Frequent fliers to Nice, France and other places that strictly control APU operating time often ask this question: how do you avoid a break power transfer going from external AC to the APU?
Answer: you don't.
The only thing that permits a no break power transfer is the IDG. Once you understand that, the procedure is easy to remember.
Everything here is from the references shown below, with a few comments in an alternate color.
G450 AC power system overview, from FlightSafety G450 Maintenance Training Manual, figure 24-10.
More about this: G450 Electrical Control / BPCU.
[G450/G550/G650 (ATA 24) No-Break Power Transfer System Overview.] The best way to avoid a BPT is to select the APU generator switch to "OFF" before starting the APU. Continue to use the AC external power cart to power the aircraft and the APU as the air source until at least one or both engines are started with IDGs online. There will be no BPT when using this technique. This information has been submitted for inclusion in the Quick Reference Handbook.
G450/G550/G650 (ATA 24) No-Break Power Transfer System Overview, The Gulfstream Journal, October 3, 2014.
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