A switch includes: at least one conducting line led through the switch; switching contacts arranged in the at least one conducting line; at least one measurement transformer arranged at the at least one conducting line; an electronic trip unit, a measurement-input of the electronic trip unit being electrically connected to a measurement transformer secondary winding of a measurement transformer; a power supply unit; at least one power transformer arranged at the at least one conducting line, the at least one power transformer including a power transformer secondary winding which is electrically connected to the power supply unit in order to supply the electronic trip unit; a signal generating unit for generating a test signal with at least a first predefined frequency, with an output of the signal generating unit being electrically connected to the power transformer secondary winding; and a self-test unit electrically connected to the measurement transformer secondary winding.