Method and apparatus for diagnosing bearing insulation impedance of a rotating electrical apparatus

A system diagnoses the operability of a rotating electrical apparatus such as a reactor coolant pump (RCP) motor. The RCP motor includes a rotatable shaft, a stator, a bearing assembly and a bearing insulator. The bearing assembly electrically contacts the rotatable shaft and the bearing insulator at least partially electrically insulates the rotatable shaft and the bearing assembly from the stator. The system includes a shaft transformer and voltage source for inducing a voltage in the rotatable shaft; a variable resistor in parallel with the bearing insulator having first and second resistances; an analog-to-digital converter for sensing a first voltage associated with the first resistance and a second voltage associated with the second resistance; and means for determining the operability or the impedance of the bearing insulator from the first and second voltages and the second resistance.