An electricity supply assembly comprises a housing assembly (30,50,150) on which an elongated mounting rail (6) of the din-type is secured, and containing an elongate bus-bar extending parallel to the mounting rail. A circuit breaker of the kind having in an underside thereof a recess (4), having a fixed element (40a,142) projecting into the recess, and a contact member (22,122) projecting from the underside of the circuit breaker at a position spaced to one side of the recess (4,104) on the side thereof opposite to that on which the fixed element (40a,142) is provided. The circuit breaker is mounted on the mounting rail by engagement of the fixed formation beneath one of the flanges (9b) of the mounting rail, and rotating the circuit breaker to bring the recess (4,104) over the mounting rail, causing the contact member (22) to pass through an opening (38) of the housing assembly into conductive engagement with the supply member.