USCG Exam PracticeAC circuits and theory
When paralleling two shipboard alternators, the operator adjusts the incoming machine's governor to match frequency and uses the synchroscope to verify phase. At which synchroscope position should the breaker be closed?
- A. When the pointer is stationary at the six o'clock position
- ✓ As the pointer creeps slowly through the twelve o'clock (in-phase) positionCorrect
- C. When the pointer rotates rapidly clockwise, indicating the incoming machine is faster
- D. At any position, provided voltage and phase sequence have already been matched
Why B is correct
NEETS Mod. 3 §1-4 specifies closing the breaker as the synchroscope creeps slowly through its in-phase (twelve o'clock) position. Closing at six o'clock means the machines are 180° out of phase, which would produce the violent current surge described in NEETS Mod. 5 §3-2. Rapid rotation indicates a significant frequency mismatch and the breaker must not be closed.