There are 2 ignition systems for each engine (Ignition Sys 1 and Ignition Sys 2). Normally, 115V AC Bus #1 feeds Ignition Sys #1 and 115V AC Bus #3 feeds Ignition Sys #2.
With the Standby Ignition Switch in NORM:
Note that losing a bus is very rare. All generators feed all buses on the 744, except during Autoland.
When the Standby Ignition Switch is selected to 1 or 2, the Standby Bus will power the selected (1 or 2) Ignition system on all four engines.
If you lose Main AC power (1 & 3), then, irrespective of Standby Ignition Switch position, Standby Power will provide power to both (#1 and #2) Ignition systems.
The Standby Switch shouldn't normally be touched on the 744. The EECs control ignition switching automatically (cycling between sys 1 and 2 on each leg and controlling single/dual ignition as required).
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