SALEM
SALEM STUDY SYSTEM
Systems > DC Power

DC Power

⚠️ DRAFT

DC Power

Function

Provides DC electrical power for control functions, instrumentation, diesel generator starting, reactor trip breaker operation, and emergency lighting. (UFSAR 8.3.2)

125V DC System

  • Three 125V battery banks with associated chargers per unit
  • Provides control power for: switchgear operation, diesel generator field flashing and starting, vital instrumentation, emergency lighting, PORV and block valve control
  • Station batteries located in separate rooms within Seismic Category I structures
  • Battery chargers maintain batteries on float charge during normal operation (UFSAR 8.3.2.1, 8.3.2.2)

250V DC System

  • Provides power for turbine turning gear and other large DC loads (UFSAR 8.3.2.1)

28V DC System

  • Three 28V battery supplies per unit
  • Provides power for reactor trip breaker undervoltage coils
  • Loss of 28V DC supply causes reactor trip breaker to open (fail-safe design) (UFSAR 8.3.2.1)

Station Battery Capacity

Exam — 2023 Q9
125V DC vital batteries are rated for 2 hours during worst-case loading (no load shed) following a Loss of All AC Power. With manual load shedding per EOP-LOPA-1, battery life extends to 4 hours (SBO coping duration). Loss of All AC Power is NOT a Design Basis Accident.
ConditionBattery DurationSource
Worst case, no load shed2 hoursVTD 314204
With EOP-LOPA-1 load shedding4 hours (SBO coping duration)VTD 314204
Exam — 2019 Q22
During a STATION BLACKOUT with vital batteries discharging at their MAXIMUM rate, the vital batteries will supply DC power to emergency equipment for AT LEAST 2 hours. Trap: 4 hours is the SBO coping duration WITH load shedding per EOP-LOPA-1 — the 2 hour value is the minimum at maximum discharge rate without load shed.
Exam — 2020 Q49
Battery discharge indication on 2RP9: during normal "float charge" mode, volt meters read ~134 VDC and amp meters read 0 amps (charger supplies all loads). On loss of all AC power, batteries begin discharging — indicated by rising amps (> 0) on 2RP9 amp meters (NOT by voltage rising above zero). By design, batteries are sized for 2 hours of operation after loss of AC power. Trap: volt meters do NOT read 0 during normal operation — they read ~134 VDC. Amp meters read 0 because the charger supplies loads. Discharge is indicated by amps rising, not volts rising. Also: 2 hours (no load shed) vs 4 hours (with EOP-LOPA-1 load shed) — do not confuse the two durations.

Ground Detection

Exam — 2023 Q46
125VDC ground detection: resistance ≤50K ohms = ground detected; >50K ohms = normal (no ground). Per S2.OP-SO.125-0004, when a ground is detected, breakers are opened one at a time until resistance reads >50K ohms, isolating the ground source. Breakers are NOT all opened at once.

Bus Transfer Interlocks

Exam — 2023 Q47
125VDC distribution includes a mechanical interlock preventing normal and backup bus paralleling. The interlock requires the normal supply breaker to be OPEN before the backup (emergency) breaker can close. Transfer to backup DC bus is ONLY permitted during Modes 5, 6, or defueled — paralleling in Modes 1-4 would make DC buses inoperable.
Exam — 2022 Q15
Normal 125VDC bus with charger supplying: ~134 VDC, 0 amps on 2RP9 (battery floating). Battery supplying: lower voltage, amps > 0. OHA B-2 (125VDC CNTRL BUS VOLT LO) alarms at ≤128 VDC. To remain at power: manually align standby (2A2 125 VDC) battery charger. DC bus cross-tie only permitted in Modes 5, 6, or Defueled.
Exam — 2022 Q48
Loss of 2C 125 VDC: all breakers powered by 2C DC remain AS-IS (no automatic trips or closures — DC control power is required to operate breaker trip and close coils). EDG will NOT start (DC starting circuits de-energized). Fast bus transfer to alternate SPT prevented (interlock requires other SPT infeed breaker open, but 24CSD cannot open without DC). Breakers do not trip open on loss of DC — they remain in their last position. This is opposite of 28VDC RTB behavior (RTBs trip open on loss of 28VDC).
Exam — 2022 Q49
125 VDC battery charger transfer: standby charger (2A2) located in 84 ft Switchgear Room (same as normal charger). Transfer sequence requires no-parallel — must de-energize normal charger (2A1) output FIRST, then energize standby charger (2A2). A and B 125VDC batteries are in the 100 ft Relay Room, but chargers are in the 84 ft Switchgear Room — do not confuse locations. Paralleling chargers is prohibited in Modes 1-4 to prevent cross-connecting 230 VAC Vital buses.
Exam — 2020 Q13
Loss of 2A 125 VDC Bus loads: 2A EDG is NOT available for start (EDG starting circuits require 125 VDC). #1 SGFP Emergency Oil Pump and Main Turbine Emergency Oil Pump are NOT powered from the 2A 125 VDC bus — they are powered from other DC voltage levels (250 VDC or 28 VDC). All three malfunctions listed (SGFP Emergency Oil Pump, Turbine Emergency Oil Pump, EDG start) can be caused by loss of SOME DC bus, but only the EDG start is affected by loss of the 2A 125 VDC bus specifically. Know which loads are on which DC bus voltage level.
Exam — 2019 Q59
Confirms 2020 Q13: with OHA B-2 "2A 125 VDC CNTRL BUS VOLT LO" and 2A Vital 125 VDC Bus at 0 VDC on 2RP9, the ONLY affected equipment from the listed malfunctions is 2A EDG is NOT available for start. #1 SGFP Emergency Oil Pump and Main Turbine Emergency Oil Pump are powered from 250 VDC or 28 VDC — NOT from the 2A 125 VDC bus. This topic tested on both 2019 and 2020 exams — know which loads are powered from each DC bus voltage level (125V, 250V, 28V).

Key Design Points

  • DC power is critical for safety functions — provides control power independent of AC
  • 125V batteries sized for station blackout coping duration (4 hours with load shedding)
  • Battery OPERABILITY verified by regular capacity testing
  • Loss of a 28V supply directly trips the associated reactor trip breaker

Tech Spec LCOs

  • TS 3/4.8 — Electrical — TS 3.8.2.1 requires three 125V battery banks and three 28V supplies OPERABLE
  • 1 battery bank inoperable: 2 hours to restore
  • 1 charger inoperable: 24 hours (with spare charger)
  • 1 of 3 28V supplies inoperable: 48 hours
Exam — 2019 Q90
LCO 3.8.2.3 battery cell parameters: with 3 connected cells at Float Voltage of 0 Volts, battery does NOT meet Category B and C limits. Required Actions C (24 hrs to restore), D (2 hrs to restore), and F (6 hrs to Hot Standby if D not met) all apply simultaneously. D and F are most limiting: 2 hours to restore + 6 hours to Hot Standby = 8 hours maximum from time of discovery. Trap: Action C allows 24 hours for Category B/C limit restoration, but Action D (connected cells at 0V) is more limiting at only 2 hours. Candidates who use 24 hours get the wrong answer by one full day.
Exam — 2018 Q20
250 VDC system loads: the Main Turbine Emergency Oil Pump is powered from the 250 VDC Battery and Charger System. The following are powered from the 125 VDC system (NOT 250 VDC): Field Excitation Circuitry for the EDGs, Vital Instrument Bus Inverters, and Emergency Lighting Inverters. Know which loads are on each DC voltage level (28V, 125V, 250V). The 250V system primarily powers large DC motors like the turbine turning gear and the Main Turbine Emergency Oil Pump.
JPM — 2018 IP-i
Station Blackout 125VDC Load Shed per EOP-LOPA-1 Checkoff Sheet 3 (COS 3-1 thru 3-3). Operator must locate DC distribution panels across two elevations and open ONLY the specific breakers listed. 64 ft elevation: 2A 125VDC Bus, 2ADC 125VDC Distribution Panel, 2B 125VDC Bus, 2DDC and 2CDC 125VDC Distribution Panels. 100 ft elevation switchgear room: 2AADC, 2BBDC, 2CCDC 125VDC Distribution Panels. Field labeling discrepancies: 2ADC3AX29 labeled "Source #1 125 VDC to AVR Excitation Cab" (procedure calls it "2 Generator Exciter Voltage Regulator"); 2DDC2AX28 labeled "21SJ162, 2SJ166, 22SJ162, 22SJ166, 2SJ158" (procedure calls it "Safety Injection Test Line Valves").

Connections