When a winding fault develops, the fault can behave like an unintended load. For example, if a short develops between turns on a winding, current can circulate through the shorted turns. That fault current creates an opposing magnetic flux.

The test source must then supply additional current to maintain the magnetic flux in the transformer core. As a result, the excitation current increases.

This is why a turn-to-turn fault can show up as an abnormally high excitation current reading. Even when the turns ratio or winding resistance results appear acceptable, excitation current may reveal that something has changed internally.

Grounded windings can create a similar effect. If a transformer winding with a grounded neutral develops a fault to ground, the faulted portion of the winding may draw current. That current, in turn, produces an opposing magnetic effect, causing the excitation current to increase.

Excitation current testing is not intended to replace other transformer tests. Instead, it adds another diagnostic view. Winding resistance evaluates the conductor path resistance. Turns ratio evaluates the winding ratio of a transformer. Power factor evaluates the insulation condition. Excitation current evaluates the magnetizing behavior of the transformer.

When used together, these tests provide a more complete picture of the transformer's condition.

Tech-Tip Takeaway: A winding fault can act like an unintended load. That load causes additional current to flow, which can appear as increased excitation current during testing.