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8.3
Checking the insulation resistance
After long storage or shutdown periods, the insulation resistance of the windings must be
measured to ground with direct voltage.
Danger to life through electric shock
During and immediately after the measurement, the terminals are in some cases at
hazardous voltages, which can lead to death when touched.
• Only check the insulation resistance if you are appropriately qualified to do this.
• Before measuring the insulation resistance, read the manual for the insulation resistance
• Never touch the terminals when making measurements or immediately after the
• Check the connected supply feeder cables to ensure that the line supply voltage cannot
Note the following requirements when you measure the insulation resistance:
• Always measure the insulation resistance of the winding to the motor enclosure when the
winding temperature is between 20 °C and 30 °C.
• When performing the measurement, wait until the final resistance value is reached (this
takes approx. one minute).
Limits
The table below specifies the measuring circuit voltage as well as the limit values for the
minimum insulation resistance and the critical insulation resistance with a rated motor
voltage of U
Stator winding insulation resistance at 25 °C
Measurement voltage
Minimum insulation resistance with new, cleaned, or re-
paired windings
Critical specific insulation resistance after a long operating
time
Note the following:
• Dry, new windings have an insulation resistance of between 100 MΩ and 2000 MΩ
(sometimes higher).
If the insulation resistance is close to the minimum value, this could be due to humidity
and/or an accumulation of dirt.
• The insulation resistance of the motor winding can drop during the course of its service
life due to ambient and operational influences. The critical insulation resistance for a
temperature of 25 °C on the winding can be calculated by multiplying the rated voltage
(kV) by the specific critical resistance value (0.5 MΩ/kV)
Example: Critical resistance for a rated voltage (V
0.6 kV × 0.5 MΩ/kV = 0.3 MΩ
1PH1 main motors
Equipment Manual, 09/2021, A5E51077676A-AA
WARNING
meter you are going to use.
measurement.
be connected.
< 2 kV.
N
8.3 Checking the insulation resistance
500 V (at least 100 V)
10 MΩ
0.5 MΩ/kV
) of 0.6 kV
N
Commissioning
93
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