Low Voltage AC - Instrument Bus

Question 1

What is the purpose of the Instrument Bus?

Answer 1

Provide electrical power for instrumentation & controls which require reliable 120vAC

Question 2

What are the normal power supplies for Instrument Bus?

Answer 2

MCC 28-2 (38-2) - U2 has 2 transformers off it

Question 3

What is the alternate power supply for Instrument Bus?

Answer 3

MCC 25-2 (35-2) through reserve instrument and RPS Tranformer

Question 4

How does the Instrument Bus transfer from normal to reserve power?

Answer 4

Normal Seeking ABT

Question 5

What type of protective relaying is on the Instrument Bus?

Answer 5

Undervoltage relay - low voltage alarm in control room

Question 6

What happens on a loss of Instrument Bus?

Answer 6

RMCS Lost
All 24/48vdc chargers lose power
SBLC lights, squib continuity, pressure, flow, and level indicators lost
SIV - loss of power to test solenoid and indicating lights for SIV vent and drains (valves stay open)
Loss of power to local HCU accum. panels and directional control valves
ANNUN DC PWR FAIL Annunciator lost
IB feed to ATWS Div I Panel (also fed from 125vdc 2A-1
Inboard and outboard MSIV AC solenoids lose power and indicating lights, but MSIV say open (DC solenoids still powered)
Select power for SRMs/IRMs lost
FWLC lost (Auctioneered with ESS Bus)
Channel “B” APRM/IRM/RBM recorders fail as is
RFP Min Flow valves loose one of powr supplies (Auct. with ESS Bus)

Question 7

TS: When must Inst. Bus be energized?

Answer 7

Modes 1, 2, and 3

Question 8

What is required for Inst. Bus to be Operable?

Answer 8

Fed from Div I ESS bus (EDG Powered) - i.e. if 25-2 is supplying bus, it is INOP