During the opening and closing sequences, an electric arc occurs between the contacts of a circuit breaker and advantage is taken of this electrical discharge to assist in the circuit interruption process, as has been discussed in detail [4, chap. 7]. In an AC electrical network, the arc is tolerated in a controlled manner until a natural current zero of the waveform occurs when the discharge is rapidly quenched to limit the reaction of the system to the interruption. With asymmetrical waveforms and for DC interruption, advantage is taken of the arc resistance for damping purposes or to generate a controlled circuit instability to produce an artificial current zero.