Speedwinner
Since APU is permitted to be started with batteries alone without external that should answer the fire protection part. APU flap is controlled by APU master so with batteries off and ext.connected flap will close. During start with batteries off the flap will open but APU will not sart and auto shut down. However different MSNs have different APUs you can read my answer on dream747's thread.
Last edited by vilas; 11th Apr 2015 at 08:50.