737NG electrical question

[Tapshi]

My FCOM, on page 6.20.12 says 'the power to inverter is controlled by standby power switch and battery switch on the overhead panel'!!
What role does the battery switch play here? :*

[babisiliop]

If you loose all your AC power, stand-by power(AC from INV& DC from BAT) takes over and electrical power is taken from main battery( 30 min)
If you don't have BAT on you get no power.

[BOAC]

My understanding is that the position of the battery switch is immaterial. The Stby Inverter is powered from the 'hot battery bus' and therefore always powered. If you put the Stby power switch to 'BAT' you could then turn off the battery which means the power drain from the rest of the battery circuit is stopped eg if you are miles way from a landing place and can operate without all the Switched battery items you could extend the battery life. You then, of course, could also switch 'OFF' the Stby inverter as well if needed. I think the switched BATT bus has some function involved with 'Auto' Stby power but I'm not sure (hence having it 'ON' normally.)

One of our 'leccie experts will be along shortly I'm sure.

[EEngr]

If I recall correctly:

BAT switch ON, Standby switch on AUTO: Battery supplies standby AC bus via inverter and standby DC if main power is lost.

BAT switch OFF, Standby switch on AUTO: Standby AC and DC buses are de-energized when main AC is lost.

Standby switch on BAT: Standby buses powered from battery regardless of BAT switch position or state of main sources. The battery will be float charged by the battery charger as long as it is supplied with AC.

Standby switch OFF: AC and DC standby buses de-energized regardless of main source states.

These are the settings one will normally encounter. There is a (complex) truth table that describes the state and source of each bus, depending on main AC bus and source states, switch positions, the type of battery system (single or dual), air-ground logic, etc. But I seem to have misplaced my copy after leaving Boeing.
:8

[EW73]

The air/ground sensing and number of batteries fitted plays no part in the switching of the Standby Power system.

At least in the 737NG airplanes I associate with....

As regards the number of batteries fitted, the primary battery supports all loads until Standby Power is initiated, then, and only then, the secondary battery is connected to effectively double the capacity of the standby system.
The secondary battery plays no part, other than being charged, until standby power is initiated.
And even then, if an APU start is initiated with standby power on, the two batteries with disconnect from each other for the duration of the APU starter cycle.
The system is determined to keep the secondary battery fully charged until it is switched into the system as it is designed!

Cheers...

[EEngr]

The air/ground sensing and number of batteries fitted plays no part in the switching of the Standby Power system.That's correct (as far as I can remember). The truth table I had describes the entire electrical system's behavior. So without it in front of me, I can only rely on a rapidly dimming memory of what the input variables were and the states that it describes. :confused:

[NSEU]

Tapshi, does your 737NG have a normal battery and an Aux Battery?

Looking at the Maintenance Manual for the single battery type NG, there is a Remote Control Circuit Breaker (RCCB) between the Hot Battery Bus and the Static Inverter. The Standby Power Control Unit (SPCU) opens this RCCB if the battery switch is selected OFF (unless the Standby Power switch is selected to BAT)

On the dual battery NGs (i.e. those with a regular battery and an Aux Battery), there is also a "DUAL BATTERY RCCB". It has different logic from the normal RCCB. However, looking at the wiring schematics, power from the Aux Battery has to go through both RCCBs (in series) to get to the Static Inverter. On the dual battery type NGs, the logic for the normal RCCB is the same as the single battery type NG.

I can't see how you can get power to the Static Inverter (with the main busses dead), if the Battery switch is not ON, or if the Standby Power switch is not in BAT.

[sky-738]

reading the FCOM, about the BAT Switch :
OFF-- removes power from battery bus, switched hot battery bus, DC standby
bus, static inverter, and AC standby bus when battery is only power
source.
looks like when in air ,if all AC is gone , when u choose the BAT switch to OFF . all you power is gone except some hot bat thing.
is that correct ????

[BOAC]

AFAIK not if you select STDBY PWR.to 'BAT' as post #4

[EEngr]

AFAIK not if you select STDBY PWR.to 'BAT' as post #4 That's the way I remember it.

I have a foggy recollection that the 737 classic is different. Perhaps there, the inverter requires BAT switch to ON in all cases. But that one was before my time.

[BOAC]

Yes - fading memories, but I think it was the same.

[sky-738]

got it , thanks to you guys