Patent ID: 8817442

Claim:
A switch for interrupting an alternating current flowing through a conductor that is led through the switch, the switch comprising: a switching mechanism configured to separate switching contacts in order to open the switch, a measurement transformer arranged at the conductor and configured to generate an analog measurement voltage corresponding to the alternating current, an electronic trip unit to which the measurement voltage is applied, the electronic trip unit configured to: compare a current derived from the measurement voltage with a predefined current threshold and, if the current exceeds the current threshold, controlling the switching mechanism to trigger a separation of the switching contacts, an iron-cored power transformer arranged at the conductor, the power transformer configured to output a transformer voltage, a rectifier circuit connected to the power transformer and configured to charge a capacitor to a predefined capacitor voltage, the capacitor configured to feed the power supply of the trip unit, the capacitor being connected to a monostable multivibrator, the output of which is set to a predefined level depending on the operation of the power transformer by signals corresponding to the transformer voltage, a switching device configured short-circuit the rectifier circuit when the predefined capacitor voltage is reached, wherein the monostable multivibrator remains set, as long as the signals from the power transformer are tapped off within a time determined by the time constant of the monostable multivibrator, and wherein the level at the output of the monostable multivibrator is used to test the operation of the power transformer.