737 standby power

[David McInnes]

Can anyone tell me of a reason why you would select the stanby power switch to BAT, in flight.. .My books are a bit vague. Also, has the battery charger got anything to do with this switch. Thanks. .OQA.

[FlapsOne]

Does this help?

STANDBY POWER Switch:. .AUTO (Guarded position)-- During NORMAL operations:. .A) #1 Transfer Bus powers the AC Standby Bus. .B) #1 DC Bus powers the DC Standby Bus. . . .- Loss of either normal source (A or B above) in-flight:

A) Static Inverter powers the AC Standby Bus (through the Battery Bus). .B) Battery Bus powers the DC Standby Bus

-- Loss of all AC power on the ground there is NO automatic transfer of power.

OFF -- Standby Buses are NOT powered; . .STANDBY PWR OFF light is illuminated

NOTE: The Static Inverter is NOT powered; and The Standby Attitude Indicator (SAI) is NOT powered.

BAT -- . .A) Hot Battery Bus powers the Battery Bus. .1) Battery Bus powers the DC Standby Bus. .2) Battery Bus powers the Static Inverter. . a) Static Inverter powers the AC Standby Bus

NOTE: In BAT position, the Hot Battery Bus powers the Battery Bus regardless of this Switch's position.

NOTE: Battery can supply power to the Standby Equipment for about 30 minutes.

[spannerhead]

With the standby power switch in auto, the auto transfer is controlled by a relay. Not unknown for this relay to stick. Indeed it is exercised very regularly by the eng's.. .If no auto transfer occurs (maybe due to a sticky relay) manual transfer is available "on batt".

[Checkboard]

IF you experience a power failure in flight, with the standby power switch in AUTO, you will get an auto power transfer - all fine and dandy. If you land in this configuration, then you will lose the standby power on touch down - i.e. at night things will suddenly go black in the cockpit!

The battery charger has nothing to do with this switch.. . [quote]Battery Charger Transformer/Rectifier

The purpose of the battery charger is to restore and maintain the battery at full electrical power. The battery charger is powered through the AC ground service bus with provisions for automatic switching to main bus 2 when the ground service bus is unpowered.<hr></blockquote>

[ 06 February 2002: Message edited by: Checkboard ]</p>

[Grendel]

The above posts are all correct and detailed but the way my instructor taught it is Auto-allows things to happen and Bat forces things to happen.

When in bat all the relays are forced to their standby positions. This allows operation on the ground of some items not normally available in auto. Sorry I can't remember what it's been awhile. Auto allows Bat position forces.

Hope this helps.

[Dead Leg...Dead ?]

Question.... Why does the APU shut down when the bat. switch is turned off?

[Nacaduck]

Dead Leg - That's because there is no fire protection with the Battery switched off.

[Redpanda]

The APU fuel solenoid is powered from the hot battery bus. Shut the battery off and the fuel solenoid fail safes to closed.....APU shuts down. The APU inlet door is then left "out of sequence" and may cause problems on the next start.

[David McInnes]

Thanx everyone. Heaps of help. . .OQA.