A relay system is provided which is designed to avoid flow of inrush current through a capacitor in a pre-charge mode wherein the capacitor is pre-charged. The relay system includes a pair of power lines, a capacitor, a series-connected assembly, a control circuit, and a relay module. The relay module includes two main switches, a main coil, and a sub-coil. In the pre-charge mode, the control circuit energizes both the main and sub-coils to turn on only one of the main switches. Before entering the pre-charge mode, the control circuit diagnoses the sub-coil. When the sub-coil is determined as being malfunctioning, the control circuit inhibits the pre-charge mode from being entered.