The paper has been prepared as an introduction to this important field of new development.Notable improvements in circuit-breaker design are discussed in particular, with reference firstly to the reduction in oil quantities and subsequently to its elimination. The principal characteristics of air-blast circuit-breakers are reviewed and comparisons between oil and oil-less circuit-breakers are madeThe cross-blast breaker, a recent American development, is referred to, but the paper deals principally with breakers of the axial-blast type developed earlier by Brown Boveri in Switzerland and the A.E.G. in Germany, both of these companies having done much pioneer development for several years with this class of apparatus and both having installations in service in this country to-day.Switching conditions affecting the rate of rise and the magnitude of recovery voltage are discussed, and cathode-ray oscillograms are presented which verify how these factors are affected by the use of resistors connected in parallel with the arc.A description of 11-kV and 66-kV air-blast circuit-breaker designs is given, and the application of these designs to service equipments is reviewed.
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