The invention relates to an arrangement in connection with a feedback control system, the arrangement comprising a controllable device (10), a measuring device (7), a controlling device (9) and a user interface by which the controlling device (9) can be monitored by means of set values. The measuring device (7) is adapted to measure a measuring value (8) from a measuring point, which measuring value is dependent on the operation of the controllable device and the controlling device (9) is adapted to monitor the controllable device (10) on the basis of the measuring value and set values. To improve safety, the arrangement comprises means (15a, 15b, 15c) adapted to feed a reference signal (16) to the measuring device (7) periodically. The controlling device (9) is adapted to compare a measuring value (18) obtained from the reference signal with the real reference value (17) of the reference signal and adapted to take a safety measure when the measuring value (18) and the real reference value (17) differ substantially from each other.