9.1 Introduction

Much literature exists relating to the historical developments worldwide associated with switchgear related technology [1 29]. A circuit breaker (CB) can be described [3] as a device used in an electrical network to ensure the uninterrupted flow of current in that network under normal operating conditions, and to interrupt the flow of excessive current in a faulty network. Under some circumstances, it may also be required to interrupt load current and to perform open-close-open sequences (autoreclosing) on a fault on others. The successful achievement of these, and indeed all, duties relies on extremely careful design, development and proving tests on any circuit breaker. Good and reliable mechanical design is required [1] to meet the demands of opening and closing the circuit breaker contacts, and an effective electrical design is also essential to ensure that the CB can deal with any of the electrical stresses encountered in service [4, chap. 7].

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...