Switching device comprising - a current transformer for measuring in a potential-free manner a current to be switched by the switching device, - a current sense amplifier, to which the unrectified measurement current of the current transformer is fed on the input side, and - a controller, to which the output current of the current sense amplifier is fed on the input side and which is configured so as to digitalise the fed output current and to perform one or more performance-related measurements on the basis of the information contained in the digitalised output current and further information from an additional voltage measurement of the current to be switched, and to output the measurement results, characterised in that - the current transformer is operated in such a way that the phase shift angle between the current to be switched and the measurement current is approximately zero, - the current sense amplifier has an operational amplifier operated without the use of phase shifting elements as differential amplifiers, to which a voltage dropping across a resistor, through which the unrectified measurement current of the current transformer flows, is fed on the input side for amplification while maintaining the original phase information of the current to be switched, and - the controller is configured to perform the performance-related measurements on the basis of the amplitude and phase information of the current to be switched, said information being contained in the digitalised output current, wherein a performance measurement, an energy measurement and/or a performance factor measurement are carried out as the performance-related measurements. - a current transformer for measuring in a potential-free manner a current to be switched by the switching device, - a current sense amplifier, to which the unrectified measurement current of the current transformer is fed on the input side, and - a controller, to which the output current of the current sense amplifier is fed on the input side and which is configured so as to digitalise the fed output current and to perform one or more performance-related measurements on the basis of the information contained in the digitalised output current and further information from an additional voltage measurement of the current to be switched, and to output the measurement results, - the current transformer is operated in such a way that the phase shift angle between the current to be switched and the measurement current is approximately zero, - the current sense amplifier has an operational amplifier operated without the use of phase shifting elements as differential amplifiers, to which a voltage dropping across a resistor, through which the unrectified measurement current of the current transformer flows, is fed on the input side for amplification while maintaining the original phase information of the current to be switched, and - the controller is configured to perform the performance-related measurements on the basis of the amplitude and phase information of the current to be switched, said information being contained in the digitalised output current, wherein a performance measurement, an energy measurement and/or a performance factor measurement are carried out as the performance-related measurements.