A sensor and a signal preprocessing unit for this sensor are connected in series in a circuit breaker. The series circuit is normally calibrated. According to at least one embodiment of the invention, the sensor is now calibrated individually, and the calibration value obtained during this process is used from then on. The signal preprocessing unit is also calibrated individually, and a calibration value obtained in this process is used individually. The two calibration values, possibly a plurality of calibration values, are used in order to obtain an overall calibration value, possibly a plurality of overall calibration values, and one such overall calibration value is used by a data processing unit which is connected downstream from the signal preprocessing unit. This allows considerably more exact detection of values of the physical variable.