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The paper presents a survey of miniature-circuit-breaker (m.c.b.) design and practice. After tracing the origins of m.c.b. technology in Europe and the USA, it examines their influence on the British concept, with particular reference to the degree of overload and short-circuit protection provided. To ensure the successful application of m.c.b.s, it is essential that they be properly integrated into distribution networks and not be merely regarded as one protective device replacing another: consequently, they must be correctly co-ordinated with the fuses or other circuit breakers with which they may be associated, and the various aspects of discrimination and backup protection are therefore studied in some detail; it is shown that the let-through energy (I2t) is the controlling quantity in this respect. Future trends may include the increasing use of the quick-acting-type m.c.b., which embodies a `cutoffÂ¿ feature and which therefore has an increased short-circuit capacity, but this design has also certain limitations. Universal application of m.c.b.s is likely to be further encouraged by the progress made in international standardisation and approval, particularly within Europe, since this will contribute towards the removal of technical trade barriers.