Polarity (mutual inductance)

These markings may be found on transformer cases beside terminals, winding leads, nameplates, schematic and wiring diagrams.

Reversed connections of paralleled transformer windings will cause circulating currents or an effective short circuit.

Unlike single-phase transformers, three-phase transformers may have a phase shift due to different winding configurations (for example, a wye connected primary and a delta connected secondary), resulting in a multiple of 30 degree phase shift between H1 and X1 bushing designations.

Where markings have been obscured or are suspect, a test can be made by interconnecting the windings and exciting the transformer, and measuring the voltages.

This may be in the form of a phasor diagram, or using an alpha-numeric code to show the type of internal connection (wye or delta) for each winding.