Switchable longitudinal voltage source (100, 101) for interconnection into a first line and a second line and for feeding in a respective longitudinal voltage (Ua, Ub) into each of the two lines, wherein the longitudinal voltage source (100, 101) comprises: - a first H-bridge circuit comprising four switches (S1-S4), said first H-bridge circuit having a first outer output terminal, a second outer output terminal and two center terminals (M1, M2), wherein the center terminals (M1, M2) can be connected to the first line at a disconnected location of said first line, wherein the disconnected location of the first line has a first line end and a second line end and a first of the two centre terminals can be connected to the first line end and a second of the two centre terminals can be connected to the second line end, - a second H-bridge circuit comprising four switches (S5-S8), said second H-bridge circuit having a first outer output terminal, a second outer output terminal and two center terminals (M1, M2), wherein the center terminals (M1, M2) can be connected to the second line at a disconnected location of said second line, wherein the disconnected location of the second line has a first line end and a second line end and a first of the two centre terminals can be connected to the first line end and a second of the two centre terminals can be connected to the second line end, and - a capacitor, the first capacitor terminal of which is connected to the two first output terminals of the two H-bridge circuits (110, 120) and the second capacitor terminal of which is connected to the two second output terminals of the two H-bridge circuits (110, 120), characterized in that - at least one first switching module is connected between the first capacitor terminal and the first output terminal of the first H-bridge circuit, - at least one second switching module is connected between the first capacitor terminal and the first output terminal of the second H-bridge circuit, - at least one third switching module is connected between the second capacitor terminal and the second output terminal of the first H-bridge circuit and - at least one fourth switching module is connected between the second capacitor terminal and the second output terminal of the second H-bridge circuit. - a first H-bridge circuit comprising four switches (S1-S4), said first H-bridge circuit having a first outer output terminal, a second outer output terminal and two center terminals (M1, M2), wherein the center terminals (M1, M2) can be connected to the first line at a disconnected location of said first line, wherein the disconnected location of the first line has a first line end and a second line end and a first of the two centre terminals can be connected to the first line end and a second of the two centre terminals can be connected to the second line end, - a second H-bridge circuit comprising four switches (S5-S8), said second H-bridge circuit having a first outer output terminal, a second outer output terminal and two center terminals (M1, M2), wherein the center terminals (M1, M2) can be connected to the second line at a disconnected location of said second line, wherein the disconnected location of the second line has a first line end and a second line end and a first of the two centre terminals can be connected to the first line end and a second of the two centre terminals can be connected to the second line end, and - a capacitor, the first capacitor terminal of which is connected to the two first output terminals of the two H-bridge circuits (110, 120) and the second capacitor terminal of which is connected to the two second output terminals of the two H-bridge circuits (110, 120), characterized in that - at least one first switching module is connected between the first capacitor terminal and the first output terminal of the first H-bridge circuit, - at least one second switching module is connected between the first capacitor terminal and the first output terminal of the second H-bridge circuit, - at least one third switching module is connected between the second capacitor terminal and the second output terminal of the first H-bridge circuit and - at least one fourth switching module is connected between the second capacitor terminal and the second output terminal of the second H-bridge circuit.