AB.115-0001 — Loss of 115V Vital Instrument Bus
Purpose
Provides operator actions for responding to a loss of a 115V Vital Instrument Bus, including identification of affected components and required valve repositioning.
Key Actions / Information
Exam — 2022 Q31
RHR valve failure modes on loss of VIB: 21RH18 fails open (2A VIB), 2RH20 fails open (2D VIB).
Exam — 2020 Q12
On loss of 2A VIB: LT-112 (VCT Level) loses indication and alarms, PI-139 (VCT Pressure) loses indication, and VCT makeup is unavailable. After restoring letdown with 2CV35 in MANUAL FLOW TO VCT: LT-114 is only available in the control room via the plant computer (not the control console). VCT overpressure protection is provided by 2CV241, VCT relief valve (75 psig setpoint), relieving to CVCS HUT. With LT-112 controller in manual, LT-114 auto function is overridden and will NOT automatically divert 2CV35.
Connections
- Related systems: RHR, 115V AC, CVCS
- Related exam questions: 2020 Q12, 2022 Q31
- Related exam: 2020 NRC Written Exam, 2022 NRC Written Exam