24-51-00
ALL ú ú 12 Page 14 ú Jun 18/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
(39)
Both BCUs independently control the shedding of the loads. The failure of one BCU does not affect the ability of the remaining BCU to provide all load shed signals. In this way, load shed is redundant.
(40)
In addition, the loads can be shed manually via the UTILITY switches on the electrical system control module.
NOTE: The associated UTILITY switch OFF light will not illuminate
____ when the ELCU is shed by the BCU.
(41)
During an overload condition, then load shedding continues (in one-second intervals) with the utility ELCUs until the overload is removed. The utility loads shed in the following order: Utility ELCU 1, ELCU 4, ELCU 2, and ELCU 3.
(42)
When a generator is running isolated (BTB open) and an overload is experienced on the remaining generators, load reduction starts with the lowest numbered unshed load associated with those paralleled busses experiencing an overload. For example, if BTB No. 2 is open and an overload occurs on the remaining paralleled generators, the trip sequence would be: Utility ELCU 1, ELCU 4, and ELCU 3.
(43)
When the airplane is in the air mode (three or more engines operating above 75% N2), the load shed current limits are as outlined in the following table.
~...............................................................
| | NUMBER OF | |
| CASE | IDGS ON | POWER FEEDER LINE CURRENT |
| | LINE | (PER PHASE) |
................................................................. | 1 | 4 | 305 +/- 8.5 amps for 5 seconds | | | | or 243 +/- 8.0 amps for 4 minutes| ................................................................. | 2 | 3 | 305 +/- 8.5 amps for 5 seconds | | | | or 243 +/- 8.0 amps for 4 minutes| ................................................................. | 3 | 2 | 203 +/- 7.5 amps for 5 seconds | ................................................................. | 4 | 1 | All loads shed, no delay | ................................................................. | 5 | 0 | All loads shed, no delay | 2..............….............…..................................1
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24-51-00
ALL ú ú 11 Page 15 ú Jun 18/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
(44)
If the APU generators are the only source of power for the airplane, and the airplane enters the air mode (three or more engines operating above 75% N2), all loads will be shed immediately.
(45)
When the airplane is in the ground mode, and only one IDG powers the airplane, load shedding will occur when the power feeder line current exceeds 305 +/- 8.5 amps for 5 seconds, or 243 +/- 8.0 amps for 4 minutes.
(46)
Load shedding should not occur when two or more IDGs are supplying power and the airplane is in the ground mode. Two or more IDGs are sufficient to adequately supply power to the plane without exceeding the load shed triggering levels.
(47)
When the airplane is in the ground mode, load shedding will occur when the external power current or APU generator load current is greater than 290 +/- 30 amps.
(48)
Load restoration occurs automatically in reverse order as load capacity is increased. The order of load restoration for one or no generators is:
(a)
One or no generators: restore no loads
(b)
Two generators: restore utility loads.
(c)
Three or four generators: restore all loads
(49)
Load restoration for APU and external power will occur automatically as load capacity is increased.
(50)
Generator load sensing is provided by current transformers (CT) in the IDGs. The current sensed by these CTs is sent to their respective GCUs for processing. The GCUs send generator load information to the BCUs by two separate means; an ARINC 429 bus and by hard wire. The hard wire signals are redundant and are for backup in the event a failure of the ARINC 429 bus is sensed.
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