The theoretical considerations in the choice of number of breaks are analysed and it is shown that, while for plain breakers multi-break construction may have its advantages, the more efficient the arc-control devices employed the less the advantages of additional breaks become with normal designs. In particular, it is shown that for a metalclad and cable system, the duty on one break of a multi-break controlled- arc breaker is almost as great as for that of a singlebreak unit. Experimental evidence in support of the theory advanced, obtained with cathode-ray equipment, is given.The development of a single-break 66-kV metalclad equipment is described, with particular reference to the interrupting ability, and test-results are given, obtained on four different short-circuit plants, showing consistent operation. The advantages to the user of the single-break oil circuit-breaker are shown by comparison with double-break designs of similar rating, and single-break layouts are described for different switching schemes and system voltages.In the Appendix an analysis is made of 3-phase cathode-ray films, showing that no leakage current is carried and that in consequence the observed reliable operation is to be expected.