From AllTestPro literature:
Surge testing requires the application of high voltages and currents, and can be a destructive test, actually generating faults. This aspect, combined with the size of surge testing equipment makes it inappropriate for troubleshooting and predictive maintenance testing, especially on larger motors.
In a healthy 3-phase motor, the windings are electrically “balanced”. These balanced electrical characteristics include passive properties, such as resistance, and active properties, such as inductive reactance and impedance. As faults develop, one or more of these electrical properties will change, depending on the type and magnitude of the fault.
ALL-TEST instruments are unique in that they perform a series of impedance-based tests (inductance, phase angle, impedance, and a current / frequency ratio) on each winding. These measurements and their balance across the three phases provide a clear picture of the condition of the motor circuit. This test and analysis method will quickly indicate the presence of a fault, even at its earliest stages. The tests are preformed at low AC voltages, so there is no stress to the windings.
ALL-TEST methods are static measurements, meaning that they are made on un-energized motors. In this regard, they are useful not only for predictive maintenance activities, but also for checking new, recently repaired, or failed motors.