In an undervoltage tripping unit of at least one embodiment, an armature acts on a tripping slide via a drum. During tripping, an auxiliary switch is closed. In order to reset the under-voltage tripping unit after tripping, a latching mechanism of an associated circuit breaker acts on the drum via a driver bolt. At the same time, a projecting element is formed on the drum and closes the auxiliary switch when the drum is rotated back. The auxiliary switch therefore need not itself be directly coupled to the latching mechanism.