5.1 Preparations for installation
For a winding temperature of approx. 25° C, the minimum insulation resistances are 20 MΩ
(U ≤ 1000 V) or 300 MΩ (U > 1000 V). The values apply for the complete winding to ground.
Twice the minimum values apply to the measurement of individual assemblies.
Note the following:
● When measuring with winding temperatures other than 40 °C, convert the measuring value
● Dry, new windings have an insulation resistance of between 100 and 2000 MΩ, or possibly
● Over its operating lifetime, the motor winding insulation resistance can drop due to ambient
Measuring the polarization index
1. To determine the polarization index, measure the insulation resistances after one minute
2. Express the measured values as a ratio:
For insulation resistances > 5000 MΩ, the measurement of the PI is no longer meaningful and
consequently not included in the assessment.
to the reference temperature of 40 °C according to the following equations from IEEE
In this case, doubling or halving the insulation resistance at a temperature change of 10 K
is used as the basis.
– The insulation resistance halves every time the temperature rises by 10 K.
– The resistance doubles every time the temperature falls by 10 K.
even higher values. An insulation resistance value close to the minimum value could be
due to moisture and/or dirt accumulation. However, the size of the winding, the rated voltage
and other characteristics affect the insulation resistance and may need to be taken into
account when determining measures.
and operational influences. Depending on the rated voltage, the critical insulation resistance
value is to be calculated by multiplying the rated voltage (kV) by the specific critical
resistance value and then converted to the current winding temperature at the time of the
measurement, see previous table.
and ten minutes.
PI = R
insul 10 min
insul 1 min
Modern measuring devices display these values automatically following the measurement.
Insulation resistance converted to 40 °C reference temperature
Temperature coefficient according to equation (2)
Measured insulation resistance for measuring/winding temperature T
Reference temperature in °C
Halving / doubling of the insulation resistance with 10 K
Measuring/winding temperature in °C
Insulation in good condition
Dependent on the complete diagnosis of the insulation
SIMOTICS TN Series N-compact 1LA8
Operating Instructions 05/2014