5.1 Preparations for installation
Limit values for the stator winding insulation resistance
The following table specifies the measuring voltage and limit values for the insulation
resistance. These values correspond to IEEE 43‑2000 recommendations.
Conversion to the reference temperature
When measuring with winding temperatures other than 40° C, convert the measuring value to
the reference temperature of 40° C according to the following equations from IEEE 43-2000.
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.
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.
● 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
Stator winding insulation resistance at 40° C
U ≤ 1000
1000 ≤ U ≤ 2500
2500 < U ≤ 5000
5000 < U ≤ 12000
U > 12000
= rated voltage, see the rating plate
= DC measuring voltage
= minimum insulation resistance at reference temperature of 40° C
even higher values. An insulation resistance value close to the minimum value could be
due to moisture and/or dirt accumulation. 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. Calculate the critical insulation resistance value depending on
the rated voltage by multiplying the rated voltage (kV) by the specific critical resistance
value. Convert the value for the current winding temperature at the time of measurement,
see above table.
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
500 (max. 1000)
1000 (max. 2500)
2500 (max. 5000)
5000 (max. 10000)
SIMOTICS TN Series N-compact 1LA8
Operating Instructions 08/2017