24-23-00
ALL ú ú 04 Page 9 ú Jan 28/02
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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757
MAINTENANCE MANUAL
When the power source load exceeds 90 kva, a switch in the BPCU closes a path to ground. This allows the GND GALLEY LOAD SHED relay to energize after a 3-second delay. (If the overload disappears within these 3 seconds, the relay will not energize.) Energizing of the load shed relay energizes the GALLEY GND LOAD SHED relay also. This removes power from the galley buses. (The galley ELCUs lose coil power.) The OFF lights in the UTILITY BUS switches do not come on after the galleys are shed. The GALLEY GND LOAD SHED and GND GALLEY LOAD SHED relays are in right miscellaneous electrical equipment panel P37.
(b)
The galley buses remain off until all hydraulic switches are off, or until an IDG comes on line (GCB closes). When an IDG comes on line, at least one bus tie breaker (BTB) opens. (To prevent parallel source operation.) This causes the R and L UTILITY BUS RESET relays (one shot relays) to reset the galley shed circuit. Manual reset, if desired, can be accomplished by cycling the ENG START LD SHED RESET circuit breaker (11R2) on P11. (If APU or external power load remains above the 90-Kva level, the galley buses will again be shed when the circuit breaker is closed.)
(c)
This load shed circuit was designed to prevent a source overload (and subsequent shedding), when essential loads are turned on just prior to engine start. (Examples of essential loads are hydraulic pumps, and fuel pumps.)
(5) Engine Start with APU (Galley and Utility Buses Shed)
(a)
This circuit is implemented by relays and interlocks external to the BPCU. Galley and utility bus power is interrupted by removing 28-volt dc power to the UBRs and galley ELCUs.
(b)
Engine start load shedding occurs when: 1) The airplane is on the ground or inflight, and 2) an engine is started using APU pneumatics (APU bleed air
valve control relay K23 is closed), and 3) the auxiliary power breaker (APB) is closed. (APU
generator supplying electrical power.) Since the IDG associated with an "off" engine is not operating, that IDG's generator circuit breaker (GCB) is open. Also, when the engine is started (and the above conditions are satisfied), the associated ENG START 2 relay energizes. The GCB and START relay positions provide a closed path for 28-volt dc to energize the UTIL BUS RESET relay of an "off" engine. Energizing the RESET relay(s) opens the path providing coil power for the UBR(s) and galley ELCU(s). All utility and galley buses that are not supplied by their associated IDG, are shed. The amber OFF light in the UTILITY BUS switch(es) will come on. However the L and R UTIL BUS OFF messages on EICAS are inhibited during engine start.
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24-23-00
ALL ú ú 05 Page 10 ú Jan 28/02
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL
(c)
The utility and galley buses are automatically reset. When the engine has started, the associated engine start relay opens. Also, when the IDG produces correct power quality, its GCB closes. These contactor actions remove 28-volt dc power from the UTIL BUS RESET relay(s). This once again closes the path that energizes the UBRs and ELCUs. Power is restored to the utility and galley buses.
(d)
If both engines are initially off, at the time of the first engine start all UBRs and galley ELCUs are de-energized. All utility and galley buses lose power. When the engine IDG comes on line (GCB closes), all UBRs and ELCUs are again energized. The operating IDG will supply its respective utility and galley buses. The APU generator supplies the other utility and galley buses. When the other engine is started, the utility and galley buses supplied by the APU generator are shed. The utility and galley buses being supplied by the operating IDG are not shed. After the second IDG comes on line, the shed buses are reset. Each IDG supplies its own utility and galley buses.
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