This test is performed during commissioning, preventive maintenance and after fault trips. The purpose of this test is to detect poor electrical connections, inter-turn shorts, gross core damage and winding problems. It provides information for mechanical assessment of transformer. On three phase transformers the results are compared between phases. The magnetising current will be much greater on the phase with the shorted turns.

In general the outer limbs of the core of a core type transformer will have similar magnetising currents whilst that of the centre limb will be slightly less. This is when no faults are present in the transformer. The magnetising current method is much more sensitive to use when seeking shorted turns and should be used to confirm the results of the turns ratio test.