This paper reviews various problems which are met in designing highvoltage air-blast circuit-breakers, and the way in which the application resistance switching confers another degree of freedom upon the designer in meeting the many conflicting requirements.It is first of all shown that a circuit-breaker must not only break its rated breaking current but must also limit the value of surge voltage which it causes while it is opening the circuit. This can be brought about by limiting the electric strength of the breaker contact gaps.An experimental method of determining the electric strength of the nozzles under air-flow conditions is then explained, followed by discussion of the means by which resistance switching enables the designer to control the breaking capacity and the electric strength of the nozzles independently.A 132-kV breaker designed in conformity with the conclusions reached is described, and the results of a full-scale test set-up are given.The paper then gives an analysis of the validity of “unit testing” applied to multi-break breakers with resistance switching, as compared with the present two-part testing applied to oil circuit-breakers in accordance with B.S. 116, Part 2.In conclusion, proposals are made for a series of type-test duties to be applied to air-blast circuit-breakers, in order to prove compliance with their breaking-capacity ratings.