Some Question on Boeing 737NG Electric System
 

Hi all PPRUNE members,

I'm currently reviewing a B737NG Electric system by CBT and it getting a little bit
confusing.

- First, in the standby power section. The CBT said that if the Standby power is
at auto position and battery is place to off and no AC generator is operate
the AC and DC standby bus will not be energize by battery. Why? Isn't the
battery switch control the switch hot battery bus only?
[Refer to the Schematics in the book " Boeing 737 Technical Guide" page 96]

- If we switch the battery off during the normal config [all IDG operate,
AC Transfer Bus operate] the engine will not shut down and the aircraft
equipment will continue to operate
[except the equipment in Switch hot bat bus]. Am I understand it right?

- Why during making the Auto land, the DC Bus tie open and separate the
Left & Right Electric sys.

That should be all the questions I have. Thanks for all of your reply.

Best Regards

It's just the way the aircraft was designed. The internal logic of the Standby Power Control Unit (SPCU) prevents power from going to the AC Standby Bus. Logic inside the SPCU controls a Remote Control Circuit Breaker (which supplies power to the Static Inverter) won't close in this configuration, and you get no hot battery power going to the Static Inverter. Therefore you have no AC Standby Power.

There is also a relay inside the SPCU, called K3, which prevents hot battery bus power going to the DC Standby Busses in your specified configuration.

It is a legal/safety requirement that equipment used for Autoland receive electrical power from separate sources. When navigating so close to the ground, you don't want one electrical failure to affect too many devices.

Regards
NSEU

The reason of Standby power in the 1st place is for Emergency use when u loose all sources of AC power,so u remain with DC power that'll last for 30 minutes if u use a single battery.Standby Power on AUTO only works if BATT switch is ON otherwise place the Standby Power switch on BATT but the switch Hot Batt is OFF.

As for the disconnect relay at Glide slope capture is to prevent in case of a faulty DC bus to affect the other,it"s a safety aspect

Thanks for all of yours reply sir.

But how does the single failure going to affect the rest of the system.
If the T/R no.1 were fail and DC bus tie were open, that's gonna make the
DC Bus 1 and DC Standby bus fail too. Why it's more safe in open than close?
Can anybody explain in this part please?

Best regards

I think the designers were more concerned about failures of components which would affect most of the DC network. E.g. Large short circuits on relatively unprotected sections of the bus system, which would affect a majority of DC-related systems if the Bus Ties were closed. Don't forget that smaller DC voltages are often used to control AC powered systems on most aircraft types, so the consequences are great.

I understand your point about losing large sections of the electrical system, but it's better than losing all your electrical system ;)

When bus isolation is initiated and, afterwards, a failure occurs on one side of the aircraft, the aircraft should remain in a controllable state, at least long enough for you to land.

Rgds
NSEU