a question about ELEC

[sunxuyang 0]

i checked the FCOM 6.20.7 acorrding to the chart on the page if i shut dwn the bus tie breaker(bus transfer) (i am on APU or ground source) the plane will lose all the elec power except the battery but i tried on NGX it doesn't seem to go like that am i wrong about the manual? thank you

[Paul_Smith 92]

The Bus transfer switch (FCOM 6.10.9) is used to connect one bus to the other, so a single power source can power both buses if necessary. The diagram on 6.20.7 is for inflight configuration so does not really reflect your situation, but it does show that ground power also connects to the buses via the ground Service relay even if the transfer bus is open. I suspect that the detail of the switch is not completely correct in that diagram because as far as I know, the APU can power each bus independantly, regardless of the transfer position.

[sunxuyang 0]

via the ground Service relay ? GSR is only to offer AC GND service bus 1 ? so the rest buses will be inoperative?you are right there is sth wrong with the APU part in the diagram

[Paul_Smith 92]

Read the diagram more carefully. There is a GSR relay on each bus, which in turn powers the AC Transfer buses which power everything else. The bus transfer switch only allows the two AC transfer buses to be connected. You can also see from the APU GEN configuration switchs that the APU can power each bus independantly (regardless of the bus trandfer switch position). There is more information over on Smartcockpits dot com including this description of the Bus Transfer switch:

AUTO (guarded position) - BTBs operate automatically to maintain power to AC transfer busses dfrom any operating generator or external powerOFF - isolates AC transfer bus 1 from AC transfer bus 2 if one IDG is supplying power to both AC transfer busses.

[Houghton11 12]

I think Paul got it above but yes...the APU powers both Transfer Bus' regardless: AMM Part II - 24-22-00 Pg. 208

To energize the 115V ac transfer buses, set both of the APU GEN switches on the P5-4 panel to the ON position. NOTE: Either of the APU GEN switches will connect the APU ____ generator to both TRANSFER BUSES. However both of the APU GEN switches must be set to the ON position to make both of the SOURCE OFF lights go off.