This chapter discusses the physics of circuit-breaker arcs and the different types used in circuit breakers.

This chapter refers to oil circuit breakers, i.e. a circuit breaker in which the contact members of the interrupting device are separated in oil. The chapter is confined to design features directly associated with the function of the circuit breaker, i.e. the interrupters, contact design, mechanisms; insulation problems, general construction and the effects of environment and features at the interface of the circuit breaker and other equipment with which it forms an entity.

This chapter deals with the technical aspects of the design of air-blast circuit breakers as seen in the early 1980s and does not establish cost or economic parameters for them. It may therefore be worthwhile to bear in mind a number of factors which have been influencing their fortunes throughout their lifetime of some 40 years and will continue to do so in the future.

The vacuum circuit breaker has fascinated the switchgear designer for many years, primarily because of the great advantages of vacuum interrupters, which are: (a) They are entirely self contained, need no supplies of gases or liquids and emit no flame or gas. (b) They require no maintenance and in most applications their life will be as long as the circuit breaker in which they are applied. (c) They may be used in any orientation. (d) They are not flammable. (e) They have a very high commutating ability and need no capacitors or resistors to interrupt short line faults. (f) They require a relatively small mechanical energy to operate them. (g) They are silent in operation.

In this chapter, circuit breaker specification and testing are discussed. For high-voltage circuit breakers, the no-load sound pressure level must not be greater than 95 dB at the most disadvantageous point, 25 m from the circuit breaker. It is appreciated that, under the conditions of interruption of a heavy short circuit, the additional release of energy could significantly increase the noise level emitted. However, because of the extreme rarity of high fault operation, no account is at present taken of such operations in the specification of noise levels.

This chapter discusses the technique of insulation design for circuit breakers.

To distinguish between 'real' (or 'conventional') current chopping and 'virtual' current chopping is, on the face of it, easy. As with many phenomena, the more deeply the processes are examined the more blurred the demarcation becomes. 'Real' or 'conventional' current chopping may be defined as the forced interruption of an alternating current at a time usually just before the natural current zero. The word 'forced' implies that the increasing resistance of the arc in the circuit breaker (defined as the instantaneous arc voltage divided by the instantaneous arc current) is the cause of the current being 'forced' to zero at a time when it would not be zero if the resistance of the circuit-breaker arc were zero when current is finite. 'Virtual' current chopping may be defined as the extinction of the arc at a current zero which is not the natural power frequency current zero. Virtual chopping can occur even in an ideal circuit-breaker in which the arc resistance is finite only when the instantaneous current is zero. Such a circuit-breaker cannot cause conventional current chopping. These definitions imply that any circuit-breaker, however stable its arc, and however free of a propensity to current chop it may be, can still give rise to virtual chopping.