In a three-phase transformer bank, polarities must be the same across all phases.

• A. True
• B. False
• C. Not Required
• D. Sometimes

Answer: (A)

Polarity alignment across windings is essential to preserve the correct phase relationships in a three-phase transformer bank. Each winding has a polarity mark (dots) that shows which end is positive at a given instant. When wiring the bank, you connect the ends with the same polarity so the voltages from all three windings rise and fall together, producing the intended 120-degree phasor separation and the correct phase sequence.

If one winding is connected with opposite polarity, that leg becomes inverted relative to the others. This disrupts the vector diagram, can reverse the phase sequence, and may cause circulating currents between windings or through the neutral, leading to overheating, nuisance tripping, or improper operation of connected equipment. Because keeping polarities the same ensures the bank behaves as a coherent three-phase source, polarities must be the same across all phases.