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Entry type: FAQ Entry ID: 39215191, Entry date: 12/22/2009

Significance of the insulation resistance

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What is the significance of the insulation resistance?







The insulation resistance is measured to check the condition of the insulation. The insulating power of the voltage-carrying parts are tested against ground with the winding connected, and against each other with the winding open (see figure below).

The insulation resistance is highly dependent on the moisture film on and the moisture content in the insulation, as well as on possibly conductive soiling.


Information from the Siemens operating instructions, which are based on many years of operational experience:




Minimum insulation resistance at a winding temperature of 25 °C


 Rated voltage

 UN < 2 kV

 UN > 2 kV

Measurement voltage / time

 500 V / 1 Minute

Minimum insulation resistance of new, cleaned or refurbished windings

 > 10 MΩ

 > 100 MΩ

Critical, specific insulation resistance after a long operational life

 0.5 MΩ/kV

 5 MΩ/kV



    The temperature is a major influencing variable on the insulation resistance of the insulation investigated.

Rule of thumb:

Each 10 °C rise in temperature halves the resistance.


In order to make insulation resistance measurements comparable, they should be converted to a single reference temperature, for example 25 °C.

This is done by multiplying the value measured at any given temperature by the conversion factor k25°C applicable to that temperature.

    Factor for converting an insulation resistance to its equivalent value at a reference temperature of 25°C
Measured Rins.55°C = 10 MΩ;
then Rins.25°C = 10 MΩ x 8 = 80 MΩ




  • If insulation resistance reaches or falls below the critical level, the windings must be dried, or the rotor must be removed so that the windings can be thoroughly cleaned and dried.

  • After the cleaned windings have been dried, it must be noted that the insulation resistance of a hot winding is lower. The insulation resistance can only be correctly assessed after the measurement has been converted to the value at the reference temperature of 25 °C.

  • If the measured value is near the critical value, the insulation resistance should subsequently be checked at correspondingly short intervals.

  • A voltage test may only be made if the insulation resistance is adequately high. The safety rules according to EN 50110-1 (VDE 0105) and the respective operating instructions of the measuring instruments must be observed during the measurement of the insulation.



Measurement sequence

  • Remove the connection cables before starting measurement, as otherwise major measuring errors must be expected.

  • Clean and dry the terminal compartment, terminal boards and terminals.

  • The measuring instrument is connected between the winding and ground. The other windings and all the poles of fitted winding temperature sensors and mounting parts are connected to ground.

  • Rotor windings are measured against the shaft.

  • The measuring time is approx. 1 minute.

  • The winding must be discharged when measurement has finished.
    With large machines, a discharge resistor must be used for this purpose.




Measuring instruments used:

Insulation tester (megger) or highly stable DC source and microammeter




Safety instructions

In some cases, the terminals carry dangerous voltages during and immediately after the measurement, and they must not be touched.

After the measurement, the terminals should always be bridged to discharge any remaining dangerous voltages.

If supply cables are connected, ensure that a voltage cannot be applied.




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