Abstract: An automatic control device having an input for inputting measured values of cyclic voltage and/or current; computing device for computing a parameter based on said values of voltage and/or current, and for comparing the computed parameter against a predefined condition; and initiating device for initiating a control function in response to the parameter meeting the predefined condition. The input of the control device is arranged to input a predefined number of samples per one cycle; and the computing device is arranged to compute the parameter with a discrete Fourier transform algorithm optimized based on fixed coefficients related to the predefined number of samples per cycle. An exemplary control device and corresponding control method can provide a significantly faster response time than the earlier general programmed solutions without increasing the related costs as much as the known digital signal processors.