B737 Transfer Bus APU / External Power

[Ka8 Flyer]

Hi folks,

On the NG, is it possible to power the AC Transfer Busses through either the APU generator or external power if the Bus Transfer Switch is in the OFF position? Wouldn't that cause the two BTB's (and the Bus Tie Relay) to open and thus not allowing current from the APB/EPC to the Transfer Busses?

Thanks,

Mark

[NSEU]

Seems to depend on whether the aircraft is on the ground or not.

Quotes from my training notes....

".... when you put the bus transfer switch to the OFF position..."

"On the ground, no change of BTB position happens if both AC transfer buses have power from the APU starter-generator"

" On the ground, no change of BTB position happens if both AC transfer buses have power from the external power source"

Hope this helps.
Rgds.
NSEU

[Ka8 Flyer]

HI NSEU,

thanks for your answer! Are there any more Air/Ground logics in the electrical system? The manual doesn't state so (OM) but who knows...

PS any hints on this Question?
http://www.pprune.org/forums/showthread.php?t=278744

Kind regards,

Mark

[RAT 5]

I'm not trying to be smart, and I shall try to find the answer, but: in the original question it could be interpreted as asking whether the APU or GPU could be put onto the TFR busses if the switch was open.

In the reply it suggested that the power would stay on the busses, i.e. already powered up, if the switch was then opened.

Does the answer match the question? I'll ask around.

[Ka8 Flyer]

Hi Rat5,

I would like to know the answer to both questions, if possible :)
Also, if it really only applies to the ground (well naturally for the external power but what about the APU?)
And does this switching include the Bus Tie Relay (which should open if the Transfer Switch is in the OFF position)

Thanks!

[NSEU]

"Are there any more Air/Ground logics in the electrical system?"

lol... of course.... The Ground Handling Bus needs to know when to power up ;)

The statements made about BTB's remaining in the last state are true. If the BTB's are open, they will remain open. If they are closed, they will remain closed.

Re the annunciator power sources. I'm sure the Boeing wiring schematics have this information (PMDG and LDS sims usually model this data correctly... although there are many variations between individual real world aircraft)

Regards.
NSEU

[sudden Winds]

The bus transfer switch only isolates the busses primarily to prevent unwanted transfer in case of electrical smokes, fumes or fire.
if you turn it off in flight or on the ground and both generator busses are powered nothing happens...if only one gen bus is powered and you position the bus transfer switch to off you´ll isolate the busses and one of them will be unpowered, that side will lose instrument panel some lts, some ignition, an IRS, etc.
That switch does not isolate the transfer bus from its generator bus.
Regards,
SW.

[Ka8 Flyer]

I've been thinking about this a little more and have a follow-up question:

As NSEU stated, the Transfer Switch will only change the BTBs' position once airborne. So what will happen at liftoff in this example:

After selecting the APU Generator to both busses, the Transfer Switch is moved to OFF. BTB1/BTB2 remain closed. The aircraft now takes off (the IDG's are never connected).

Wouldn't this scenario cause both BTB's to open once airborne? Ok, thankfully the NG will connect the IDG's auomatically if the APU generator fails. Is this what happens?

[blackmail]

hello gentlemen
concerning the b737ng elec sytem, by the way much improved over the classics' elec system, one principle is : gpu & apu can never share the electrical load distribution, regardless of the position of the bus transfer switch. it is the winner takes all. all power sources must be manually switched on & the last one takes the load & disconnect the previous one. also, no parallelling possible of power sources, for ac system.
now what's the difference, if the bus transfer switch is open?
first, the dc bus tie R9 relay will be open. this relay opens anyway at glide slope intercept if flight director &/or autopilot is engaged. [ is a classic fail scenario in the sim for the devil(sim instructor, no offence taken) in the back]. the tr3 xfr relay will connect to whatever tfr bus is powered, independently from the tfr bus switch(normally from tfr bus nr 1)
the gpu will connect normally, even with the bus transfer switch open.
for the apu, you must connect both apu switches in order for powering the whole system. you do this all the time anyway, even with the bustfr switch in auto. the difference being that in a normal situation( bus tfr auto), one apu switch connects the whole system & the second apu switch just extinguish the associated source light. in flight apu generator can power the whole system, except for some load shedding, galley busses & main busses.
hope this helps,
bm

[Ka8 Flyer]

Blackmail,
Isn't it the Bus Tie Relay that opens during G/S intercept, thus isolating DC Bus 1 from DC Bus 2.
According to my notes, the TR3 Xfer Rly can only switch between "Left" and "Right", thus selecting either AC Xfr Bus1 or AC Xfr Bus2 as TRU3's power source.


Also, your statement indicating that it is possible to connect external power with the transfer switch in the off position seems to contradict what NSEU stated. For this to work, BTB1/2 would already have to be in the closed position...
Not trying to sound smart just trying to understand the thing :confused:

[blackmail]

hello ka8,
yes,sorry, it was a typo. i corrected my above post. i have a home cbt with a freeplay elec system & you can connect the total ac system on the gpu, with the transfer bus switch in the off position.

kind regards,

bm

[Ka8 Flyer]

Hi Blackmail,
thanks for your feedback. Okay I have now come up with a new theory:

As long as the APU generator or the external power are connected (note, both APU generator connect switches have been toggled), nothing will happen to the BTBs if the Transfer Switch is moved to Off. If only one side has been connected to the APU, the other side will trip if you move the Transfer Switch. (Say you select the left APU generator switch and set the Transfer Switch to Off, BTB1 will remain closed but BTB2 will open, isolating AC Transfer Bus 2)
This should be common to both Ground and Air ops. As to how the bus tie rly behaves, I don't know. My _guess_ is that it always opens with the Transfer Switch in the Off position as it doesn't de-energize any bus as long as everything is working correctly.