Circuit breaker (10, 20, 30, 40, 50) configured for interrupting a direct current in a current path of a high voltage DC transmission system, the circuit breaker comprising:- a first switch having a disconnecting contact; a second switch; wherein the first and second switches (1,2) are operatively connected in series in the current path; a capacitor operatively connected in parallel with the first switch; wherein:- the capacitor is selected such that, when the first switch starts to open, the current in the current path experiences a series inductor-capacitor network to cause a change in the current from a direct current waveform to an alternating current waveform; and a switch opening control:- operating to start to open the first switch while the second switch is closed thereby to cause the current flowing in the current path to charge the capacitor and to experience the series inductor-capacitor network hereby the charging of the capacitor causes the current to subsequently reduce; and operating to open the second switch at a time delay after starting to open the first switch thereby to interrupt the current at a low current or a zero current in the alternating current waveform; characterised in that the disconnecting contact of the first switch has a contact velocity sufficient so that the voltage across the capacitor is below a withstand voltage of the first switch at substantially all times, and wherein the circuit breaker further comprises; at least one further first switch in series with the first switch and both in parallel with the capacitor; wherein the switch opening control operates to start to open each first switch (1, 1') while the second switch is closed. a first switch having a disconnecting contact; a second switch; wherein the first and second switches (1,2) are operatively connected in series in the current path; a capacitor operatively connected in parallel with the first switch; wherein:- the capacitor is selected such that, when the first switch starts to open, the current in the current path experiences a series inductor-capacitor network to cause a change in the current from a direct current waveform to an alternating current waveform; and a switch opening control:- operating to start to open the first switch while the second switch is closed thereby to cause the current flowing in the current path to charge the capacitor and to experience the series inductor-capacitor network hereby the charging of the capacitor causes the current to subsequently reduce; and operating to open the second switch at a time delay after starting to open the first switch thereby to interrupt the current at a low current or a zero current in the alternating current waveform; characterised in that the disconnecting contact of the first switch has a contact velocity sufficient so that the voltage across the capacitor is below a withstand voltage of the first switch at substantially all times, and wherein the circuit breaker further comprises; at least one further first switch in series with the first switch and both in parallel with the capacitor; wherein the switch opening control operates to start to open each first switch (1, 1') while the second switch is closed. the capacitor is selected such that, when the first switch starts to open, the current in the current path experiences a series inductor-capacitor network to cause a change in the current from a direct current waveform to an alternating current waveform; and a switch opening control:- operating to start to open the first switch while the second switch is closed thereby to cause the current flowing in the current path to charge the capacitor and to experience the series inductor-capacitor network hereby the charging of the capacitor causes the current to subsequently reduce; and operating to open the second switch at a time delay after starting to open the first switch thereby to interrupt the current at a low current or a zero current in the alternating current waveform; operating to start to open the first switch while the second switch is closed thereby to cause the current flowing in the current path to charge the capacitor and to experience the series inductor-capacitor network hereby the charging of the capacitor causes the current to subsequently reduce; and operating to open the second switch at a time delay after starting to open the first switch thereby to interrupt the current at a low current or a zero current in the alternating current waveform; characterised in that the disconnecting contact of the first switch has a contact velocity sufficient so that the voltage across the capacitor is below a withstand voltage of the first switch at substantially all times, and wherein the circuit breaker further comprises; at least one further first switch in series with the first switch and both in parallel with the capacitor; wherein the switch opening control operates to start to open each first switch (1, 1') while the second switch is closed.