A circuit-breaker includes a fluid-tight cutout chamber in which are disposed a fixed contact as well as a sliding movable contact and which encloses a dielectric in the liquid state kept under pressure. The movable contact is brought and kept selectively against the fixed contact by releasable disconnecting means, in opposition to ever-present elastic disconnecting means tending to separate the two contacts and comprising hydraulic elastic means responding only to the volumetric elasticity of the compressed liquid in the circuit-breaker. The dielectric in the liquid state is preferably a compressible liquid having a compressibility at least ten times greater than that of oil and may be a liquified gas dielectric.