At the end of the day latetonite , either you are wrong or I am wrong. It is important to understand this system so I am open to the possibility that I am wrong and am ready to change my mindset. At this point in time however you have only provided analysis of the Boeing docs that is contradictory to mine and several other posters here, one of whom appears to be an Electrical Engineer.
I am going to pursue this with my own companies Engineers to get a definitive answer. If you are open to changing your mindset on how the system works I will come back on here with the outcome, if you are going to doggedly stick to your own personal analysis of the schematic ( which is at odds with everyone else on here) I will not continue discussing it here as it is pointless and puts mis information on the thread that may confuse new pilots trying to learn.
If the Engineers conclude that you are right I will post that here.

Framer,
I asked you to pm me an email address, where I can send you the wiring diagram AND the reference to my mythical TR3. I cannot post it on the tread.
And yes, if it helps, my background is electrical, electronical engineering with military specialisation in avionics.

But check with your engineers.

Framer,

You are correct. The HBB and SHBB are powered from the battery or charger. TRU 3 does not supply either, in any configuration.

MyBoeingFleet's simplified electrical schematic, and WDM 28 VDC buses schematic clearly show this.

I am not claiming to be an expert on all things maintenance, but I have held a B737NG AMEL type rating for over 10 years, including the Electrical category.

lateonlite,

Perhaps you are looking at the K3 standby DC alternate relay and thinking that TR3 can supply DC power through it to the Hot battery buses in the event of a battery/charger failure??

The K3 relay is relaxed open. It energises closed to allow the battery or the charger to power the DC standby busses. This happens when the standby power switch is in AUTO and either or both DC bus 1 and AC transfer bus 1 have no power. The K3 relay will not be energized by a battery and/or charger failure, and therefore TR3 cannot power the hot battery busses. The SPCU controls all this.

The only source of power to the hot battery buses is the charger or the battery. If both fail you have lost the hot busses. The AUX battery was installed to prolong emergency power as well as provide a third source of power to these buses. This provides 3 levels of redundancy to the hot battery busses.

GB,

I am not referring to K3, as this one, correctly, closes when either DC bus 1, or/and AC transfer bus 1 has no power, i.e. generators not available.

In normal flight however, with the battery switch on, stby pwr not in bat position, K2 is closed and connects TR3 to the bat bus via SPCU.

TR3 is normally powered by Transfer bus 2, alternatively by Transfer bus 1.
If TR 3 fails, or has no power, K1 closes, to energise the battery bus from the battery or the battery charger.

The battery bus, and the switched hot batt bus via the batt switch, are now energized, so fire detection and extinguishing is available.
This was the main issue in the loss of battery and batt charger, as I understand.

latetonite,
To be honest I'm not even sure what point you are arguing any more :confused:
Why are you talking about the bat and dc standby buses? You said:
Hence we were talking about TR3 feeding the hot buses. Look at FCOM2 6.2.18.

But all this is by the by. The scenario is normal AC power with the BATTERY DISCHARGE light illuminated. If that light is on, the battery is being drained and therefore what you do know for sure is that anything connected to the battery is threatened.

Framer,

You are still correct.

lateonlite,

You have essentially repeated my post and I agree with everything you state.

Can you cover off on exactly how TR3 can power the hot battery busses??? I don't need a circuit diagram or any reference, just the relay/switch numbers and what circumstances cause them to allow TR3 to power the hot battery buses??.....As Framer has said this will clarify for youngsters out there willing to learn.

I now thinking Latetonite is an engineering troll.

GB: answer to your inquiry is in my post #44. For your conveniance I paste it again:
In normal flight however, with the battery switch on, stby pwr not in bat position, K2 is closed and connects TR3 to the bat bus via SPCU.
K2 is the normal batt bus relay switch.

I have trouble understanding what makes this so difficult to comprehend.

And the hot battery busses??? You adamantly state that TR3 can power them...

Something about walking???

Framer,

You are still correct.

Hot bat bus via R1 in normal ops, via R326 in stby..please stay polite if you want to learn something.

B737900er....Correct. This is definitely a gee up.

To make you more upset, the battery discharge light will not illuminate when AC transfer bus #1 is powered. So basicly only on the ground with no engine running and no external power.

Funny - I always assumed that the triangles in the electrical circuits in the diagrams represented diodes. According to 'late' they do not?

Hey look! EEng who claims to be an ex Boeing engineer agrees

BOAC: Referring to the link i posted in #44, Page 2, DC schematic, the diodes are pointing in the right direction.

There is no link in post 44.
Latetonite, does it make you uncomfortable that so many other people disagree with your interpretation of the system?
I asked my companies Engineers and they agree that the HBB and SHBB are fed by the battery charger as a primary source and by the battery as a back up. There is no third source.

ltn means post #34 and now has the difficult task of explaining how the 'correctly fitted diode' would allow current flow in both directions.....................................

Do we have 'son of SSG' here?

He certainly has no understanding of circuit diagrams or basic electrical theory, and has an obvious inability to admit he is wrong.

Tipping the 'ol wrist a bit, me suspects.

Dear latetonight

You said during normal ops the K2 relay is closed and TR3 powers the Battery Bus. And you say the HBB & SHBB will be powered by TR3 once the battery goes flat and the charger is not working.

The only way I can see that (using the diagram given by you) is if the K1 relay (between the Battery Bus and the Battery) closes and the current somehow jumps a one-way diode which is pointing UP which says current can only flow form the Battery to the Battery Bus and not reverse.

So I would like to understand how is it possible for K2 and K1 relays to be closed both together and if that were to occur how would the current bypass the diode??

Also I find you the rudest here and you the one asking someone to be polite. Sorry but that’s just hilarious.

Dear Framer:

I had this a week back just after takeoff. With -5 amps on the Metering panel. I called maintrol and they told me that my battery would go flat in 1 hour. My flight was over 2 hours long. Thus did an ATB.

After landing I was told that -5 wouldn’t have drained the battery and that if I had chosen to continue I could have as the battery wouldn’t have drained that fast as it wasn’t a huge value. But they did say that if the battery did drain out the HBB and SHBB would be unpowered. I shall put up a PDF file which shows the equipment that are powered by the HBB and SHBB.

While on the R/T I got wrong info from maintrol that my Battery would drain out in 60 mins, as -5 amps would have taken it much longer to drain. Nonetheless safety are happy that I came back as I was headed to Delhi where it was CAT 3 conditions so no tea or biscuits with management as of now. Though once I have the official report I shall publish it here.



I though have one last thing to add. The FCOM says

With the BAT Switch in OFF –
• removes power from battery bus and switched hot battery bus when operating with normal power sources available
• removes power from battery bus, switched hot battery bus, DC standby bus, static inverter, and AC standby bus when battery is only power source.



So now to me that means regardless of normal power or not if the BAT is OFF the battery bus will also not be powered along with the SHBB and HBB.
Thus on the same lines once the Battery is flat and the charger not working would the battery bus be powered even though we have normal AC power???

As per me once the Battery goes flat and the charger not working the SHBB, HBB and the Battery Bus also would be unpowered.

What do you think?

Dear Sirani,

Many of my ab initio students think I am rude. As a matter of fact, quite a few of the captains I train think so.
But most of them admire me after being on line for a while.
And yes, they would not RTB like you mentioned in your experience.
But that is me, after 36 years of airline flying.
Nowadays, it seems to be a different way of doing things.
That does not mean I trust the new breed of so called paper captains.
Please do not take this as an offence. After all, your assertiviness did bot offend me either.

Hi Sirani,
Thanks for letting us in on your experience.
In my opinion it was a good idea to RTB. In another thread about this same topic is a link to an accident report of an AA 757 who lost all battery power after electing to continue in 2008. When the battery is dead you lose a lot of systems. I think you would enjoy reading the report in the other thread even though it was in a 757.
Cheers

Framer,

Found this on the net .. don't know how authentic it is but gives a list of what is powered by what...

http://www.sjap.nl/NG%20Electrical%2...20Services.pdf

I agree coming back was the right thing to do especially with CAT 3 conditions in Delhi. Read the report on the B757, thanx. Scary.

Hi Sirani,


I think the Battery Bus will still remain powered if you lose transfer bus 2 and the battery is discharging. The key busses you will lose are the Hot Battery Bus and the Sw. Hot Battery Bus. In this situation your decision was good if you were in a single battery equipped aircraft. In a dual battery aircraft both buses should remain powered via the aux. battery charger. The system will last much longer than the 30 mins. as quoted in the loss of all generators scenario and being on battery power. I have a 737 guide which gives a number of 2 hours but it actually could be quite longer than this if you could minimize the load on the battery. I would treat this scenario as a land soon but not land ASAP. Soon will have a different meaning for each captain. This is why we are paid to make decisions as Captains. Not all things are black and white and this scenario gives you little to no guidance from the QRH. The Transfer Bus off checklist is also leaving you hanging out to dry with little guidance.

A depleted battery will also cause a dual IRS DC FAIL..this will tell you that your HBB and SHBB are dead ..in this case you better get your wheels on earth quickly because if you have an engine or APU fire the only thing you can do is pull the handles and hope for a miracle

Elmok: Many small commercial Aircraft with turboprop engines are not equipped with fire extinguishing systems, e.g., it is an option.

Thing you are getting scared for the wrong reason.

can you dispatch with a 737 without fire extinguishers ? this is not an "option" on a Boeing. a fire under your wings is a serious matter.

Update to my case as you may have known... the battery was just replaced before my flt. It was sent to the manufacture for diagnostics and they didn't find anything wrong with it and engineering insists the charger was connected properly. I think its being covered up by my company. No further action. But they are happy i came back.


Dear WYOMINGPILOT,

even with a 2 battery a/c the aux battery only couples when there is loss of a/c power. but here a/c power is available so the aux battery probably won't connect and u have power only from one batt

??????

Dear elmokh,

Thanx.. i really didn't want to find out what would and wouldn't have worked if the battery went dead..

but a dual IRS DC Fail.. scary...:uhoh:

Not sure I follow you there. With AC power there would be no impact on an otherwise normal operation.

Some previous colleagues had a dual IRS DC Fail and continued flying for quite some time.
It was a battery overcharge condition that, according the engineers, would have exploded if it was left much longer.
(It was a 73 classic)

The important point we've got to is once the battery runs out the ability to fire the squibs and put out engine fires is lost. Err, I think that would make my mind up on it's own.


30 minutes at normal loading is going to mean a lot longer at a light discharge, but the light doesn't come on for light discharges......


I think it's time to go take a look in the books again for me..

Did i read somewhere that the spar fuel SOV has it's own small battery in the spar? intended to shut the SOV if the fire handle is pulled without electrical power? (73NG)

Kefuddle,

I guess u r right.. with the DC FAIL and ac power it would still work.

But I didn't want to find out what would work and what wouldn't if the battery failed (depleted).

Spar Valve Battery
 

Did i read somewhere that the spar fuel SOV has it's own small battery in the spar? intended to shut the SOV if the fire handle is pulled without electrical power? (73NG)

There's a little panel at the bottom of the P6 panel that that battery lives behind.

Since it is not an emergency , Call/ACARS your Airline/MCC and ask what they prefer you do,if reasonable do as requested.
CRM is not limited to the flight and cabin crews.

Call/ACARS your Airline/MCC

The majority do not have this luxury.

Strange..last 3 operators I worked for had it.
Maybe its a lowcost airline thing?