2. Ground Handling Relay (Fig. 1)_____________________
A. There are two interchangeable ground handling relays (GHRs), which are located in the P714 panel in the main equipment center. The GHRs are 28-volt dc controlled, 3 pole, double throw, center off type relays. The No. 1 BCU controls these relays to select the power source for the ground handling bus. The two power sources are the No. 1 external power and No. 1 APU generator.
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MAINTENANCE MANUAL
GROUND
HANDLING
RELAYS
SEE C
SEE A
SEE B
GROUND
MAIN DECK CARGO SERVICE
HANDLING BUS RELAYS
RELAY 2 SEE D
SEE C
MAIN DECK CARGO
HANDLING BUS
RELAY 1
SEE C
P714 AUX/EXT
NO. 1 POWER CENTER
A
P715 AUX/EXT NO. 2
POWER CENTER
B
GROUND
SERVICE
GROUND HANDLING RELAY
AND MAIN DECK
CARGO HANDLING
BUS RELAY
NOTE: TERMINAL LEADS NOT D
C
SHOWN FOR CLARITY.
Main Deck Cargo Handling Bus, Ground Handling and Ground Service Relays Location
Figure 1
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MAINTENANCE MANUAL
3. Main Deck Cargo Handling Relay (Fig. 1)
______________________________
A. There are two interchangeable main deck cargo handling relays, which are located in the P715 panel in the main equipment center. The main deck cargo handling relays are 28-volt dc controlled, 3 pole, double throw, center off type relays. The No. 2 BCU controls these relays to select the power source for the ground handling bus. The two power sources are the No. 2 external power and No. 2 APU generator.
4. Ground Service Select Relay (Fig. 1)
___________________________
A. There are four interchangeable ground service select relays (GSSRs), which are located in the P714 panel in the main equipment center. The GSSRs are 3 pole, double throw type relays. The BCU No. 1 and the ground service switches on the flight attendants panel control the relays for selection of power for the ground service bus. The three possible sources of ground service power are as follows:
(1)
AC bus No. 1
(2)
APU generator No. 1
(3)
External power No. 1
5. Electrical Load Control Unit (Fig. 2)
____________________________
A. There are eight electrical load control units (ELCUs) used for load management and control. Two ELCUs are located in each of the P514, P614, P515, and P615 panels. The ELCUs are controlled by a combination of two utility switches in the electrical system control module on the P5 overhead panel and the two BCUs. The ELCU will automatically trip if a fault occurs internally or on a load bus within its load distribution system.
6. Instrument Bus Voltage Sense Unit (Fig. 3)
_________________________________
A. There are two instrument bus voltage sense units (IBVSUs) in the P6 power distribution panel. Two flight instrument transfer buses are provided to supply power to the captain's and first officer's flight instruments. These are normally connected to the ac bus 3 and ac bus 2 respectively. If power is lost to ac bus 3 or ac bus 2, then the affected instrument bus will automatically transfer to ac bus 1 as an alternate power source. Bus transfer between the primary and alternate sources is accomplished by the two IBVSUs.
7. Operation
_________
A. Functional Description (Fig. 4 and 5)
(1) Each main generator bus, which is the connection between the generator circuit breaker (GCB) and bus tie breaker (BTB), receives power from two paths: one from its associated generator through the GCB and the other from the other generators, external power sources, or auxiliary generators through the sync bus, the split system breaker (SSB) and the BTBs. The main generator bus will remain energized if one path is lost, however it will be de-energized if both paths are lost.
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