For –300/500’s (the Classics! 
)
Hej Richie
As soon as both CTR pumps are turned off the scavenge shut-off valve opens and a 20 minute timer starts. Utilising #1 FWD pump pressure the scavenge pump “sucks” CTR tank fuel (or air) into the #1 MAIN tank for the duration of the 20 minutes. The scavenge SOV regulates fuel pressure to the scavenge pump (motive flow pump), but I’ve never found out what kind of transfer flow rate it achieves … it just “sucks”
Would the pump run for 20 minutes…
Yes, that’s what it’s supposed to do.
… and continue to use fuel from the centre tank…
Do you mean the extraction of fuel from the CTR tank or #1 ENG … the scavenge pump operates on fuel pressure provided by #1 FWD pump and draws fuel from the CTR tank to the #1 MAIN tank.
…or does it only run when the fuel quantity is below a certain value?
I’ve not discovered or been made aware of any "limiting" fuel quantity governing the jet pump’s operation – the criteria is for both CTR pump to be switched to OFF (and #1 FWD pump on of course, else no motive flow
). It is also possible to keep activating the the scavenge pump if you so desire ...
Where did you see “positioning the bus transfer relay [sic – switch] to off would cause the transfer buses to lose power”? By selecting OFF you isolate the #1 and #2 DC buses (via their respective TR’s), prevent the battery charger from switching to it’s alternate source of power (#2 MAIN bus) and prevent TRFR to GEN bus switching. The relay logic is to switch a de-powered TRFR bus (due associated GEN bus failure) to the opposite powered GEN bus – this is to protect the aircraft’s “essential” AC/DC loads. The TRFR buses cannot power each other.
STBY PWR switch in AUTO – Yes and yes; you are right as long as TR3 is powered or the BATT switch is on, and on the ground the BAT position overrides the air/ground safety logic. What else is BAT good for? Well … it’s a good way to drain the battery
->
… otherwise the standby power test, doing a battery start (powers the right ignitors), calling for the fuel truck on Comm#1 (instead of going for a walk)
, refuelling on battery power, checking out how much fuel is onboard (cold aircraft) the lazy way … awww c’mon … who wants to pull out all those floatsticks
STBY PWR switch in OFF – the STBY AC and STBY DC buses will be de-powered