Abstract: An electromagnetic circuit breaker comprising a solenoid coil and an armature actuatable by the coil. A tube of nonmagnetic material within which is a movable magnetizable core, the tube having a pole piece at one end toward which the armature is attracted on predetermined overload conditions, the core being biased toward the end of the tube away from the pole piece.An inertial mass is carried by the armature and is pivotally and eccentrically connected to a link at one end. The link has another end pivotally mounted to a fixed support.Thus, when the armature is attracted to the pole piece the inertial mass is caused to rotate at an angular velocity greater than the angular velocity of the armature, so that the current in the coil required to overcome the bias on the armature with substantially no time delay, when the gap between the core and the pole piece is at its maximum, is substantially higher than the current required for a similar device but which omits the inertial mass.