B737 NG Electrical Questions

[Ka8 Flyer]

Hi folks,

I've got a couple of questions regarding the electrical system of the NG:

1) According to my schematics, the battery switch position has no effect on the battery bus - it is powered whenever TR3 is powered or the battery's voltage is sufficient. Is this correct?

2) Can the electrical load be shared by external power and an IDG? Say you connect external power and then select the left generator. Would this disconnect external power completely or would the left transfer bus be powered by its IDG while the other transfer bus receives its power from the external power?

Thanks!

[Rainboe]

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[Ka8 Flyer]

Dear Rainboe,

I respectfully disagree: you can have the left AC Transfer Bus get its power from the left IDG while the right AC Transfer Bus is powered by the APU Generator...
Now my question concerns external power mixed with an IDG (naturally not on the same side)

Regarding the first question: The battery bus can also be powered by the battery, right? But for this to happen the battery switch must be ON? (And of course TR3 must be unpowered)

[vopz]

try this web site ,you will find the answers for all questions regarding the 737 ng electrical system
http://www.click2cbt.com/demos/ElectricalDiagram.htm

the battery switch position off denergise the battery bus if the stand by switch is in auto position.
yes the electrical load can be shared by the external power and idg.
best regards

[Ka8 Flyer]

"the battery switch position off denergise the battery bus if the stand by switch is in auto position."

Well according to the site you posted, the stby power switch has no effect on the battery bus - it is powered as long as the battery switch is in the ON position.

[Ka8 Flyer]

Rainboe, it seems we are misunderstanding each other.

vopz stated that the battery bus is deenergized if the battery switch is in the OFF position AND the stby power switch is in AUTO.
He also posted a website with a simple simulation of the electrical system.

After trying that site, I noticed that the stby power switch does NOT have an effect on whether or not the battery bus is powered instead, according to that site, the battery bus is powered as long as the battery switch is in the ON position; if the switch is in the OFF position, the battery bus is de-energized. The simulation therefor acts differently than what vopz stated. That's all what my last reply stated.

And, to resume, I still have no clear knowledge of when the battery bus is powered. My schematics are too simple; vopz and the simulation contradict each other and I haven't received a clear reply as to what conditions must be met for the battery bus to be powered.

Kind regards

Mark

[BOAC]

Originally Posted by Ka8

My schematics are too simple;

- I quite agree with you - the 'simple' Boeing schematic SUGGESTS that TR3 would power the battery bus in those conditions, yet 'vopz' presents a circuit diagram with an unknown switch between TR3 and the B Bus (and NO Standby Power Control Unit?). As far as I can see it is all going to depend on how the Standby Power Control Unit functions wrt the battery switch? It needs some brave soul (not me) to turn off the battery switch after start and see what happens. Even trying it in the sim will not be conclusive. Any avionics whizzes know the answer?

I cannot, however, at the moment see any practical use for this function (except for one of those 'anally retentive' tech quizes)?

[vopz]

i want to say that the battery switch off position denergises the battery bus if the stand by switch is in auto position.
if you put the stand by switch in the bat position the battery bus will be energised even if the battery switch is in the off position.
in addition i would like to inform you that you cannot see it on the shematic,the best thing you can do to clear your mind is to try it on the sim.

[stator vane]

simple diagrams---yes!

they never show the gerbal in the cage that switches the buses automatically when something goes wrong!!!!

and i've never seen any buses with windows!!!