The movable portion (35) of a vacuum circuit-breaker comprises a circular spring (46) sliding on an outer bearing surface (41) in most positions, that is suitable for entering a recess (40) on relaxing in order to define a maximally open position for the vacuum circuit-breaker in which locking is thus provided. The movable portion (35) may be made up of two sections (38, 39), of which one, made of copper, is a very good electrical conductor, and is directed towards the stationary contact (37) and receives a connection element for connection to an outside electrical circuit; the section (38) may be made of steel in order to promote sliding of the spring without wear.