A320 APU

[flame_bringer]

Sarius
If the start contactors are disconnected the apu wouldnt go up to 20% it just wont crank.
BCLs will only disconnect the start contactors during rat deployment while the red rat legend is on to inhibit apu start during rat deployment.
It is the ECB which aborts the start by disconnecting the start contactors upon 20% if batteries are not selected.

[sarius]

i see... so it's the BCL logic call that signals the ECB to shut down the APU.

but once the APU is up and running, will it do an auto shutdown if the batteries are switched off?

[flame_bringer]

No it won't shutdown as long as the APU GEN is online.
However if you switch off the APU generator or delatch the Bus Ties whilst the Batteries are off then it shuts down because ECB power supply will be lost.

[sarius]

got it! thanks!!

[flame_bringer]

Glad you got it keep asking and researching thats the best way to learn .

[Rishabh0005]

Till now there is no perfect answer for this thing?? Is there any new idea about that?
​​​​​​Why the two batteries should be on while apu starting or while it is running on ground when ground power is connected?

[abilgilang]

Caution:
Do not operate the apu when the main batteries are off. If you operate the apu in these conditions, you can cause these results:
-
if there is an apu fire, the apu fire-bottles will not operate
-
there will be no de-oiling of the apu during shut-down. This will cause smoke in the cabin
-
there will be no cool-down sequence of the apu
-
the igv will not close fully. This can cause problems when you start the apu again

[Acie]

Originally Posted by abilgilang

there will be no de-oiling of the apu during shut-down. This will cause smoke in the cabin

Depending on the APU oil level, this may also trigger low oil level message on ECAM upon APU start.