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