How do you verify proper phase rotation when wiring a generator to a bus?

• A. Use a phase sequence indicator to confirm the correct phase order before closing the breaker.
• B. Measure the temperature of the windings to determine rotation.
• C. Listen to the generator hum to deduce phase order.
• D. Check the color of wires to determine phase sequence.

Answer: (A)

Verifying phase rotation is about making sure the generator’s output arrives on the bus in the same phase order as the bus. When you parallel a generator with a live bus, the voltages must rise in the correct sequence (A, B, C) so the phase relationships line up and there’s no circulating current or uneven loading that could stress equipment.

Using a phase sequence indicator gives a direct, reliable check of the order of the three output lines and confirms it matches the bus. By verifying the correct sequence before closing the breaker, you ensure proper paralleling and minimize risk of damage or instability on the system. If the sequence isn’t right, you can swap two leads on the generator (or rewire) to correct it and recheck.

Other methods don’t reliably reveal phase order: winding temperature doesn’t indicate rotation, listening to the hum isn’t a valid diagnostic of phase sequence, and wire color alone isn’t a dependable indicator since codes can vary. The essential practice is to confirm the phase sequence with a phase sequence indicator before connecting to the bus.