How do you verify proper phase rotation when connecting a transformer bank?

• A. Use phase rotation indicators or measurement of phase angles to confirm the correct vector group.
• B. Rotate the windings manually until the colors match.
• C. Phase rotation cannot be checked until after energizing.
• D. It does not matter for a transformer bank.

Answer: (A)

Verifying proper phase rotation ensures the three-phase voltages appear in the correct sequence at the connection point, which is essential for downstream equipment and protective relays to operate in the intended direction and timing. The reliable way to do this is to use a phase rotation indicator or to measure the phase angles with a suitable meter and confirm the observed sequence matches the transformer's intended vector group (for example, the expected A-B-C order on the connected side). If the observed rotation is off, adjust the winding connections or tap configurations so the sequence aligns with what the system requires before energizing. This pre-energization check prevents motors from running in wrong directions and avoids relay or protection miscoordination. Rotating windings by hand is unsafe and not how phase rotation is set; phase rotation cannot be properly verified after energizing, and the idea that it doesn’t matter is incorrect.