Tests that take place aren't destructive.
Megger - IEEE43 -2000
A Megger used to measure the insulation resistance of wires and coils , can be compared to ground or between phases , and we can find the index of polarization.
Hipot - IEEE95
This is a dielectric strength test , this is used for high voltage insulation submit much higher than would be found in normal operation. This voltage is subjected for a specified time and this can check the integrity of the insulation by monitoring the resulting leakage current . This test can be performed on AC or DC.
Balance impedances and phase angles - IEEE120 , IEEE389
This allows the use of the latest technology in measurement equipment to detect the presence of contaminants by the capacitance variations in the coils and the existence within these incipient shorts.
Resistive Balance - IEEE118
Allows you to determine the condition of the joints within the engine . Usually these values are very low and it is necessary the use of adequate equipment.
Surge - IEEE522
To verify the adequacy of insulation subjected to high voltage winding controlled according to established standards. Reference standards used in testing : IEEE1415 , IEEE112 , IEEE115 , Nema MG1 , EASA AR100
Polarization Index
It is the relationship between the measured insulation resistance to a minute and 10 minutes after applying a test voltage . A low polarization index show us that there is a high leakage current caused by dirt and moisture.