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 C.  The external power is applied to the airplane buses by the external power receptacles. The receptacles are found on the lower right side of the airplane, aft of the nose gear wheel well. The electrical system control module, on the P5 overhead panel, has two switches to manually control the external power.
 D.  APU power is applied to the airplane buses by the two APU generators. The electrical system control module has two switches to manually control the APU power.
 E.  Engine-driven generator power is supplied by the four integrated drive generators (IDG). One IDG is attached to each engine. The electrical system control module has four switches to manually control the IDG power.
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

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747-400
MAINTENANCE MANUAL

 F.  The ground handling bus is used for airplane ground operations. The electrical power for the ground handling bus is supplied by external power or APU power. When APU and external power are available, external power is automatically supplied to the ac ground handling bus. The external power must be turned off to let the APU generator supply the ground handling bus.
 G.  The ground service bus is used for ground and flight operations. The ground service bus can be supplied by:
 External power
 APU generator power
 Engine-driven generator power

 The ground service bus is automatically supplied by the ac bus No. 1. When the ac bus No. 1 is off, the ground service bus can be supplied by APU or external power. The ground service bus switch is found on the flight attendants panel at the left door 2. When the ground service bus is supplied by APU or external power, the ac ground handling bus will be energized by the same source. When APU and external power are available, external power is automatically supplied to the ground service bus.
 H.  The battery switch must be set to OFF when electrical power is removed from the airplane. If the battery switch is set to ON with no ac power on the airplane, the batteries can become discharged in 30 minutes. The APU will not start if the APU battery discharges for 10 to 15 minutes.
 I.  For the external power source, we recommend that you use two different 90KVA external power sources (one external power source for each external power receptacle). This procedure can cause a single external power source to go off line.
 J.  Automatic load shedding may occur if airplane load demand is larger than the airplane overload protection threshold. Partial or total removal of electrical power from galleys and utility buses will automatically occur during load shedding.
 K.  Power will be automatically restored to the loads when an additional source of electrical power is provided.
 (1)  
If load shedding occurs during ground operations, you should increase the electrical power available to the airplane by one of the following methods:

 (a)  
Add a second 90KVA ground power source.

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A
747-400 MAINTENANCE MANUAL
 (b)  
Add an APU generator source.

 (c)  
Add an engine powered generator source (IDG).

 L.  
You will see an interruption in power service when you change these power sources:

 From one APU generator, to two APU generators
 From one external power source to two APU generators
 NOTE: You can avoid an interruption in power when transferring from
____ one external power source to APU power by switching to APU power in the following manner. If the airplane is being powered by external power 1, engage APU generator 2. If the airplane is being powered by external power 2, engage APU generator 1. This action will cause the split system breaker (SSB) to open, and the APU generator to synchronize with the external power. You can then engage the remaining APU generator without an interruption in power.
 M.  The SSB has to open before the added power source can be connected to the buses.
　
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