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The designer of low-voltage switchgear is continually confronted with two conflicting demands: first, the requirement for reducing the space occupied by the switchgear; and second, the requirement to increase the interrupting capacity of the circuit breakers which must be installed in the allotted space of the switchgear enclosures. However, higher interrupting capacity is accompanied by the need to be able to close against higher momentary ratings. This requires a more powerful operating mechanism, which usually occupies a greater portion of the allotted space. The a-c solenoid, which is still the conventional power source for low-voltage circuit breakers, requires exceedingly high currents to supply even marginal closing effort in the small available space. It is natural, then, that high-pressure hydraulics, which has proved so successful in aeronautical and automotive practice and more recently1,2,3 has proved equally successful in high-voltage circuit breaker operators, should be extended to the low-voltage circuit breaker field where it can solve the problem of great power in small space and because of its ability to respond without heavy shock to abruptly changing loads during breaker closing stroke. This paper describes the design and development of a simple and unique hydraulic system which has been co-ordinated with a time-tested and proved mechanically trip-free linkage arrangement adapted to fulfill the requirements imposed by the extremely arduous load characteristic of low-voltage high-current-interrupting-capacity air circuit breakers.