View Full Version : B737NG Bat Bus power source

1st Mar 2009, 10:31

consider B737NG:
-aircraft inflight or on ground
-electric system runs normal, all busses powered

now, place battery switch to OFF

the books say:
removes power from
1-switched hot bat bus
2- battery bus

my question::confused:
why the battery bus?
should it not continue to be powered by TR3!!!!

your help is appreciated.


1st Mar 2009, 11:45
The electrical system is really quite complicated, isn't it? I've always found electrics one of the hardest areas. Only got Classic manual to hand at the moment- this is what I dredged up:

Normal power for the Battery Bus is from TR3 via the Standby Power Control Unit. In the event of failure of AC Transfer Bus 1 or DC Bus 1, power is supplied through the DC Battery System, I think the Batteries are supplying Battery Bus and Hot Battery Bus whether the Transfer Buses are powered on not powered whatever the position of the Standby Power Switch. 737 Ops Manual 6.20.13. There seems to be an inconsistency there!

1st Mar 2009, 12:19
There is no switched hot bat bus. You mean the switched battery bus? Disconnected bij battery switch. Battery bus now powered through standby system if normal AC is available. If not, the battery bus is isolated.

1st Mar 2009, 12:23
there is no switched hot battery bus??? i think there is on the Ng's have a look again ops manual/electrical/ page 20 out of 24.

the only time i can think that would happen is if the BAT switch is off and the standby power switch is in OFF or BAT.

1st Mar 2009, 12:28
Hello there trot

I pondered about the same thing some time ago. What is written in the manuals are of course true, but the displayed schematics are a bit misleading.

If you see the schematics in front of you there is a switch between TR3 and the battery bus that opens when you select the battery switch to OFF.

You can check out this diagram for visualization:

ElectricalDiagram (http://www.click2cbt.com/demos/ElectricalDiagram.htm)

So.. in your stated conditions you indeed get:

- Switched hot bat bus unpowered
- Bat bus unpowered

I hope that helps :ok:

1st Mar 2009, 12:35
helps protect the DC standby system if there is a main batt fire god forbid. lets the aux batt take the strain for a limited time.

1st Mar 2009, 12:37
A strange name, I agree, but trot is right. The 737-800 indeed has a switched hot battery bus.

1st Mar 2009, 12:39
so does the 700

2nd Mar 2009, 08:53
tahnks folks,

great help.:ok:
unfortunately i could not open the electric diagram posted by Knorway to see where and how the battery switch affects the TR3--battery bus connection.
the FCOM shows a symbol between the TR3 and battery bus, but no explanation to its purpose.
again, thanks for all the replies.


2nd Mar 2009, 09:50
An alternative schematic: SmartCockpit - Airline training guides, Aviation, Operations, Safety (http://www.smartcockpit.com/pdf/plane/boeing/B737/systems/0015/)

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