can be used as a decision value for example the number of which indicates how often the value u03b4fr exceeds a threshold within a predetermined time span or within a predetermined number of investigated regulated value deviations hence the actual method of computation is not essential for determining the decision value ew but it is important that an averaged absolute value of deviation be determined it is important to use the amount so that both regulated value deviations such as are caused by the occurrence of a fault and those brought about by the disappearance of this fault are taken into account the averaging is important so that not every rapidly transitory higher regulated value deviation results in the output of a fault signal fs by the comparator unit 21 which in particular compares the respective current decision value ew with a decision quantity threshold value and which outputs the said fault signal when the decision value exceeds the decision quantity threshold value in the exemplary embodiment the averaging is carried out by means of a digital low pass filter as explained further below with reference to step s2 of the flow diagram of fig2 for this a low pass constant corresponding to a time constant of a few tens of seconds in respect of a corresponding integrating element is used a method such as is now described with reference to fig2 can be performed by means of the functional units according to fig1.