how to test a 3 phase transformer with a megger?

To test a 3-phase transformer with a megger, first ensure the transformer is de-energized and disconnected from the power supply. Connect the megger leads to the primary and secondary windings, testing each phase individually. Measure insulation resistance by applying the megger’s rated voltage (typically 500V or 1000V) for about one minute. Record the resistance values; they should be above 1 M?. A significantly lower reading indicates insulation deterioration, requiring further investigation or maintenance.