A power supply unit for an Intelligent Electronic Device for low voltage or medium voltage electric power applications characterised in that it comprises: - a power transformer stage, which is operatively coupled to a feeding conductor to harvest electric power from said feeding conductor, said power transformer stage being adapted to provide a first voltage; - a first storage stage, which is electrically connected to said power transformer stage to receive said first voltage, said first storage stage being adapted to store electric energy and provide a first supply voltage, said power transformer stage being adapted to provide a first charging current to charge said first storage stage, when a current flows along said feeding conductor; - a first step-down conversion stage, which is electrically connectable/disconnectable to/from said first storage stage, said first step-down conversion stage being adapted to receive said first supply voltage and provide a third voltage having a lower amplitude than said first supply voltage, when said first step-down conversion stage is electrically connected with said first storage stage; - a switching stage adapted to electrically connect/disconnect said first step-down conversion stage with/from said first storage stage; - a second storage stage, which is electrically connected to said first step-down conversion stage, said second storage stage being adapted to store electric energy and provide a second supply voltage, said first step-down conversion stage being adapted to provide a second charging current to charge said second storage stage, when said first step-down conversion stage is electrically connected to said first storage stage; wherein said switching stage comprises at least a switching device electrically connected between first supply terminals of said first storage stage and output terminals of said switching stage and a driving circuit to drive said switching device, wherein said driving circuit is adapted to receive control signals from a control unit and enable/disable a switching operation of said switching device in response to said control signals, wherein said switching device naturally switches in a conduction state, if said control signals are no more available. - a power transformer stage, which is operatively coupled to a feeding conductor to harvest electric power from said feeding conductor, said power transformer stage being adapted to provide a first voltage; - a first storage stage, which is electrically connected to said power transformer stage to receive said first voltage, said first storage stage being adapted to store electric energy and provide a first supply voltage, said power transformer stage being adapted to provide a first charging current to charge said first storage stage, when a current flows along said feeding conductor; - a first step-down conversion stage, which is electrically connectable/disconnectable to/from said first storage stage, said first step-down conversion stage being adapted to receive said first supply voltage and provide a third voltage having a lower amplitude than said first supply voltage, when said first step-down conversion stage is electrically connected with said first storage stage; - a switching stage adapted to electrically connect/disconnect said first step-down conversion stage with/from said first storage stage; - a second storage stage, which is electrically connected to said first step-down conversion stage, said second storage stage being adapted to store electric energy and provide a second supply voltage, said first step-down conversion stage being adapted to provide a second charging current to charge said second storage stage, when said first step-down conversion stage is electrically connected to said first storage stage;