24V (Safety Extra Low Voltage) power supply with batteries for buffering are warranty for an uninterrupted power supply. The PSD System forms the interface between Railway Power Supply and Public Power Supply. These different voltages may cause dangerous situations under load conditions (e.g. starting trains) because of the coupling through the ground (earth). The usage of SELV for the power supply of the PSD system (control units and power devices) avoids additional bonding systems.