Abstract: An actuating mechanism for a circuit breaker includes a first piston movable in a first cylinder, against the force of a first compressible member, by admitting hydraulic fluid, and a second piston movable in a second cylinder, against the force of a second compressible member, by admitting hydraulic fluid. Hydraulic fluid can transferred from the second cylinder into the first cylinder via an interconnecting fluid passage, and released from the first cylinder via an outlet fluid passage. The first piston is arranged to be connected to a circuit breaker in a switchgear so as to open and close it. Importantly, the first piston is directly mechanically connected to the first compressible member, ensuring that all the energy stored in the compressible member is available to open the circuit breaker.