I was always taught, and now teach, that when you get the Master Caution ELEC that you start from the top of the electrical panel and work down. In doing so whatever light you come to first is the QRH checklist you run. Consequently you would then discover that if the DRIVE light is illuminated, that is the root cause of the ELEC Master Caution, the secondary effect of that malfunction is the SOURCE OFF illuminating. You then ask for the QRH DRIVE checklist and complete that. It is important to do that first as leaving this could cause damage until it is disconnected, start the APU when directed to do so by the QRH. There is no need to complete the SOURCE OFF checklist as this is a secondary effect of the DRIVE malfunction.
Last edited by Johnny F@rt Pants; 2nd Mar 2020 at 06:47.