This work proposes an analysis on a specific topic regarding ballast design and lamp operation in dimming conditions, in compliance with current regulations. Particular issues regarding electrode operating conditions are referred. The SoS definition, currently adopted in IEC standards, is analysed and applied to T5 commercial ballasts, in order to verify reference results. Similar SoS lines are then obtained for two different TL5 fluorescent lamps (HE 21W and HO 39W), using a developed dimmable electronic ballast, based on a magnetic control technique. The analysis and discussion of these results represents a necessary step for the optimization of magnetically-controlled electronic ballasts.