2018 Q60 — LOOP / EDG Output Breaker / RHR SEC Mode

✓ C. Correct. For Part 1, when the LOOP occurs at 12:00, 2A and 2B 4KV Vital Buses will sense an Instantaneous UV condition which will cause ALL three SECs (2A, 2B and 2C) to enter Mode II - Blackout Only (2/3 Vital Buses sensing Instantaneous UV condition). Since the 2C SEC has gone into Mode II, this will cause the running 2C EDG Output Breaker to OPEN. As the SEC 2C continues its sequencing, the 2C EDG Output Breaker will reclose. For Part 2, 21 RHR Pump is powered from 2A 4KV Vital Bus. With 2A SEC in Mode II, the 21 RHR will NOT be automatically loaded (during the 2A SEC sequencing). Consequently, at 12:15 (after SEC 2A has completed its sequencing), 21 RHR pump is stopped.

✗ A. Incorrect but plausible. For Part 1, since 2C EDG was already running loaded during the LOOP, the candidate could incorrectly conclude that 2C EDG would remain closed as SEC 2C strips the loads off of 2C 4KV Vital Bus and then automatically loads the blackout loads. Part 2 is correct.

✗ B. Incorrect but plausible. For Part 1, since 2C EDG was already running loaded during the LOOP, the candidate could incorrectly conclude that 2C EDG would remain closed as SEC 2C strips the loads off of 2C 4KV Vital Bus and then automatically loads the blackout loads. For Part 2, the RHR pumps are automatically sequenced on their respective Vital Buses when the SECs are in Mode I (SI only) and Mode III (SI & Blackout). Consequently, the candidate could incorrectly conclude that following a LOOP that an RHR pump that had been running in Shutdown Cooling will automatically restart after the SECs complete their sequencing.

✗ D. Incorrect but plausible. Part 1 is correct. For Part 2, the RHR pumps are automatically sequenced on their respective Vital Buses when the SECs are in Mode I (SI only) and Mode III (SI & Blackout). Consequently, the candidate could incorrectly conclude that following a LOOP that an RHR pump that had been running in Shutdown Cooling will automatically restart after the SECs complete their sequencing.

Ref: NOS05EDG000-12 (Emergency Diesel Generators) | LO: NOS05EDG000-12 | Source: New | Cognitive: Higher