there is single locking and double locking - one locking is without alarm arming (central lock button) when the fuel cover is not locked (US cars) and the doors can ba opened from inside with double pull on inside handles... the other locking is when the alarm is activated, the fuel cover is locked and the doors can NOT be opened from inside.

In case of the electrical failure, the mechanical lock is still operational to open the vehicle from the driver's door, and then the system needs to be energized in order to unlock the rest of the doors