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A 737-300/400/500MAINTENANCE MANUAL
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A 737-300/400/500MAINTENANCE MANUAL
IRS XFR RLY 2(E1-2)
IRS Transfer ControlFigure 13
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A
737-300/400/500MAINTENANCE MANUAL
14. IRS Power Interface (Fig. 14)
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A. Power for the IRS system is derived from the standby bus, electronics power No. 2 bus, 28-volt dc bus No. 1, and switched hot battery bus. Control is through the electronic circuit breaker panels P18 (for system No. 1) and P6 (for system No. 2).
B. The system is energized by closing all IRS and DAA circuit breakers on the two circuit breaker panels. System power includes 115-volt ac primary power and 28-volt dc backup power.
C. When airplane power fails, the IRU automatically switches to battery power. IRS No. 2 stays on battery power for a maximum of 340 |40 seconds (5 minutes) and turns off if primary power is not restored within this period. IRS No. 1 stays on battery power indefinitely.
NOTE: 340 seconds was chosen to meet the requirement of 5 minutes
____ minimum. Because the time delay relay has a tolerance of |10%, the actual delay is between 5 min 6 sec and 6 min 20 sec.
D. A power supply within the IRU furnishes operating power for the MSU and ISDU. It supplies two status indication signals for the MSU, mode selection signals to the input electronics and a power-up initialization signal for the entire IRS.
15. IRS Fault Sensing (Fig. 15)
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A. When a fault is detected in the IRU, dc from the IRU illuminates the fault light on the MSU and also applies high logic to the master caution unit. After a brief time delay, master caution unit logic applies a ground to the master caution lights through the annunciator and dimming module, thereby illuminating the lights.
B. The lights will also illuminate if the battery current detector senses that the IRU is on hot battery bus power, or if the dc monitor senses a level of less that 18 volts.
C. The lights are extinguished by depressing either master caution light lens assembly. A recall can be made by pressing the left or right master caution light lens assembly a second time, provided that the fault still exists.
16. IRS Cooling System
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A. It is important that inertial reference units have an adequate cooling air supply since overheating will result in failure of the IRU's.
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