A power supply circuit for an on board energy source or storage device, and particularly for a super-capacitor storage unit having a plurality of interconnected super-capacitors and a first and a second terminal, includes a positive bus connecting the first terminal to a positive pole of a DC high voltage feeding source and a negative bus connecting the second terminal to a negative pole of the high voltage feeding source; and means for disconnecting the super-capacitor storage unit from the high voltage feeding source after detecting circulating fault currents due to insulation losses to ground, which include a high speed circuit breaker and a differential relay that are connected in series along the negative bus, between the second terminal and a grounding point, the differential relay generating an activation signal of the high speed circuit breaker in response to the circulation of a fault current.