the insulation faults unbalanced voltages, are no insulation monitors according to DIN EN IEC 61557-8
Response Value RA Measuring Current IM
Insulation resistance RF in which the ISO monitor responses under defined conditions
DIN EN 0100 Part 610 usually at least 100 / V.
Maximum current between the electrical system and ground that can flow from the ISO, limited by the internal resistance Ri.


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