Question 58 QMED02 - Electrician-Refrigerating Engineer

The Correct Answer is B. ### Explanation for why Option B is correct: The method described (using synchronizing lamps) is a classic technique for visual synchronization. When paralleling two AC alternators (or an alternator to a live bus), synchronization requires three conditions to be met simultaneously: 1. **Equal Voltage** 2. **Equal Frequency** 3. **Correct Phase Sequence and Phase Angle (ideally 0° phase difference)** The synchronizing lamps are connected across the circuit breaker poles such that when the voltages of the running bus and the incoming alternator are exactly in phase ($0^\circ$ difference), the voltage across the lamps is zero. * When the voltages are $180^\circ$ out of phase, the voltages add up, causing the lamps to be at maximum brightness (fully lit). * When the voltages are in phase ($0^\circ$), the potential difference across the lamps is zero, and they are completely dark. The statement says the lamps "grow dim and are totally darkened as the synchroscope pointer approaches the $0^\circ$ position." This indicates that the phase angle between the incoming alternator and the bus is approaching and achieving zero degrees. This exact moment (lamps dark, synchroscope at $0^\circ$) is the precise condition required for successful synchronization, meaning **the circuit breaker can be closed** to connect the alternator to the grid or bus. ### Explanation for why the other options are incorrect: **A) the synchroscope is defective or broken:** This is incorrect. The synchroscope is corroborating the lamp indication (pointer approaching $0^\circ$). If the synchroscope were broken, it would likely be erratic or stuck, but its reading is confirming the ideal phase condition shown by the lamps. **C) the incoming alternator is running too slowly:** This is incorrect (or at least incomplete). The rate at which the lamps dim and brighten (the beat frequency) indicates the difference in speed (frequency). While the alternators *must* be running at a slightly different speed (to observe the phase shift and wait for $0^\circ$), the *total darkness* indicates that the frequencies are momentarily equal *and* the phase angle is $0^\circ$. If the incoming alternator were running "too slowly," the phase angle would continuously shift away from $0^\circ$ immediately after the brief moment of darkness, but the current indication is that the exact moment for closure has arrived. **D) the alternator voltages are 180° apart:** This is incorrect. If the voltages were $180^\circ$ apart (opposite phase), the lamps would be at their **maximum brightness** (fully lit), as the voltage across them would be the sum of the two alternator voltages. Total darkness signifies $0^\circ$ phase difference.