Airstairs Normal/Stby switch... Standby position?

[JetA1]

Hello all,

I understand the airstairs operation.
Looking at the below;

When the external switch is placed to standby and the extend/retract switch left in neutral what physically happens?
Is the entire switched hot batt bus powered, so all the flight deck lights up?
Is it draining any power at this point just by being in standby? Or nothing until extend/retract is commanded, which then completes a circuit and power is drawn?

Thanks for info!
All info appreciated

[maro12]

Hi JetA1,

When you place NORM/STBY switch to STBY (Ref.SSM 52-61-11), one pair of the contacts of the switch connect ground to K8 SWITCHED HOT BAT BUS RLY coil inside SPCU (Ref.SSM 24-31-12, D11714A pin 1). K8 coil gets power from Battery, through BAT BUS CB on J9 (C1340) and SW HOT BAT BUS CB on the front face of SPCU (C169), therefore assuming C1340 or C169 are not open (both closed), when you place NORM/STBY switch to STBY, K8 will close energising SW HOT BUT BUS SECT 1, SECT 2, SECT 3. Anything connected to these buses will be energised.

You can read on SSM that placing Airstair external switch to STBY is the same as closing contact set 4 in BAT SWITCH S5 (BAT SW to ON).

NOTE: References above are generic, please confirm with SSM specific for your Aircraft effectivity.

Best Regards,
Maro12