The purpose of a circuit breaker is to ensure the unimpeded flow of current in a network under normal operating conditions, and to interrupt the flow of excessive current in a faulty network. It may also be required to interrupt load current under some circumstances and to perform an open-close-open sequence (auto-reclosing) on a fault in others. The successful achievement of these duties relies upon the availability of good mechanical design to meet the demands of opening and closing the circuit-breaker contacts, and good electrical design to ensure that the circuit breaker can satisfy the electrical stresses.

For many applications, two-pressure air systems have been replaced by SF₆ puffer systems, which overcome the need to store large quantities of high-pressure gas for prolonged periods. Instead, pressurisation to produce sufficient flow is achieved by piston compression during contact separation. At higher current levels, arc heating is utilised to 'block' the nozzle in a controlled manner (as indicated in Chapter 2), in order to preserve valuable arc-quenching gas until the appropriate recovery period. There are four distinct types of puffer interrupters, various manufacturers preferring each different form. The simplest is the monoflow system utilising unidirectional flow from the piston chamber. This type of puffer concept has been superseded by more sophisticated forms designed, amongst other factors, to reduce contact vapour and its deleterious effects upon performance.

The proliferation of interrupter types, based not only upon a given arc-control and quenching method but also upon different operating principles, has highlighted the need for computer-aided-design packages to assist during initial design evaluation. The possibility of also being able to provide guidance to prospective clients concerning the predicted performance of the circuit breaker with respect to particular conditions which might prevail in the clients network should also not be overlooked, and could be particularly attractive for isolated 'stand alone' networks with unique characteristics. The usefulness of computer aided-design packages is therefore clear, and has been recognised through the formation of a CIGRE Working Group concerned with arc modelling, whilst both industry and universities worldwide are making progress in establishing practically useful modelling and computational packages.

The distribution function may be regarded as consisting of two stages. The first stage is used to bring the medium-voltage supplies to the centres of use, whilst the second stage conveys the medium-voltage supply to the centres where it is converted to a low voltage for industrial or domestic use. Typically, although not exclusively, primary distribution voltage levels lie in the range 66-145 kV, whilst secondary distribution may be at 3.6-36 kV. The precise levels may vary from country to country and also from region to region within a country. Although 11 kV is usual for secondary distribution in the UK, the North Eastern Electricity Board has a substantial distribution at 20 kV. The switchgear for these purposes in the past have traditionally utilised either insulating oil or air as the arc-quenching media, whereas more recently vacuum or SF6 is being increasingly used. In order to appreciate the impact being made by SF₆ technology upon distribution switchgear, it is necessary first to discuss the requirements imposed upon the circuit breaker.

The development and commissioning of SF₆ circuit breakers and gas insulated components involves testing electrically, dielectrically and mechanically to meet standard specifications as stipulated by existing standards (IEC, ANSI) and also any additional requirements of the consumer. The advent of SF₆ circuit breakers with the closer coupling between arcing and mechanical drive has led to additional test requirements and the introduction of more sophisticated instrumentation.