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Your motor insulation has a negative temperature coefficient. This means that as the motor temperature rises, like in normal operations, the insulation resistance to current flow reduces. If insulation temperature goes down, like after a shutdown, the resistance goes up. This change can be dramatic. For every 10-degree Celsius rise in temperature, the resistance is cut in half. For every 10-degree drop in temperature, the resistance is doubled. Be careful that you are not trying to trend insulation resistance without performing a temperature correction of the readings. Performing a polarization index test during a significant temperature change will provide additional data for evaluation.
To view a case study showing an insulation integrity problem, visit the PdMA Corporation Web site at http://www.pdma.comand click on the Insulation Fault Zone button.