(51)
External and APU power CTs provide current information directly to the BCUs for load shedding protection. Whenever the external or APU load exceeds 290 amps (on each generator or receptacle), load shedding occurs in the same order as with IDG load shedding.
(a)
Load shed can occur during taxi if the APU generators are supplying the electrical power. The load shed occurs, accompanied by considerable loss of cabin lights, when the engine speed is increased while taxiing. The load shed only occurs when three or more of the engines are increased above 73% N2. Cabin illumination returns to normal when the engine speed is reduced.
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747-400 MAINTENANCE MANUAL
(b)
AIRPLANES WITH RB211-524 SERIES ENGINES; Load shed can occur during taxi if the APU generators are supplying the electrical power. The load shed occurs, accompanied by considerable loss of cabin lights, when the engine speed is increased while taxiing. The load shed only occurs when three or more of the engines are increased above 63% N2. Cabin illumination returns to normal when the engine speed is reduced.
(52)
Utility load management operates the same as galley load management except that power for the utility ELCUs comes directly through the left and right UTILITY switches and not through control relays. Power from the switches is delivered to normally closed load management relays in BCU 1 and BCU 2.
(53)
If no overload condition exists, the load management relays will be closed allowing 28 volts dc to be applied to the ELCUs. With 28 volts applied, the ELCUs need a ground to close. This ground is provided by the load management relays in the opposite BCU. With this type of hookup each BCU is capable of initiating load reduction. This prevents a failure of a single BCU from disabling the load reduction system.
(54)
For the BCUs to initiate load reduction, they first determine the electrical system configuration and the appropriate load for that configuration. If demand exceeds capacity, the BCUs then energize the appropriate load management relays.
(55)
To determine the electrical system configuration, the BCUs use auxiliary contacts in the SSB, BTBs, GCBs, APBs, and XPCs. This allows the BCUs to monitor the number of generators operating and whether they are isolated or paralleled.
(56)
Generator load is monitored in the BCUs and compared to generating capacity (capacity is determined by the number of generators operating). If the load exceeds capacity, the BCUs initiate load reduction by energizing the appropriate load management relays until the balance between capacity and load is restored.
(57)
Ground power loads are different in that BCU 1 senses APU generator 1 and external power 1 loads directly from the CTs. BCU 1 compares this load to a fixed reference of 290 amps. BCU 2 does the same for the right side of the sync bus. Both BCUs share load information with each other over an ARINC 429 bus.
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MAINTENANCE MANUAL
(58)
Automatic load restoration occurs when loads have been shed as a result of an overload condition or during loss of all generators and subsequent normal operation of at least two generators is resumed. (Normal operation of one generator following loss of all generators does not result in load restoration.) Loads are restored in the reverse order of load shedding, and at one-second intervals. This load restoration is designed not to cause another overload. However, if the restoration process should cause an overload condition to occur, load restoration is halted and load shedding occurs until the overload condition is removed. Following load shedding, load restoration does not occur until an additional generator or generators are restored.
(59)
Ground power automatic load restoration occurs when loads have been shed as a result of an overload condition during APU generator and/or external power source operation on the ground and subsequently, the opposite APU generator or external power source is added, or an opposite IDG source is added. Loads are restored at one-second intervals in the reverse order of shedding.
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