CIRCUIT BREAKER

A circuit breaker includes a tripping device (20) in which a magnetic core (50) tapered at (54) and urged to the right by a compression spring is contained in a vessel (46) of viscous fluid in a magnetic frame (40). An armature (22) surrounds the core and is connected to a toggle mechanism forming part of the operating mechanism of the circuit breaker. Under conditions of moderate overloads, the armature and the core inter-act and the armature is retracted slowly so that the tripping device provides a time delay before it operates. Under severe fault conditions the armature (20) is activated directly by the magnetic flux generated by the coil (42) and the effect of the core (50) is of less significance.