24-20-00
 ALL  ú ú 07 Page 9 ú Jun 10/90
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (g) IDG Power Operation to External Power Operation - Two External Power Sources Available (SSB Open) 1) With both external power sources available and with one IDG (or two IDGs in parallel) powering each half of the sync bus, momentarily pushing either EXT PWR switch will cause the adjacent IDG(s) to synchronize with the associated external power source. When the frequency and phase angle are within acceptable limits, the associated XPC closes, momentarily paralleling IDG power and external power before automatically tripping the adjacent GCB(s).
 2)  The other half of the system can be brought onto external power in a similar way by momentarily pushing the other EXT PWR switch to the ON position.
 (h) IDG Power Operation to External Power Operation - Two External Power Sources Available (SSB Closed) 1) IDG(s) Connected to Both Sync Bus Halves a) With both external power sources available and with one IDG (or two IDGs in parallel) on each half of the sync bus, momentarily pushing either EXT PWR switch causes the SSB to trip and the adjacent IDG(s) to synchronize with the associated external power before automatically tripping the adjacent GCB(s). b) Once the configuration of two external power sources available with one supplying its adjacent buses, SSB open, and IDG(s) supplying the opposite two buses is established, removing the opposite external power source will not cause the SSB to automatically close. Shutting down the opposite engine(s) causes the associated buses to go dead. If the opposite external power source remains available, but is not brought on line, shutting down the engines also results in two dead buses.
 c)  The other half of the system can be brought onto external power in a similar way as described above with SSB open by momentarily pushing the other EXT PWR switch to the ON position.
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

 24-20-00
 ALL  ú ú 06 Page 10 ú Feb 10/90
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
 2)  IDG(s) Connected to Opposite Sync Bus Half Only
 a)  With one or both IDGs on one half of the sync bus only (no IDGs in operation on the other half of the sync bus), momentarily pushing the EXT PWR switch opposite to the connected IDG(s) causes the IDG(s) to synchronize with the opposite external power source. When the frequency and phase angle are within acceptable limits, the opposite XPC closes, momentarily paralleling IDG power and external power. When the XPC closes, the SSB trips, leaving IDG(s) to power one sync bus half and external power to power the other sync bus half.
 b)  When two external power sources are available (one external power source supplying its adjacent sync bus half and IDG power supplying the opposite sync bus half), removing the opposite external power source and/or shutting down the opposite engine(s) does not cause the SSB to automatically close. The associated buses will go dead.
 c)  After this transfer, momentarily pushing the other EXT PWR switch initiates a power transfer as described above with SSB open.
 3)  IDG(s) Connected to Adjacent Sync Bus Half Only
 a)  With one or both IDGs on one half of the sync bus only (no IDGs on the other side), momentarily pushing the EXT PWR switch adjacent to the connected IDG(s) causes the IDG(s) to synchronize with the assocaited external power source. When the frequency and phase angle are within acceptable limits, the associated XPC closes, momentarily paralleling IDG power and external power. When the XPC closes, the adjacent GCB(s) will trip.
 b)  After this transfer, momentarily pushing the opposite EXT PWR switch causes the SSB to trip and the opposite XPC to close.
 (i)  IDG Power Operation to External Power Operation - One External Power Source Available (SSB Open) 1) With one external power source available and with one or
 two IDGs on the adjacent half of the sync bus, placing the available EXT PWR switch in the ON position causes the IDG(s) on the adjacent side to synchronize with the external power source. When the frequency and phase angle are within acceptable limits, the XPC closes, momentarily paralleling the IDG(s) with the external power source before automatically tripping the adjacent GCB(s).
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