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For AC Induction motors, winding failures can start and end as turn and coil shorts that do not break through the ground-wall insulation, regardless of the root-cause of the failure. Therefore, if you are only performing an insulation to ground test then you will miss these types of faults. Insulation to ground tests only detect resistive paths between the stator core and the conductors adjacent to the stator core.
Motor Circuit Analysis (MCA) is a de-energized, non-destructive test that evaluates the condition of the motor connections, stator, and rotor. MCA testing can be performed from the output side of the motor starter or motor drive, therefore, no need to open up and disconnect phase leads at the motor for routine testing purposes.