B737 -electrical question

[owl-attack]

Hello everybody

This must be elementary for most of you. But I do not know the answer

1.When placing the DC meters selector (on the AC/DC metering panel) to BAT BUS or STBY PWR, why is the amperage indication blank (no value) and only voltage indication shows. Why no amperage? Is there no current flow to the equipments concerned from BAT BUS?

2. I have been told that BATTERY DISCHARGE light illuminates (ofccourse when there is excessive battery disch) when you start the APU with battery power (no AC power available), but the Master caution will not illuminate if the APU starting is the cause of BAT DISCH light to illuminate. What logic does this have?

Kindly help me with this
Thank you in advance

[latetonite]

I take it you are not a pilot.

[TURIN]

I take it you are not a pilot.

Of course, he/she would have told you so in the first line. ;)

[owl-attack]

I am a cadet. Would you mind giving me the answer

[owl-attack]

I haven't got an answer from them tutors.

If you know, I'd like to learn

[CHfour]

If your instructors don't know then it's probably safe to assume you don't need to know either. There's more than enough stuff to learn on a type rating so just focus on what you need to know and you'll be fine.

[owl-attack]

I have finished my type rating, and right now undergoing line flight training.

I trying to understand the aircraft better

[Lancelot de boyles]

After about 13 years off and on, of messing with the B737, interesting questions pop up which get one thinking. At least for some of us; (I can't really understand the need to respond without actually providing any input, but I guess some people feel the need...:=)

To answer the first part-

1. An alternate book of words (Bulfer- cockpit companion) mentions that these give a volts reading only, as they are not a source of power. Thinking from a different point of view, I'm taking this to mean that the meter is measuring at a point where there is no load, and so no attempt is made to measure/display amps.

To measure amps requires a load.

As to the logic of the master caution, it's interesting, and I would be interested to know why from an engineering point of view, but I'm reasonably satisfied about the rationale.

You are starting the APU on battery power. It's a given that there is a significant draw, and that this is normal in this circumstance-

Battery as only source of power
Deliberate APU start
Not abnormal

PS. Don't take my responses as Gospel!

[NSEU]

1.When placing the DC meters selector (on the AC/DC metering panel) to BAT BUS or STBY PWR, why is the amperage indication blank (no value) and only voltage indication shows. Why no amperage? Is there no current flow to the equipments concerned from BAT BUS?

When are you testing the voltage/current in this situation? What power source is on the aircraft?

I had a look at the wiring schematics, but after following wires on a dozen pages, I was none-the-wiser. Which airplane, exactly, are you talking about? 737 Classic? 737NG?

Regarding the Caution light, (EDIT), on the NG wiring schematic 24-31-11, a wire is shown going from the APU ECU to the P5 (overhead) panel. When the APU is started, a signal on this wire stops the CAUTION ouput from the panel.

[NSEU]

The following is written in the NG Maintenance Manual (It appears to be ambiguous)

Voltage (DC VOLTS) and load (DC AMPS) show on the
alphanumeric display when you put the selector in any of the
TR positions or any of the BAT positions.
Only voltage shows when you put the selector to the BAT BUS
or STBY PWR positions.

(EDIT) I see (below) I've included data for the BAT meters position (I had confused it with "BAT BUS", but I'll keep the data for reference.

You see the voltage and current output of a battery with
the DC meter selector in the BAT position or AUX BAT position.
If the battery’s charger has power, you see the output voltage of
the battery or its battery charger, whichever is more.


The main
battery charger is in the charge mode when you see a positive
DC AMPS indication while the DC meter selector in the BAT
position. Use the AUX BAT position to monitor the auxiliary
battery charger.
The main battery charger cannot go into the charge mode
during any of these conditions:
* Fueling station door open
* APU start
* Standby power switch (P5-5) in the BAT position
* Standby power switch (P5-5) in the AUTO position, battery
switch ON, and DC BUS 1 and AC TRANSFER BUS 1 do not
have power
* Main battery overheat.
The auxiliary battery charger cannot go into the charge mode
during any of these conditions:
* Standby power switch (P5-5) in the BAT position
* Standby power switch (P5-5) in the AUTO position, battery
switch ON and DC BUS 1 and AC TRANSFER BUS 1 do not
have power.
* Auxiliary battery overheat.

[owl-attack]

Thank you so much Lancelot de Boyles and NSEU. I really appreciate your teachings


I'm taking this to mean that the meter is measuring at a point where there is no load, and so no attempt is made to measure/display amps.

To measure amps requires a load.

So does this mean with normal AC power available & powering the aircraft, BAT BUS is not powering any equipments???. I am not sure of this because, recently in the sim, we had multiple amber lights on the overhead panel (with no indication on the electrical panel of any failure) with normal AC power available. Later on after the session was over, our examiner told us this was a battery bus failure.


You are starting the APU on battery power. It's a given that there is a significant draw, and that this is normal in this circumstance-

Battery as only source of power
Deliberate APU start
Not abnormal


Thank you very much this makes sense

When are you testing the voltage/current in this situation? What power source is on the aircraft?

I am not testing the voltage but was just curious why is there no amperage indication, it does not even show ZERO, the indication is blank.
The aircraft is running with normal AC power sources from its respective IDGs . This question is w.r.t NGs

My questions here: I understand there is no load on the battery bus, thus no amperage. But why is this the case? With normal AC power available, isn't the BAT BUS supplying its respective systems. I have had a situation in the SIM with BAT BUS failure & normal AC power available and not failed (informed to us later on by the examiner that this was an example of BAT BUS failure after the sim session was over) we had multiple system failures. So I am under the assumption that BAT BUS still powers many systems even with AC power available and supplying the aircraft and thus there has to be a load/ampergae indication

Regarding the Caution light, (EDIT), on the NG wiring schematic 24-31-11, a wire is shown going from the APU ECU to the P5 (overhead) panel. When the APU is started, a signal on this wire stops the CAUTION ouput from the panel.

Thanks NSEU this makes sense

[RVF750]

OK. Remember and look again at the main electrical system schematic from your course, or in the Part B.

You do need to clarify that LOGIC in a BOEING is nothing to do with computers but simply relay switching and suchlike. The NG is basically a digitally redrawn and simplified 737 airframe with mostly the same crap stuffed in it from before or modernised versions of the same crap. Essentially it's a simple old beast with a few cleaver boxes driving the screens.....

So, is the Battery is attached to the DC Bus? Is it driving the DC bus or are the TRUs driving it? Where is the current flowing from or to? The Battery is removable from the bus so is connected via relays. To know the voltage of the battery, you must sense the voltage from the battery side of this switched relay.

TRU output? Battery charger output? Remember a Battery is effectively a NiCad 24v device. to keep it tip top it is fed a slightly higher voltage to keep it fully charged.

Your car alternator feeds 14V to your 12v car battery. Measuring with a Volt meter when it's running will show 13.5-14.5v usually. Notice how you have to change the settings to see the load? DC load needs to go through the meter whereas voltage can be measured in parallel.

Remember, the 737 is not rocket science, just a loose collection of many different systems often designed in different decades flying in loose formation most of the time.

Good luck!

[owl-attack]

Thank you for your response RVF 750

I am not talking about the Battery, I am talking about the BAT BUS and its indications on the AC/DC metering panel

[RVF750]

Same in some ways as you can't measure Voltage and current on a DC fed piece of copper at once with the same meter. just saying...

It's a sad fact that so many 737 pilots seek more detailed information in vain. I'm in the same boat for a few things, but the internet is an amazing place.....

The trouble with engineers documentation is it is difficult to understand - certainly for me! Best of luck and I applaud your thirst for knowledge!

[NSEU]

I am not talking about the Battery, I am talking about the BAT BUS and its indications on the AC/DC metering panel

Sorry, I made this mistake, too. I was confusing BAT with BAT BUS (so the Maintenance Manual is not as ambiguous as I originally thought).

It appears that there is no amperage shown (ever) with the switch in BAT BUS or STBY POWER because it is like this by design. There will be current flow of course with the right circumstances, but, for example, in the case of the BAT BUS, current measurement may not be so easy:
The BAT BUS is broken into 3 sections. Looking at the wiring schematics, the BAT BUS voltage reading appears to be looking at BAT BUS Section 2 only. Voltage will be common to all 3 (parallel) sections (assuming all 3 contacts of the relays are functioning), but the current will be different in each BAT BUS section depending on the load in each branch.

It would probably all make sense to you if you could see the wiring schematics but, sorry, I can't post diagrams here (for copyright and other reasons). The K1 and K2 relays (on Wiring Schematic 24-61-11) are wired up in such a way that it wouldn't be possible to put an ammeter in the circuit at a single point to get a reading of the current in all three BAT BUS Sections in all normal/abnormal situations. Under normal situations, TRU 3 feeds all three BAT BUS sections (and you have the amperage data in the "TRU 3" meter position anyway). When the BAT BUS is powering the DC Busses under abnormal situations, the "BAT" meter position will offer general current information (but note that the BAT may be powering lots of other busses too under certain circumstances: e.g. Standby DC, Hot Bat, Hot switched BAT).

Anyway, I hope this helps more than it hurts

Cheers
NSEU

[owl-attack]

Thanks a LOT, NSEU

This makes a lot of sense, I know that this is how it is designed, but i wanted to know why would it be designed that way to give no AMP indication. I understand it now

your explanation makes a lot of sense to me now

Thank you once again. And thank you for your patience and not ridiculing me like others for wanting to learn even if its silly to some

Thank you again