View Full Version : 737-800 Electrics
19th Sep 2008, 13:06
Would anyone tell me what the practical difference is between the battery bus and the switched hot battery bus on the 737-800?
I know there must be an operationally significant difference since they have after all divided them into two different buses, but I can't see it.
They are both operated by the battery switch in the same manner and in the same situations.
All inputs are very welcome :)
19th Sep 2008, 13:27
If the battery switch is off but the standby pwr switch is set to bat, will then the battery bus be energized together with the dc standby bus and the ac standby bus?
If so we have one sole situation where one can power the battery bus but not the switched hot battery bus.
Not that it helps me towards an answer, but maybe it helps you guys towards one.
I certainly hope so :)
19th Sep 2008, 13:51
BATTERY SWITCH (Battery Bus Switch)
-the battery switch must be ON to power the battery bus except......
- the battery bus can be powered with the BAT switch off by moving the Standby power switch to the BAT position.
BATT SWITCH OFF
- removes power from the battery bus and switched hot battery bus when operating with normal power sources available (standby power switch in OFF or the AUTO position)
-removes power from the switched hot battery bus,battery bus,static inverter,AC Standby Bus,DC Standby Bus when battery(s) are the only power source
BATT SWITCH ON
- energizes battery bus, switched hot battery bus and electrical meters module on the overhead P5 Panel
- energizes relays that provide automatic switching of standby electrical system (bat bus,static inverter,AC & DC Standby buses) to battery power with loss of normal electrical power.
Hope this Clears things up for you :ok:
B1 and Electron Free
19th Sep 2008, 18:21
Thanks guys, I appreciate it, but it still doesn't answer my question from what I see.
Why aren't the items in the switched hot battery bus instead coupled to the battery bus?
They are energized in the exact same situations, the only exception being one (BAT SW - OFF and STBY PWR SW - BAT).
Of course there is a thought out reason for the design. It's just that I can't see it.
20th Sep 2008, 08:14
My understanding of the basic electrical system of the NG, with regard to the battery switch is as follows:
With the Battery switch OFF - the switched battery bus is not powered, except in the case of the standby power switch in BAT. But the hot battery bus continues to be powered unless the battery is disconnected!
With the Battery switch ON - all battery busses are powered.
Battery switch ON - can operate APU starter and electric stairs.
Battery switch OFF - those things won't be powered, but other things like the two airplane clocks will continue to be powered (although with no display).
Short, but I think you will get my drift...
20th Sep 2008, 08:51
Firstly, I haven't worked the -800, but I know about the classics. I would imagine that they are not too different from each other, from what I've read so far.
My understanding, is that under normal conditions, ie. Batt on, Stby Pwr switch auto, in Air mode, the hot bat buss was fed via the battery, but the battery bus, confusingly, is fed via some relays from TRU3 (115V Bus2). I think the reasoning is that if you lost one power source, it's so configured, that you wouldn't lose all the instruments, radios, lighting etc. in one go, so giving you the chance to start the APU and get some power back, or reconfiguring the DC system according to the QRH.
Hope that helps a bit.
21st Sep 2008, 04:23
I don't think the Standby Power system knows or cares whether the airplane is airborne or not!
21st Sep 2008, 09:31
Again thank you for your responses. It seems you all know the system schematics, but my question may have been written unclearly.
Still my question stands: Why does the 737-800 have a battery bus and a switched hot battery bus?
The only situation where just one of the two is energized is (as stated by several of you) when the battery switch is off and the standby power switch is set to bat. But when would one do this?
I myself cannot think about any situation.
So in all normal cases the battery bus and the switched hot battery bus are powered at the exact same time (i.e. when the battery switch is on).
(Even though the battery bus is energized via TR3 when the transfer buses are energized, it turns off when the battery switch is set to off).
21st Sep 2008, 12:22
"I don't think the Standby Power system knows or cares whether the airplane is airborne or not!"
Sorry, but from a Standby Power point of view, it most certainly does. Depending on the model, and customer options, like two batteries for instance, the Standby power switch "Auto" function can be directly related to an Air/Gnd function. That's why I always used to see the avionic guys putting the 737's in Air mode to test the system - every day. Under normal conditions, the Standby Busses are powered from one of the transfer busses (1 I think). If your in the air and lose that bus it will automatically turn on the Inverter and connect the battery to the Standby Bus. Now I freely admit, I'm not familar with the NG, but I can't belive it's all that radically different.
KristianN -It's purely so there is a separation of systems and to enable control of bus switching etc.
21st Sep 2008, 12:47
The hot battery bus is connected to vital emergency equipment so they are live all the time ie engine fire extinguishers.
Switched battery bus does other emergency equipment like the emergency exit lights when they are in the armed position so when ac power is lost they illuminate.
To get an understanding of the difference its prob best to look at the uses of the hot and normal battery bus.
22nd Sep 2008, 02:26
Separation of the systems? But if one power source goes offline these two buses and their associated systems will stay energized or de-energized in the same situations. So the question is: What is then the point of separation?
And control of bus switching? Is there any procedure you know of where one switches as stated above to energize the battery bus and de-energize the switched hot battery bus?
24th Sep 2008, 23:51
Its to do with elec bus isolation when trying to conserve ships main batteries or depowering busses one by one to eliminate smoke source.Following dual gen failure with no APU,youd be down to batt bus,sw hot batt,stby busses,INV and hot batt bus..to prolong ships batteries(nowhere to land) you may elect to depower further in an effort to save them if in day VMC on top..its complex and rarely taught.737 must have APU for ETOPS so...
25th Sep 2008, 12:18
The hot battery bus cannot be de-energized. Its always live unless it is physically disconnected at the teminals.
With all generators inop and standby power off systems powered bat switch on:
standby comp lt
white dome lt
instr flood lt
Capt pitot heat
engine fuel shut off valve
AC pack valves
Manual press control
cabin alt warning
Capt EFIS control panel
Capt outboard DU
Left ILS, GPS
Standby instruments clocks
engine hyd sov
stby rudder sov
inbd antiskid sys
air/gnd sensor sys
landing gear indicators
APU fire detection
APU fire bottle
ENG fire detection
ENG fire bottles
Cargo fire bottles
Stall warning sys
Master caution sys
Switching the battery off you would lose all of the above which are the essentials to keep the ac flying. The exceptions being the fire detection and protection sys as they are connected to the hot battery bus.
As mentioned before seperating the systems is used for load shedding and to isolate a smoke/fire source. There isn't anything in the QRH which requires you to switch off the switched bat bus(bat switch) as you would lose all listed above. If the prob came down to one of the above systems causing a major problem you could pull some CB to isolate it.
24th Oct 2008, 01:08
Thanks for the answers guys, but I still haven't seen a valid answer here.
24th Oct 2008, 01:41
Battery bus – Normally powered by TR3, alt power is battery. Powered when the battery switch is ON or the standby power switch is BAT.
Switched hot battery bus - Only powered when the batt switch is turned on.
24th Oct 2008, 01:51
Thanks. I know the schematics.
But again. Why separate them?